Foundation Fieldbus
Fourth Edition
Foundation Fieldbus
Fourth Edition
by Ian Verhappen
and Augusto Pereira
Notice
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Because neither the author nor the publisher has any control over the use of the information
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author nor the publisher makes any representation regarding the availability of any referenced commercial product at any time. The manufacturer’s instructions on use of any commercial product must be followed at all times, even if in conflict with the information in this
publication.
Copyright © 2012
ISA—The International Society of Automation
All rights reserved.
Printed in the United States of America.
10 9 8 7 6 5 4 3 2
ISBN: 978-1-934394-76-2
Ebook ISBN: 978-1-937560-40-9
PDF ISBN: 978-1-937560-81-2
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ISA
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P.O. Box 12277
Research Triangle Park, NC 27709
www.isa.org
Library of Congress Cataloging-in-Publication Data in process.
For my wife Michele, who has supported me throughout my career and through the
original development of this book and its revisions, while also raising our daughters
Ashley and Madeline. You are the glue that keeps it together.
Thank you also to everyone who has supported me during my Fieldbus development
and continued growth, providing opportunities to expand my knowledge and apply
what I have learned. You have made it possible to be able to share this knowledge.
—Ian Verhappen
For my wife Margareth and my sons Sergio and Fabio, who gave me the support to
write this book and helped me during the revisions.
Thanks to everyone who taught me the Fieldbus concepts since the early years and all
the people who, during the several projects that I have been involved in, followed my
suggestions for getting their projects working successfully.
—Augusto Pereira
Table of Contents
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xi
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii
Ian Verhappen, P. Eng., CAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix
Augusto Pereira, Eng. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi
Chapter 1 — Fieldbus Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1
1.2
1.3
Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Application Layer . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2 User Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 Testing and Registration . . . . . . . . . . . . . . . . . 14
1.1.4 Interoperability Test System . . . . . . . . . . . . . . 15
1.1.5 Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.1.6 Topologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Communications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Parameter Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1.3.1 EDDL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.3.2 FDT/DTM . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1.3.3 Field Device Interface (FDI) . . . . . . . . . . . . . . 35
Chapter 2 — Fieldbus Cabling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.1
2.2
2.3
2.4
2.5
Segment Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
High-Speed Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Grounding/Earthing . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Surge Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cable Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
54
60
64
65
Chapter 3 — Fieldbus Power Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.1
3.2
3.3
3.4
3.5
3.6
Intrinsic Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fieldbus Intrinsically Safe Concept . . . . . . . . . . . . . . . . .
3.2.1 Architecture with FISCO installed in
the DCS cabinet . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 Redundant FISCO . . . . . . . . . . . . . . . . . . . . .
Fieldbus Non-Incendive Concept
(FNICO/FISCO Ex ic) . . . . . . . . . . . . . . . . . . . . . . . . . .
High Energy Trunk – Fieldbus Barrier. . . . . . . . . . . . . . .
DART (Dynamic Arc Recognition and Termination) . . .
Selecting the Right Power Supply . . . . . . . . . . . . . . . . . .
TABLE OF CONTENTS
72
74
78
81
81
83
85
87
vii
Chapter 4 — Documentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.1
Segment Loading Calculation . . . . . . . . . . . . . . . . . . . . 100
Chapter 5 — System Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.1
5.2
Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 Client-Server VCR Type . . . . . . . . . . . . . . . .
5.1.2 Report Distribution VCR Type . . . . . . . . . .
5.1.3 Publisher–Subscriber VCR Type . . . . . . . . . .
5.1.4 “Fail Over” Strategies and Design
Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
104
105
105
109
112
Chapter 6 — Commissioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.1
6.2
Physical Layer Checks . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 Cable Testing. . . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 Electronic Commissioning . . . . . . . . . . . . . .
6.1.3 Configuration Commissioning . . . . . . . . . . .
6.1.4 FOUNDATION Fieldbus Digital
Communication Certification. . . . . . . . . . . . . . . . .
6.1.5 Typical Installation Problems . . . . . . . . . . . .
Device Configuration . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1 Configuration of the Resource Block . . . . . .
6.2.2 Configuration of the Transducer Block . . . . .
6.2.3 Configuration of the Analog Input
Function Block . . . . . . . . . . . . . . . . . . . . . . . . . . . .
129
131
133
133
134
139
146
148
148
149
Chapter 7 — Troubleshooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
7.1
7.2
7.3
Optimization Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.1 Physical Fault Symptoms . . . . . . . . . . . . . . . .
Communications and Configuration . . . . . . . . . . . . . .
Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
153
156
157
158
Chapter 8 — Operations & Maintenance . . . . . . . . . . . . . . . . . . . . . . . 161
8.1
8.2
Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
Chapter 9 — New Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
9.1
9.2
viii
Fieldbus Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Wireless & Remote I/O (WIO) . . . . . . . . . . . . . . . . . . . 172
TABLE OF CONTENTS
9.3
9.4
Wireless . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
Host System Interoperability. . . . . . . . . . . . . . . . . . . . . 177
Appendix A — Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
Appendix B — Fieldbus Foundation Specification List . . . . . . . . . . . . 183
Appendix C — Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Appendix D — Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
Appendix E — FF Segment Design Example Exercise. . . . . . . . . . . . . 199
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
TABLE OF CONTENTS
ix
List of Figures
Figure 1-1
Digital control system architecture . . . . . . . . . . . . . . . . . . . 2
Figure 1-2a
OSI model compared with Fieldbus model . . . . . . . . . . . . . 2
Figure 1-2b
Fieldbus data transfer packets. . . . . . . . . . . . . . . . . . . . . . . . 3
Figure 1-3
Manchester encoding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 1-4
Device description hierarchy . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 1-4a
Analog Input Block (AI). . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 1-4b
Analog Output Block (AO) . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 1-4c
PID Block (PID) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 1-5
Fieldbus bridge capability . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 1-6
Maximum length of Fieldbus network. . . . . . . . . . . . . . . . 18
Figure 1-7
Fieldbus network with repeaters. . . . . . . . . . . . . . . . . . . . . 19
Figure 1-8a
Physical layouts – Single combined segment . . . . . . . . . . . 20
Figure 1-8b
Wiring practices – Cable efficiency . . . . . . . . . . . . . . . . . . 21
Figure 1-9
FOUNDATION Fieldbus node addresses. . . . . . . . . . . . . . . . . 23
Figure 1-10a Function block scheduling and macrocycle . . . . . . . . . . . . 26
Figure 1-10b LAS algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Figure 1-11
SCADA layer Fieldbus traffic management . . . . . . . . . . . . 28
Figure 1-12
Radar level gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Figure 1-13
Valve signature with best fit line . . . . . . . . . . . . . . . . . . . . 31
Figure 1-14
Field device interface communications . . . . . . . . . . . . . . . 34
Figure 1-15
FDI Flow Device and Host . . . . . . . . . . . . . . . . . . . . . . . . 37
Figure 1-16
Device Package showing documents to be updated . . . . . . 38
Figure 2-1
Short circuit protection “sizing” . . . . . . . . . . . . . . . . . . . . 48
Figure 2-2
Spur overcurrent failure indication . . . . . . . . . . . . . . . . . . 49
LIST OF FIGURES
xi
Figure 2-3
Fieldbus connector blocks . . . . . . . . . . . . . . . . . . . . . . . . . 50
Figure 2-4
Fieldbus wiring with conventional terminal blocks . . . . . . 51
Figure 2-5
Terminator inside junction box . . . . . . . . . . . . . . . . . . . . . 51
Figure 2-6
Termination guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Figure 2-7
HSE profile functional areas . . . . . . . . . . . . . . . . . . . . . . . 54
Figure 2-8
Ethernet wiring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Figure 2-9
Wiring H1 devices to a linking device . . . . . . . . . . . . . . . . 58
Figure 2-10
Integrated fieldbus system . . . . . . . . . . . . . . . . . . . . . . . . . 59
Figure 2-11
Continuity of ground. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Figure 2-12
Plant ground and instrument ground . . . . . . . . . . . . . . . . . 61
Figure 2-13
Cable shield grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Figure 2-14
Recommended fieldbus grounding . . . . . . . . . . . . . . . . . . 62
Figure 2-15
High frequency capacitive ground . . . . . . . . . . . . . . . . . . . 63
Figure 2-16
Equipotential bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Figure 2-17
Segregation of cable classes . . . . . . . . . . . . . . . . . . . . . . . . 66
Figure 2-18
Segregating cables in trays . . . . . . . . . . . . . . . . . . . . . . . . . 67
Figure 2-19
General cable installation guideline . . . . . . . . . . . . . . . . . . 68
Figure 3-1
Protective systems incendive limits . . . . . . . . . . . . . . . . . . 75
Figure 3-2
Typical FISCO network . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Figure 3-3
FISCO calculation for area classification IIC. . . . . . . . . . . 79
Figure 3-4
FISCO calculation for area classification IIB . . . . . . . . . . . 80
Figure 3-5
FISCO repeater wiring to field . . . . . . . . . . . . . . . . . . . . . . 81
Figure 3-6
Typical fieldbus power conditioner . . . . . . . . . . . . . . . . . . 83
Figure 3-7
Typical fieldbus barrier installation . . . . . . . . . . . . . . . . . . 84
Figure 3-8
High-energy trunk calculation . . . . . . . . . . . . . . . . . . . . . . 85
Figure 3-9
Typical spark behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
xii
LIST OF FIGURES
Figure 3-10
DART extinguished spark . . . . . . . . . . . . . . . . . . . . . . . . . 87
Figure 3-11
Power supply selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Figure 4-1
Network diagram with repeater . . . . . . . . . . . . . . . . . . . . . 91
Figure 4-2
Fieldbus data sheet: individual device . . . . . . . . . . . . . . . . 93
Figure 4-3
Fieldbus data sheet for multiple devices . . . . . . . . . . . . . . 94
Figure 4-4
Digital communication signal symbols . . . . . . . . . . . . . . . 96
Figure 4-5
Multivariable Device Representation on P&ID . . . . . . . . . 96
Figure 5-1
Fieldbus VCR communications . . . . . . . . . . . . . . . . . . . . 104
Figure 5-2
Host configuration screen . . . . . . . . . . . . . . . . . . . . . . . . 110
Figure 5-3
Loop configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Figure 5-4
Multiple loop function block scheduling . . . . . . . . . . . . 115
Figure 5-5
Segment bandwidth calculation. . . . . . . . . . . . . . . . . . . . 117
Figure 5-6
Typical fieldbus architecture . . . . . . . . . . . . . . . . . . . . . . 118
Figure 5-7
Fieldbus Foundation Network with control in the field . 119
Figure 5-8
Macrocycle – control in valve . . . . . . . . . . . . . . . . . . . . . 120
Figure 5-9
Fieldbus Foundation Network with control in the DCS . 121
Figure 5-10
Macrocycle – control in host . . . . . . . . . . . . . . . . . . . . . . 122
Figure 5-11
Control in output device . . . . . . . . . . . . . . . . . . . . . . . . . 124
Figure 5-12
Control in input and output device. . . . . . . . . . . . . . . . . 125
Figure 5-13
Control in Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
Figure 5-14
Control in the DCS – when there are delays . . . . . . . . . . 127
Figure 6-1
H1 network analysis tools . . . . . . . . . . . . . . . . . . . . . . . . 130
Figure 6-2
Electrical cable test meters . . . . . . . . . . . . . . . . . . . . . . . . 132
Figure 6-3
Reel of Fieldbus cable . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Figure 6-4
Correct H1 packet waveform . . . . . . . . . . . . . . . . . . . . . . 135
Figure 6-5
Change in base frequency and amplitude . . . . . . . . . . . . 135
LIST OF FIGURES
xiii
Figure 6-6
Effects of inductive components on waveform . . . . . . . . 136
Figure 6-7
Complete signal distortion. . . . . . . . . . . . . . . . . . . . . . . . 137
Figure 6-8
Check sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Figure 6-9
Correct transmitter installation . . . . . . . . . . . . . . . . . . . . 139
Figure 6-10
Correct installation of Fieldbus junction boxes . . . . . . . . 140
Figure 6-11
Field device grounding error . . . . . . . . . . . . . . . . . . . . . . 140
Figure 6-12
Cable cross-section exceeds 40% of conduit area. . . . . . . 141
Figure 6-13
Failure to maintain required mechanical separation . . . . 142
Figure 6-14
Coiled signal cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
Figure 6-15
Corrosion caused by liquid entry . . . . . . . . . . . . . . . . . . . 144
Figure 6-16
Corrosion in a junction box. . . . . . . . . . . . . . . . . . . . . . . 144
Figure 6-17
Excess cable length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
Figure 6-18
Two installation errors . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
Figure 6-19
Device display on first connection. . . . . . . . . . . . . . . . . . 147
Figure 7-1
MTL diagnostic system . . . . . . . . . . . . . . . . . . . . . . . . . . 155
Figure 7-2
P+F on-line diagnostics solution . . . . . . . . . . . . . . . . . . . 155
Figure 7-3
Turck on-line diagnostic solution. . . . . . . . . . . . . . . . . . . 156
Figure 7-4
R. Stahl diagnostic module . . . . . . . . . . . . . . . . . . . . . . . 157
Figure 7-5
PID Function Block internal functions . . . . . . . . . . . . . . 159
Figure 8-1
Emerson handheld communicator. . . . . . . . . . . . . . . . . . 163
Figure 8-2
Beamex Fieldbus calibrator . . . . . . . . . . . . . . . . . . . . . . . 164
Figure 8-3
Fieldbus signal jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
Figure 9-1
SIS (Safety Instrumented System) user layer extensions . . 170
Figure 9-2
Example SIS application analog 2 out of 3 voter . . . . . . . 172
Figure 9-3
Device Mapping Diagram (Channel Mapping
of other Protocols to FF Flexible Function Block) . . . . . . 174
xiv
LIST OF FIGURES
Figure 9-4
National Instruments USB H1 modem . . . . . . . . . . . . . . 177
Figure E-1
Simplified P&ID of a distillation tower . . . . . . . . . . . . . . 200
Figure E-2
Fieldbus system design – plot plan . . . . . . . . . . . . . . . . . 200
Figure E-3
Instrument location drawing . . . . . . . . . . . . . . . . . . . . . . 201
Figure E-4
Fieldbus system design – area classification . . . . . . . . . . . 201
Figure E-5
Junction box location drawing. . . . . . . . . . . . . . . . . . . . . 207
Figure E-6
Instrument Segment Drawing 01-Seg-1 . . . . . . . . . . . . . . 208
Figure E-7
Instrument Segment Drawing 01-Seg-2 . . . . . . . . . . . . . . 209
Figure E-8
Instrument Segment Drawing 01-Seg-3 . . . . . . . . . . . . . . 210
Figure E-9
Segment 1 macrocycle calculation . . . . . . . . . . . . . . . . . . 215
LIST OF FIGURES
xv
List of Tables
Table 1-1
Fieldbus Function Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Table 2-1
Fieldbus cable characteristics. . . . . . . . . . . . . . . . . . . . . . . . 41
Table 2-2
Cable type specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Table 2-3
HSE class summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Table 3-1
Characteristics of network-energized devices . . . . . . . . . . . 69
Table 3-2
Networks’ power supply requirements . . . . . . . . . . . . . . . . 70
Table 3-3
Equipment classification guide . . . . . . . . . . . . . . . . . . . . . . 73
Table 3-4
Default function block information for all designs . . . . . . . 74
Table 3-5
FISCO parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Table 3-6
Characteristics of FISCO and FNICL networks . . . . . . . . . 78
Table 4-1
System decision analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Table 4-2
Network decision analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Table 4-3
Device criticality decision matrix . . . . . . . . . . . . . . . . . . . . 99
Table 4-4
Connector decision analysis . . . . . . . . . . . . . . . . . . . . . . . . 99
Table 4-5
FISCO installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Table 4-6
IS/NIS installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Table 4-7
Configuration worksheet/checklist . . . . . . . . . . . . . . . . . . 101
Table 5-1
VCR types and their uses . . . . . . . . . . . . . . . . . . . . . . . . . 106
Table 5-2
Configuring a network for safety vs. availability . . . . . . . . 108
Table 5-3
Fieldbus operating mode priorities.. . . . . . . . . . . . . . . . . . 111
Table 5-4
Fieldbus alarm levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Table 5-5
Macrocycle requirements for different configurations . . . 124
Table 6-1
Examples of identifying signal and block type . . . . . . . . . 148
Table 7-1
IF communication errors. . . . . . . . . . . . . . . . . . . . . . . . . . 171
LIST OF TABLES
xvii
Table 8-1
Instrument Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
Table 8-2
Device characteristics summary . . . . . . . . . . . . . . . . . . . . . 206
xviii
LIST OF TABLES
Ian Verhappen, P. Eng., CAP
B.Sc. Environmental Science and B.Sc. Chemical
Engineering
Certificate in Oil Sands Technology
FF Certified Professional and Certified FF Instructor
ISA Certified Specialist in Analytical Technology
ISA Certified Automation Professional
Ian Verhappen is an ISA Fellow and a Professional Engineer, and is Director
of Industrial Automation Networks Inc., a global consulting firm focused
on “Making Industrial Networks Easy.” Prior to starting Industrial Automation Networks, Verhappen worked as the Director of Industrial Networks for
Measurement Technology Limited (MTL) where he was responsible for their
global digital communications product line. He has been working in the
automation industry since 1987 and has been actively involved in FOUNDATION Fieldbus technology since 1995, when he led the first Host Interoperability demonstration project at Syncrude Canada Ltd., where he worked for
20 years.
Verhappen is an acknowledged expert in FOUNDATION Fieldbus technology
and is an active member of the global standards community. He has
authored numerous technical papers on Fieldbus and is a regular columnist
for several industry trade journals including Manufacturing Automation,
Industrial Networking, Offshore, and Process West. He is co-author with
Augusto Pereira of ISA’s popular book Foundation Fieldbus. Verhappen also
wrote the chapters on Industrial Networking for the 4th Edition of the
Instrument Engineer’s Handbook, published by Taylor and Francis Group, and
the Fieldbus chapter for ISA’s book, A Guide to the Automation Body of
Knowledge.
He has been the keynote speaker at numerous conferences around the
globe, where he has shared his knowledge of industrial networking and field
level networks. Verhappen is also an FF Certified instructor and the developer of the FOUNDATION Fieldbus Certified courses at the Southern Alberta
IAN VERHAPPEN, P. ENG., CAP
xix
(Canada) Institute of Technology (FF Certified Training Center) as well as
an instructor in IDC Technology’s on-line diploma program.
In addition to serving as chairman of the Western Canada End User Council, Verhappen was chairman of the Fieldbus Foundation Global End User
Advisory Council from 2002 to 2006, reporting directly to the Board of
Directors twice per year and in the process helping set the direction of
Fieldbus technology. Under his guidance, the End User Advisory Council
prepared the “Engineering Design Guide, FOUNDATION Fieldbus Document
AG-181” for which he was the editor. AG-181, now in Revision 2, has been
translated into German, Japanese, Chinese, and Russian and is widely used
as the basis for many corporate and project Fieldbus specifications.
Verhappen has been active on a number of Fieldbus specifications committees, including being an outside expert reviewer for the Safety Fieldbus
Committee and Program Manager of the HSE Remote I/O development
team. Verhappen is a past Vice-President Standards & Practices, a Managing
Director on ISA’s Standards & Practices Board, Chair of ISA-103 (FDT),
and is past Vice-President Strategic Planning for ISA. He is the 2011–2012
Director of the ISA Communications Division. In addition, he is the Canadian Chair of IEC 65E, 65B and the TC65 Committee as well as a participating member of Canada’s IEC 65A and 65C and ISO TC1 WG7
subcommittees.
In addition to his expertise in industrial network technology, Verhappen is
also a trained HAZOP and Risk Assessment facilitator, having conducted
such investigations for several billion-dollar projects.
Verhappen has served as project lead, engineer/designer or external review
consultant for a number of companies in industries around the world,
including pulp and paper, mining, food processing, water and wastewater,
oil sands processing, petrochemicals, and refining.
xx
IAN VERHAPPEN, P. ENG., CAP
Augusto Pereira, Eng.
•
B.Sc. Electronical Engineer by FEI – Faculdade de
Engenharia Industrial (1975).
•
Degree in Mathematics and Physics by Universidade
Católica de Santos.
•
Many courses, in Brazil and in the United States, of Automation and
Hardware.
•
Since 1994, he has been involved in more than 241 automation projects
with digital protocols in Brazil, Canada, Argentina, Chile, Colombia,
Venezuela, Cuba and Peru.
•
He worked at Dow Chemical, at Smar, at Emerson Process, at Yokogawa
South America and as the Technical and Marketing Director in Pepperl+Fuchs South America.
•
He worked as the Professor of Automation Techniques of the Course of
Electronic Engineering of the Engineering College of the city of Sorocaba – São Paulo State.
•
He was the Professor of Projects with FOUNDATION Fieldbus of the
Course of Post-Graduation in Process Control of UNIUBE (University
of the city of Uberaba – Minas Gerais State, Brazil).
•
He was President of District 4 (South America) of ISA (International
Society of Automation) from 1998 to 2000.
•
In October 2011 he was elevated to the distinguished grade of ISA Fellow in recognition of his improvements in Fieldbus instruments and
automation design. The grade of ISA Fellow is granted to acknowledge
outstanding achievements in scientific and engineering fields.
•
Nowadays, he works as an MBA Professor of IT and Advanced Administration course of the college Fatec, in the city of Sorocaba – São Paulo
State, Brazil. Professor of the Post-Graduation Courses of the Brazilian
Universities: Professor of Mauá, from the city of São Caetano do Sul –
São Paulo State, Professor of Federal of Espírito Santo State, Professor
of Universidade Santa Cecília (Prominp), from the city of Santos – São
AUGUSTO PEREIRA, ENG.
xxi
Paulo State, and from ISA District 4 and also Consultant of the LEAD
Project, from Petrobras, in the city of Rio de Janeiro.
•
xxii
Currently, he is the ISA District 4 Director of Events and Exhibitions.
AUGUSTO PEREIRA, ENG.
1 — Fieldbus Layers
FOUNDATIONTM Fieldbus has several different “layers.” This chapter discusses
three of these layers:
1.
Physical Layer: The various topologies and types of data blocks used
by FOUNDATION Fieldbus.
2.
Communication Layer: How Fieldbus uses and assigns device registers.
3.
Parameter Classes: The function or role of the information generated
on the network.
This chapter provides the background on the how and what of Fieldbus. So
let’s start. What is Fieldbus?
Fieldbus is a bi-directional digital communication network that enables the
connection of multiple field instruments and processes and operator stations (HMI: Human-Machine Interfaces). They carry out control functions
and enable monitoring by means of supervision software. Figure 1-1 shows
how these three layers (Field, Fieldbus, and Supervisory System) interrelate.
The FOUNDATION Fieldbus protocol was based on the ISO/OSI seven-layer
communications model, although it does not include all layers. It can be
divided into the Physical Layer (dealing with instrument connection techniques) and the Communication Stack (dealing with the digital communication among the devices). These are the OSI layers used by FOUNDATION
Fieldbus. Figure 1-2a represents how the different components of the FOUNDATION Fieldbus protocol maps to the OSI seven-layer model.
The Physical Layer is OSI layer 1, the Data Link Layer is OSI layer 2, and
because FOUNDATION Fieldbus is a relatively simple network protocol with
little cross-network communication, OSI layers 3 through 6 are not used.
The Fieldbus Message Specification and Fieldbus Access Sublayer are part
of OSI layer 7, and the Application Layer and the User Layer in which
Function Blocks are defined reside above this. The Fieldbus Communication Stack is comprised of layers 2 through 7 of the OSI model.
FIELDBUS LAYERS
1
Figure 1-1 — Digital control system architecture
SUPERVISORY
SYSTEM
LOCAL AREA NETWORK
FIELDBUS
FIELD
Figure 1-2a — OSI model compared with Fieldbus model
FIELDBUS MODEL
OSI MODEL
USER
LAYER
APPLICATION LAYER
USER
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FIELDBUS MESSAGE
SPECIFICATION
FIELDBUS ACCESS
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TRANSPORT LAYER
NETWORK LAYER
DATA LINK LAYER
DATA LINK LAYER
PHYSICAL LAYER
PHYSICAL LAYER
PHYSICAL LAYER
As a message is transmitted from one device to another on the network, it
must pass through all of the OSI layers, and in the process, the data packet
2
FIELDBUS LAYERS
is developed, as shown in Figure 1-2b, where the numbers in the figure represent the approximate number of 8-bit octets used to transfer the user data
up and down the stack.
Figure 1-2b — Fieldbus data transfer packets
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Figure 1-3 represents Manchester encoding, which is how the actual data is
encoded in the H1 FOUNDATION Fieldbus network. Manchester encoding
adds a time reference signal to the data signal to determine the signal
boundaries. One way the protocol increases the level of noise immunity
versus other communication techniques is that it looks for a transition
every 32 ±10% microseconds to see if there is a change in state, up or down.
If there is no change within this “gate,” then there is no communication on
the network. Because FF only looks for a transition during this short time
period, the amplitude of the signal itself is not the critical element in determining if there is a message to send.
FIELDBUS LAYERS
3
Figure 1-3 — Manchester encoding
Data
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The Data Link Layer (DLL) is a mechanism to transfer data from a node to
the other nodes that need the data. The Data Link Layer also manages the
priority and order of such transfer requests, as well as data, address, priority,
medium control, and other parameters, all related to message transfer.
Only one device on a link is allowed to use the medium (Physical Layer) at
a time. The Link Active Scheduler (LAS) controls medium access.
1.1 Topology
1.1.1 Application Layer
The Application Layer provides an interface for the device’s application
software. This layer defines how to read, write, or start a task in a remote
node. The main task of this layer is to define syntax for the messages.
4
FIELDBUS LAYERS
The main components of the Application Layer are the Fieldbus Access
Sublayer (FAS) and the Fieldbus Message Specification (FMS).
The FAS uses the scheduled and unscheduled features of the Data Link
Layer to provide a service for the Fieldbus Message Specification (FMS).
The types of FAS services are described by Virtual Communication Relationships (VCR).
The VCR is like the speed dial feature on your memory telephone. There
are many digits to dial for an international call—an international access
code, country code, city code, exchange code, and the specific telephone
number. This information only needs to be entered once and then a “speed
dial number” is assigned. After setup, only the speed dial number needs to
be entered for dialing to occur.
In a similar fashion, after configuration, only the VCR number is needed to
communicate with another Fieldbus device.
Just as there are different types of telephone calls, such as person-to-person,
collect, or conference calls, there are different types of VCRs. VCRs and
their management are covered in more detail in Chapter 5.
Fieldbus Message Specification (FMS) services allow user applications to
send messages to each other across the Fieldbus using a standard set of message formats.
FMS describes the communication services, message formats, and protocol
behavior needed to build messages for the User Application.
Data that is communicated over the Fieldbus is described by an “object
description.” Object descriptions are collected together in a structure called
an object dictionary (OD).
The object description is identified by its index in the OD. Index 0, called
the object dictionary header, provides a description of the dictionary itself
and defines the first index for the object descriptions of the User Application. The User Application object descriptions can start at any index above
255.
FIELDBUS LAYERS
5
Index 255 and below define standard data types such as Boolean, integer,
float, bitstring, and data structures that are used to build all other object
descriptions.
A Virtual Field Device (VFD) is used to remotely view local device data
described in the object dictionary. A typical device will have at least two
VFDs: a Network and System Management VFD and a User Application
VFD.
Network Management is part of the Network and System Management
Application. It provides for the configuration of the communication stack.
The Virtual Field Device (VFD) used for Network Management is also used
for System Management, and provides access to the Network Management
Information Base (NMIB) and to the System Management Information
Base (SMIB). NMIB data includes Virtual Communication Relationships
(VCR), dynamic variables, statistics, and Link Active Scheduler (LAS)
schedules (if the device is a Link Master). SMIB data includes device tag
and address information and schedules for Function Block execution.
1.1.2 User Layer
The User Layer defines the way of accessing information within Fieldbus
devices so that such information may be distributed to other devices or
nodes in the Fieldbus network. This is a fundamental attribute for process
control applications.
The architecture of a Fieldbus device is based on blocks, with the Function
Block, which as the name implies is an object-based function designed to
execute a range of control functions that are responsible for performing the
tasks required for the current applications, such as data acquisition, feedback and cascade loop control, calculations, and actuation. Every Function
Block contains an algorithm, a database (inputs and outputs), and a userdefined name, typically the loop or tag name since the Function Block tag
number must be unique in the user’s plant). Function Block parameters are
addressed on the Fieldbus by means of their TAG.PARAMETER-NAME.
A Fieldbus device includes a defined quantity of Function Blocks of which
at least one block must be instantiated or defined.
6
FIELDBUS LAYERS
Function Block. The FOUNDATION Fieldbus Function Block, especially its
models and parameters—through which you can configure, maintain, and
customize your applications—is a key concept of Fieldbus technology.
What is a Function Block? A Function Block is a generalized concept of the
functionality in field instruments and control systems, such as analog input
and output as well as PID (Proportional-Integral-Derivative) control. The
FOUNDATION Fieldbus specification, FF-890, “Function Block Application
Process—Part 1,” gives fundamental concepts, while Part 2 and later parts
give various Function Block details.
The Function Block Virtual Field Device (VFD) contains three classes of
blocks: Resource Block, Function Block, and Transducer Block.
Resource Block. A Resource Block shows what is in the VFD by providing
the manufacturer’s name, device name, Device Description (DD), and so
on.
The Resource Block controls the overall device hardware and Function
Blocks within the VFD, including hardware status.
Tip 1 — The mode of the Resource Block controls the mode of all
other blocks in the device.
Transducer Block. A Function Block is a general idea while the Transducer
Block is dependent on its hardware and principles of measurement. For
example, a pressure transmitter and magnetic flow meter use different measurement principles but provide an analog measured value. The common
part is modeled as an AI (Analog Input) Block. The difference is modeled as
Transducer Blocks, which provide the information on the measurement
principle. A Transducer Block is linked to a Function Block through the
CHANNEL parameter of the Function Block.
In addition to converting the signal between a digital number and a physical signal (milliVolts, capacitance, frequency etc.) or output (pressure, current, etc.), Transducer Blocks are becoming ever more important because
they are also the blocks used to capture and store all the diagnostic and
maintenance-related data for a device. A number of Standard Transducer
FIELDBUS LAYERS
7
Blocks have been defined, including the Common Block (to define the
minimum requirements for all Transducer Blocks) and Temperature, Pressure, and Advanced Positioner Blocks. The Advanced Positioner Block is a
requirement for partial stroke testing, which is needed for Safety Instrumented Fieldbus applications. The Flow Transducer Block is likely to be
released in 2012.
It is end-user demand and economics that are driving the need for Standard
Transducer Blocks since, without a standard interface to the maintenance
data contained within each device, it is a cumbersome task to take full
advantage of the diagnostic capabilities of a digital transmitter, using modern software and asset management systems.
Transducer specifications are generally defined by the device developers.
The transducer specifications establish the base scope of transducer functions. A device may have additional functions, but it must contain the
functions specified in the specification to be interoperable within the given
specification.
Function Block. A Function Block is a generalized model of measurement
and control. The three Function Block classes are:
1.
Standard Block, as specified by the Fieldbus Foundation.
2.
Enhanced Block, a Standard Block with additional parameters and
algorithms but still fully defined by the appropriate FF specifications
3.
Extended, Open Block or a Vendor-Specific Block, designed by individual vendors with parameters not defined by the FF specifications
but rather by the device DD file. Extended blocks must contain the
Standard Block parameters so basic connectivity and communications
will always be possible.
The Function Blocks MAI (Multiple Analog Input), MAO (Multiple Analog
Output), MDI (Multiple Discrete Input), MDO (Multiple Discrete Output), and FFB (Fully Flexible Function Block), defined in Parts 4 and 5 of
the Function Block Application Process specifications, were developed as
part of the High-Speed Ethernet (HSE) process. The “M-series” of blocks
are able to transfer a group of eight PV (process variable) signals as a single
message on the Fieldbus Network and because HSE is fully backwards com8
FIELDBUS LAYERS
patible with H1, a number of H1 devices, such as temperature multiplexers,
are taking advantage of the MAI block.
The most novel of the new blocks, however, is the Fully Flexible Function
Block (FFB), as it is able to be fully programmed by the end user, using any
of the IEC 61131-1 programming languages.
Like all object-based Fieldbus Function Blocks, the FFB is a “wrapper” for
the actual functions that reside and execute inside of it. The Fieldbus specifications define a set of parameters that must be common to all Function
Blocks to ensure interoperability and communications between the various
blocks, devices, and host system. Since each component of the Fieldbus
specification is treated as an object and is, to some extent, similar to a subroutine or function call in a computer program, it is possible for each manufacturer to write its own code for the object to execute, as long as the
results are presented in the predefined format. It is this lack of definition for
the function itself that makes the FFB possible.
The FFB can be configured by the end user with any of the IEC 61131-1
languages to whichever function is required. Thus, a device supporting the
FFB can be configured or programmed for a variety of purposes, from protocol converter to a nano-PLC that performs batch/recipe operations or
complex multivariate control calculations, such as artificial neural networks
or fuzzy logic.
The FFB specification contains many useful Function Blocks; however, the
one developed to help Fieldbus in the manufacturing industry, where discrete control is more prevalent, is the device controller (DC) block, which is
intended to control any two- or three-state physical device. The device controller accepts a set point and causes the device to drive to that set point.
Time is allowed for the transition, but alarms are generated if the physical
device fails to reach the desired state or loses that state after the transition is
complete. The DC block has inputs for control of the set point by external
logic or commands from a host, as well as permissive, interlock, and shutdown logic functions. An operator may temporarily bypass a faulty limit
switch after visual confirmation of the state of the physical device. The
parameter DC_STATE displays one of 14 states that describe the current
control condition, while the parameter FAIL gives specific reasons for failures.
FIELDBUS LAYERS
9
Unfortunately, the interfaces to program FFB are not yet fully interoperable. This means that an FFB from Manufacturer A must be programmed
and configured by the host and software tools of Manufacturer B, and vice
versa. However, once the FFB has been prepared and compiled through
DD Services (the binary file that is used by field devices and hosts to execute the information from the DD file), it can be executed by any system
supporting the FFB block type.
FFB technology was successfully demonstrated at the International Specialty Products facility in Lima, Ohio, in May 2005. The demonstration
consisted of converting one of the three filter beds in the process from control in the host to field-based control, using linking devices containing FFBs
from two manufacturers. The first FFB controlled the 10 quick opening/
closing valves (250 milliseconds) on one side of the filter, and then control
was transferred to the second linking device and its FFB to control the second bank of 10 valves. After that, control was passed back to the host to
control the third filter bed’s operation.
Figure 1-4 shows not only how the various function blocks work together
but also the different parameters that are used in each of the Standard,
Enhanced, and Extended Blocks available in a device. Simplistically, the
Universal parameters define the basis for the Standard Blocks, Enhanced
Blocks build on this concept, and then manufacturers can further expand
on the Enhanced Blocks with their own enhancements.
Tip 2 — The Function Block extensions provided by manufacturers are not defined by the Foundation, so they may not be the
same between two different manufacturers.
10
FIELDBUS LAYERS
Figure 1-4 — Device description hierarchy
Universal
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Transducer Blocks
Function Blocks
Despite the fact that the enhancements are not defined by the Fieldbus
Foundation, they will be supported by all host systems capable of reading
the associated DD and Capabilities Files.
A block has a series of parameters, which have continuous indexes. It is
because these indexes are continuous that the DD revisions for devices
must match between a field device and its associated Host system. If a
newer DD revision device is associated with an older DD revision in the
Host, the links between where the Host thinks a parameter resides in memory and the actual device memory address no longer match. For example if
the AO Block in DD revision 01 starts at memory address 600 when revision 02 is published, the AO Block may now start at memory address 645
meaning that if the Host system is looking for the parameter at address 600,
it is likely to get a data type mismatch as a minimum and certainly the
incorrect data.
Table 1-1 shows these various blocks as defined by the Fieldbus Foundation
in the indicated part of the specification.
FIELDBUS LAYERS
11
Table 1-1 — Fieldbus Function Blocks
Part-2 Blocks
Standard Blocks
AI
Analog Input Block
DI
Discrete Input Block
ML
Manual Loader Block
BG
Bias/Gain Station Block
CS
Control Selector Block
PD
P, PD Controller Block
PID
PID, PI, I Controller Block
RA
RATIO Station Block
AO
Analog Output Block
DO
Discrete Output Block
Part 3
Enhanced Blocks
DC
Device Control Block
OS
Output Splitter Block
SC
Signal Characterizer Block
LL
Lead Lag Block
DT
Dead Time Block
IT
Integrator (Totalizer) Block
(More blocks are under development)
Part 4
Multiple I/O Blocks
MDI
Multiple Discrete Input Block
MDO
Multiple Discrete Output Block
MAI
Multiple Analog Input Block
MAO
Multiple Analog Output Block
Part 5
FFB – Flexible Func- IEC 61131 Blocks
tion Blocks
As Figures 1-4a through 1-4c show, a Function Block has input, output, and
contained parameters. Figure 1-4a is a typical Analog Input (AI) Block, 1-4b
is an Analog Output (AO) Block, and 1-4c is a PID Block. Data generated in
a block is made available from an output parameter, which can be linked to
the input parameter of other Function Blocks. The Fieldbus Foundation
does not define how each manufacturer is to implement the algorithms to
complete the functions shown in these figures, but rather defines the func12
FIELDBUS LAYERS
tionality, parameters, and “connections,” such as channel out, etc., between
each of the Function Blocks. This allows each manufacturer to differentiate
their product from their competitors’ through increased efficiency and features, such as improved signal conditioning and diagnostics.
Floating-point parameters have a valid range of ±1.2 × 1038 to ±3.4 × 1038,
as well as (in some cases) the values -Inf or +Inf.
Discrete Blocks have 256 valid enumerated states, which means that in addition to simple logic 0 or 1, they can also be used to represent specific states,
such as open, closed, true, false, start, stop, running, etc.
In the case of an AO Block, if the actual device reaches either of its open or
closed limits, the block will set the corresponding limit in the status element of the associated back-calculation output parameter.
This tells the PID Block to not push the output further in that direction,
thus preventing reset windup in the loop.
The operator normally sets the PID loop mode from the Block mode
parameter in the PID Block. Control is stopped by setting this parameter to
MAN. However, if the operator wishes to “hand operate” the AO Block, it
is better to remain in AUTO mode and enter the set point instead.
Figure 1-4a — Analog Input Block (AI)
FIELDBUS LAYERS
13
Figure 1-4b — Analog Output Block (AO)
Figure 1-4c — PID Block (PID)
1.1.3 Testing and Registration
All FOUNDATION Fieldbus devices that have a DD file need to pass two separate suites of tests before they can achieve the Fieldbus “check mark” of the
Fieldbus logo with a check mark in the lower right-hand corner, which confirms compliance with the relevant suite of test specifications. One of the
FF “check mark” tests is for the Communications Stack, while the other
tests for device interoperability. Although it is not an exact indication of
14
FIELDBUS LAYERS
what each of these test suites checks, a simplified way of thinking of the two
test suites is that the Conformance test checks for media access/control OSI
Layers 2-7, while the Device Interoperability Test or ITK checks device conformance to the User Layer.
The Fieldbus Foundation has partnered with the Fraunhofer Institute, based
in Karlsruhe, Germany, for completion of the Conformance Test System
that checks the Communications Stack, which does not change much over
time. Every manufacturer then uses an approved stack to build their Fieldbus device, which is then sent to the Fieldbus Foundation for interoperability testing.
1.1.4 Interoperability Test System
The ITK is conducted at the Foundation offices in Austin, Texas, where the
device is connected to the test suite so that approximately 500 different
tests can be run. The pass rate for these tests is 100%, so if just one test fails,
the manufacturer, after making any modifications to correct the problem(s),
needs to repeat all the tests. Devices that successfully complete the tests are
given the FF “Check Mark” and are then listed on the Fieldbus web site as
such, along with the DD file that was used for the test. A device that passes
ITK 5.1 and supports the Field Diagnostics Profile to support enhanced
Device Description and graphics will have this feature separately listed on
the registration certificate.
The ITK is normally revised approximately every 18 months and as of late
2011 is on revision 6. Prior to any FF product receiving a “check mark” it
must go through a rigorous quality assurance process to verify that any
products getting this approval are truly interoperable.
The Fieldbus Foundation test procedure must pass through all the steps
below before the specification is approved:
1.
A need for the specification is identified from a request by industry,
and once the Board of Directors agrees that this is a worthwhile activity, a call for participants is distributed to all Foundation members.
This call for volunteers also includes a request for volunteers to lead
the activity as Editor, Project Leader, etc.
FIELDBUS LAYERS
15
2.
Once the new standard development committee has been formed, the
committee meets and agrees on the project scope, the project leader
prepares a project plan, outlining the deliverables and estimated timeline for the project to the Technical Steering Committee for review
and approval.
3.
The first step in the actual development process is for the development team to create a set of Use Cases to clearly define the problem
or problems that are going to be solved by the new standard.
4.
From these Use Cases the team develops a Draft specification that
fully describes how the products can be built to solve the Use Cases.
5.
The Foundation issues a “Call for Prototypes” to request that at least
two independent suppliers build products in compliance with the new
Draft Preliminary Specification.
6.
The prototypes and the Foundation’s test kit are then brought
together to test against this specification to make sure that everyone
interprets the specification in the same way, and once they have done
so, with any resulting questions being resolved by the development
team, it is accepted that the theory described in the specification will
actually work. It is this step in the specification approval process that
is key in verifying device interoperability because it confirms by three
separate sources that they have all interpreted the specifications in the
same way and met the requirements as defined in the original Use
Cases.
7.
A Preliminary Specification is then made ready for distribution to
members and the Technical Steering Committee for review and final
approval.
8.
The Technical Steering Committee (TSC) (Fieldbus Foundation’s
Standards Board) reviews any comments received, and after all the
Fieldbus Foundation members have a final chance to comment, the
TSC then approves the document as a Final Specification.
This entire process typically takes more than two years.
16
FIELDBUS LAYERS
1.1.5 Physical Layer
This is the Fieldbus layer connected with instrument devices in the field.
The standardized data transmission speed of the H1 network is 31.25 Kbps;
as stated by the standard, all other speeds shall be used for high-speed interconnection of bridges and gateways (see Figure 1-5).
Figure 1-5 — Fieldbus bridge capability
BRIDGE CAPABILITY
USER
LAYER
100 Mbps Fieldbus
COMMUNICATION
“STACK”
PHYSICAL LAYER
Bridge
31.25 Kbps Fieldbus
Devices
The standard determines the following rules (among others) for the speed of
31.25 Kbps:
1.
A Fieldbus instrument shall be able to communicate with the following number of instruments:
• From 2 to 32 instruments in a non-intrinsically safe connection and
power supply separated from the communication wiring.
• From 2 to 16 instruments receiving power by the same communication wiring in an intrinsically safe connection.
• From 1 to 24 instruments receiving power by the same communication wiring in a non-intrinsically safe connection.
Note: Most host systems are restricted to 16 devices per network (64 devices per 4
port card) or are otherwise restricted by the number of parameters they can manage
per port. The result is that physical power is not always the limiting factor in the
FIELDBUS LAYERS
17
number of devices that can be added to a network. More often the limiting factor for
fast macrocycle installations is bandwidth.
Tip 3 — This rule does not forbid the connection of more instruments than the specified number. Such limits have been established, considering a consumption of 9 mA ± 1 mA, with a
power supply of 24 VDC, intrinsic safety barriers with an output
of 11–21 VDC, and a maximum current of 80 mA for the instruments
located within the hazardous area.
2.
The length of the entire bus segment with the maximum number of
instruments operating at a speed of 31.25 Kbps shall not exceed
1900 m in the section of the trunk plus all spurs (Figure 1-6).
Figure 1-6 — Maximum length of Fieldbus network
1900 m max.
Fieldbus Segment
BUS
Terminator
+
-
Terminator
Signal
Isolation
Circuit
Fieldbus
Power Supply
Control or
Monitoring
Device
Field Devices
Tip 4 — This rule does not forbid the use of longer lengths, provided that the electrical characteristics of the instruments are
observed.
3.
18
The maximum number of repeaters for regenerating the waveform
between two instruments cannot exceed four (Figure 1-7).
FIELDBUS LAYERS
Repeaters are used to expand a Fieldbus network. Repeaters can be either
energized or de-energized. When four repeaters are used, the maximum distance between any two devices in a segment is 9500 m.
Figure 1-7 — Fieldbus network with repeaters
1900 m
REP1
1900 m
REP2
1900 m
REP3
1900 m
REP4
1900 m
Terminator
DISTANCE CAN BE INCREASED WITH REPEATERS
MAXIMUM NUMBER OF REPEATERS = 4
4.
A Fieldbus system must be able to remain in operation while a device
is being connected to it or disconnected from it.
5.
Failure in any communication element (except for short-circuit or low
impedance) shall not affect communication for more than 1 ms.
6.
The polarity of systems using twisted pairs shall be respected, their
wires shall be identified, and the polarity shall be observed in all connection points. According to the Fieldbus standard, devices themselves are not to be polarity-sensitive, but this is not always the case.
7.
For systems with a redundant physical medium:
• Each channel must comply with the network configuration rules.
• A non-redundant segment cannot be between two redundant segments.
• Repeaters must also be redundant.
• The identification number of the channels must be maintained in
the Fieldbus, that is, the Fieldbus channels must have the same
numbers as the physical channels.
8.
Cable shields shall not be used to conduct power.
FIELDBUS LAYERS
19
1.1.6 Topologies
Several topologies may be used in Fieldbus design. Figure 1-8 shows the
topologies that will be detailed below. Power supplies and terminators are
not shown so that the figures can be more easily understood.
Bus with Spurs Topology. The Bus with Spurs topology uses a single bus to
which devices and spurs are directly connected. Several devices may be connected to each spur. The total spur length is limited according to the number of spurs and the number of devices per spur. This is summarized in
Table 1-2. This spur length table is not absolute. It merely serves as a guideline for designing networks.
Figure 1-8a — Physical layouts – Single combined segment
Control Highway
Input/Output
Boards
T
JB T
JB
Host
Combine multiple
drops off single
Fieldbus cable from
interface room
20
FIELDBUS LAYERS
Figure 1-8b — Wiring practices – Cable efficiency
Multiconductor
Cable
H1 Port
FF
JB
JB
Multi-pair cable to
conventional JB at location
in Field Operating unit then
trunk continues to each FF
JB and associated spurs
FF
JB
FF
JB
FF
JB
Point-to-Point Topology. In the Point-to-Point or Daisy Chain topology, all
devices used in the application are connected in series. The Fieldbus trunk
is routed from one device to the next, being interconnected to the terminals
of each Fieldbus device.
Table 1-2 — Maximum spur lengths
Total
devices
per
network
One
device per
spur, m
(ft)
Two
devices
per spur,
m (ft)
Three
devices
per spur,
m (ft)
Four
devices
per spur,
m (ft)
Maximum
total
length,
m (ft)
1–12
120
(394)
90
(295)
60
(197)
30
(98)
439
(1440)
13–14
90
(295)
60
(197)
30
(98)
1
(3)
384
(1260)
15–18
60
(197)
30
(98)
1
(3)
1
(3)
329
(1080)
19–24
30
(98)
1
(3)
1
(3)
1
(3)
220
(720)
25–32
1
(3)
1
(3)
1
(3)
1
(3)
10
(32)
FIELDBUS LAYERS
21
Caution: Point-to-Point topology is rarely used since the failure of one
device in the network will result in total network failure.
Tree Topology. Tree topology concentrates the connection of several field
devices to couplers/junction boxes. Because of its distribution, the Tree
topology is also known as a “Chickenfoot” or Star configuration.
End-to-End Topology. End-to-End topology is used to directly connect two
devices. The connection may be entirely located at the field (a transmitter
and a valve with no other devices connected) or to connect a field device (a
transmitter) to the Host device.
Mixed Topology. Mixed topology, as the name implies, mixes the three most
commonly used topologies connected to one another. However, the maximum length of a segment, including the spurs to the total length, shall be
observed. Figure 1-8a shows how this topology might be configured by
combining individual spurs with several multiple drop field device couplers.
Many installations are also taking this one step further by running multiconductor H1 cables to a conventional junction box at a convenient location, either on the edge or centrally located within a unit operation area,
and then extending individual trunk cables to one or more Fieldbus device
coupler assemblies/enclosures strategically located in closer proximity to
the devices themselves, thus minimizing total installed cable cost. This is
shown in figure 1-8b.
Bridges are used to connect Fieldbus segments operating at different speeds
(and/or physical layers such as wires, fiber optics, radio, etc.) in order to
obtain a large network. A bridge is shown in Figure 1-5, connecting the H1
and High-Speed Ethernet (HSE) networks.
A gateway depending on the manufacturer can be used to connect one or
more H1 segments to other types of communication protocols, such as Ethernet, RS-232, MODBUS, Modbus/TCP, Profibus, etc.
22
FIELDBUS LAYERS
1.2 Communications
Figure 1-9 shows how the registers in a Fieldbus device are assigned.
Figure 1-9 — FOUNDATION Fieldbus node addresses
0x10 — V(FUN)
0xF7 — V(FUN)
+ V(NUN)
0xF8 — 0xFC
0xFD — 0xFF
Address for Link Master Class devices
Address for Basic Class devices
Default address for devices with cleared address
Address for temporary devices such as a handheld communicator
Not used
LM Class Devices
V(FUN)
Not Used
V(NUN)
V(FUN) + V(NUN)
BASIC Class Devices
0xF7
0xF8
0xFC
0xFD
Default Address
Temporary Devices
0xFF
Every Fieldbus device has a unique 32-bit hardware address identifier made
up of a 6-byte manufacturer code, a 4-byte device code, and a serial number. This makes it possible to uniquely distinguish each device from the others. The Fieldbus Foundation assigns the manufacturer codes, while the
manufacturer assigns the device type code and sequential serial number.
A temporary device, such as a handheld communicator, has a node address
in the temporary range. The Link Active Scheduler (LAS) has a node address
of 0x04 or the series of lowest addresses. If a device has an address in the
gap V (NUM), it will never be able to join the network. The V (FUN) and V
(NUN) parameters are accessible through Network Management.
FIELDBUS LAYERS
23
Several Data Link (DL) addresses are reserved for specific purposes. For
example, devices can share the same system-wide Data Link Service Access
Point (DLSAP) for alarm reception.
Foundation devices are classified into device classes: BASIC, Link Master
(LM), and Bridge. An LM class device has the ability to be the LAS, while
BASIC class devices do not have this functionality. In addition to LM capability, a Bridge class device has the functionality to connect networks.
One and only one device in a network can be the LAS at any one time;
therefore, at least one LM (or Bridge) class device is needed in a link. LM
devices will try to acquire the LAS role when no LAS exists on start-up or
when the current LAS fails.
Tip 5 — The LM device with the lowest node address becomes
the new LAS for the network.
Other secondary or Backup LM devices observe the LAS activity and can
assume the primary or Master LAS role if the operating LAS fails.
The LAS manages the scheduled communication part of the synchronized
data transfer between Function Blocks.
A Function Block output parameter is a Publisher of data, and other Function Blocks that receive this data are called Subscribers. The LAS controls
periodic data transfer from a Publisher to Subscribers using the Network
Schedule.
Other network communications take place in an asynchronous way. The
LAS is responsible for giving all nodes on a link a chance to send messages.
The third role of the LAS is to maintain network communications. The LAS
does this by giving the token to all devices detected by the LAS. When a
new device is added to the network, it must be recognized by the LAS and
added to the token rotation list, which is called the Live List.
24
FIELDBUS LAYERS
A Fieldbus device may have user applications that are independent from
each other and do not interact. A Fieldbus device consists of Virtual Field
Devices (VFD) for such individual applications.
A Fieldbus device has at least two VFDs. One is the Management VFD,
where network and system management applications reside. It is used to
configure network parameters, including VCRs, as well as to manage
devices in a Fieldbus system. The other is a Function Block VFD, where
Function Blocks exist. Most field devices have more than two Function
Block VFDs.
A measurement or control application consists of Function Blocks connected to each other. Function Blocks are connected through Link Objects
in the Function Block VFD. A Link Object connects two Function Blocks
within a device, or a Function Block to a VCR for Publisher or Subscriber.
A Function Block must get input parameters before its algorithm can be
executed. Its output parameters must be published after algorithm execution. Therefore, algorithm execution and Publisher-Subscriber communication must be orchestrated when blocks are distributed among devices. The
System Management and Data Link Layer cooperate to achieve this by
using the Link Scheduling (LS) time that is distributed and synchronized by
the Link Active Scheduler (LAS).
Other network communications take place in an asynchronous way. The
LAS is responsible for giving all nodes on a link a chance to send messages.
This asynchronous or acyclic communication should constitute the majority of the macrocycle time on a network. Figure 1-10a shows a typical network with two independent loops.
Note that any device that is either not performing an internal calculation or
participating in a publish/subscribe communication, which means it is part
of the control loop and must either publish (share) its process variable or
subscribe (read) the process variable from another device in its loop, is able
to receive the Pass Token and participate in a client server communication.
FIELDBUS LAYERS
25
Figure 1-10a — Function block scheduling and macrocycle
Scheduled Cyclic
Communication
(DLL)
Scheduled Function
Block Execution
(SM)
Unscheduled
Communication
AI110
PID110
AO110
DI101
DO101
loop 110 period of execution
Cyclic
Function Block Execution
Cyclic Communication - Publish
Acyclic Communication
Acyclic
Alarms/Events
Maintenance/Diagnostic Information
Program Invocation
Permissives/Interlocks
Display Information
Trend Information
Configuration
Figure 1-10b represents the algorithm used by the LAS to determine the
next action it needs to take while ensuring that all deterministic communications happen at their assigned time.
Figure 1-10b — LAS algorithm
,VWKHUHWLPHWR
GRVRPHWKLQJ
EHIRUHQH[W
&RPSHO'DWD"
1R
:DLWXQWLOLWLV
WLPHWRLVVXHWKH
&'
6HQGLGOH
PHVVDJHVZKLOH
ZDLWLQJ
,VVXH
&'
<HV
,VVXH3UREH1RGH7LPH
'LVWULEXWLRQRU3DVV7RNHQ
26
FIELDBUS LAYERS
1.3 Parameter Classes
Block parameters are classified into three classes: input, output, and contained parameters. Function Blocks can have all of these classes, while the
Resource Block and Transducer Blocks have only contained parameters.
An input parameter is an input of a Function Block and can accept one output parameter of another Function Block. Its data type must be the same as
the output parameter.
An output parameter is a record consisting of a value (analog or discrete) and
its status.
A contained parameter is neither input nor output. It is accessible only
through an on-demand Read or Write request. Its data type can be any of
those defined by the Fieldbus Foundation.
Input parameters, output parameters, and some contained parameters are
records with an associated status. Status shows whether the value of this
parameter is useful or not. If the value is useful, the status is GOOD. If the
value is not useful, the status is BAD. The status can be UNCERTAIN
when the block is not 100% confident that the value is useful. Blocks have
an option to interpret UNCERTAIN as GOOD or BAD.
The acyclical traffic time may be user-defined and configured by means of
the configuration software (typical macrocycle times are 250 ms).
The above discussion concentrates on communications at the device or H1
level. However, all this data must be shared with other parts of the corporation, including the Distributed Control System1 (DCS), panel operators via
the Human-Machine Interface (HMI), and other parts of the corporation as
appropriate. However, the security and integrity of the Supervisory Control
and Data Acquisition (SCADA) Layer must be ensured and a variety of
1.
Historically, DCS has meant Distributed Control System. However, in
the new era of digital communications made possible with Fieldbus
technology, the more appropriate term is Digital Control System, and
this term will be used hereafter in this text.
FIELDBUS LAYERS
27
methods are available to do so. The most common method, however, is to
use firewalls and switches.
This is not different from what is done to protect a corporate local area network (LAN) that is connected to the Internet through the use of firewalls,
switches, and related technologies. The net effect is just like with the ISA-95
model, there are different “levels” within the enterprise that are connected
to each other. Figure 1-11 represents this concept of layers with the lowest
layer being the process itself, then layer or level 1 the sensors (this is where
the H1 network resides), Layer 2 the controllers, Layer 3 the Historians, and
Layer 4 the corporate or business LAN.
Figure 1-11 — SCADA layer Fieldbus traffic management
Figure 1-11. Layered corporate architecture.
Desktop
PC’s
Corporate LAN
Data
Historian
Layer
Web /
WAP
Server
Firewall
Firewall
Host /
SCADA Layer
As shown earlier, “layers” of networks, such as H1 and perhaps an Ethernetbased system for remote I/O and data paths, are within each of these ISA95 levels as well. Data paths are important to prevent information overload
to various components or interfaces/devices on the network. The best
example of this is device diagnostic information. The panel operator needs
to know only the process measurement and its status; hence all the other
28
FIELDBUS LAYERS
information should be routed through a separate system that can be better
integrated with a facility’s computerized maintenance system. Figure 1-11
also shows how these two data paths may exist on the same Control/
SCADA Layer. This data “splitting” is possible because Fieldbus uses Publisher-Subscriber relationships to transfer data between memory registers/
devices. In this case, the process data is subscribed to by the operator interface while the diagnostic data is subscribed to by the Condition Monitoring
System.
How should all this new information be used? There is a great deal of activity under way on a number of fronts related to, but not part of, the Fieldbus
effort that will enable the integration of this information into the larger corporate network environment. Some of these initiatives include:
•
OPC – Open Connectivity technology, as developed by the OPC
Foundation (www.opcfoundation.org) and formerly known as Object
Linking and Embedding (OLE) for Process Control, is a technology to
enable real-time transfer of data from one system to another through
the use of standardized call functions. The OPC Foundation is developing this open standard.
•
XML – Extensible Mark-up Language is the most complex form of language used to display information via the Web. XML is object-based,
and therefore each piece of information can potentially be self-defined,
thus making it viewable by anyone anywhere.
1.3.1 EDDL
The majority2 of today’s “smart” analog instruments use EDDL (Electronic
Device Description Language), often shortened to DD (Device Description)
in conversation. EDDL is the base technology underlying the HART (Highway Addressable Remote Transducer), Profibus PA, and FOUNDATION Fieldbus protocols by defining a significant part of their User Layer.
The original EDDL specification as defined by IEC 61804 Part 2 contained
little support for graphics; thus, the interface and degree of integration
2.
Many discrete devices also use EDDL; however, they are not yet in the
majority.
FIELDBUS LAYERS
29
between field devices and hosts was not only limited, but the level of integration and information that could be presented to a user varied with each
installation, depending on the host being used.
One technology that was developed to attempt to overcome these limitations was FDT/DTM (Field Device Tool/Device Type Manager). The FDT/
DTM technology uses EDDL as the basis for its definitions so that device
manufacturers can define the “look and feel” of how the user accesses the
device information for other than Process Variable (PV) and related signals,
such as those used to configure and calibrate the device.
The HART Communications Foundation, Profibus International, and the
Fieldbus Foundation also realized that users required improved interfacing
with their devices and as a result have developed, through a joint activity, a
number of graphical support functions to EDDL while maintaining 100%
backwards compatibility with the original specifications prepared in the
early 1990s. The key new capabilities of EDDL include the following:
•
Lets images be presented with installation/maintenance information.
•
Uses charts and menus to present and input data more consistently.
•
Uses graphs as X-Y plots to plot one variable against another, such as a
radar waveform (see Figure 1-12) or a valve signature (see Figure 1-13).
•
Provides basic mathematics for a first-order equation of the form
y=mx+b as also seen in Figure 1-13.
•
Offers a history/archive to provide information over time.
Other, less obvious enhancements that will be invisible to end users have
also been made to improve the ability of devices and host systems to communicate.
Encouraged by the success of this first project, the three organizations that
rely on EDDL technology are now working with the OPC Foundation to
add more features and capabilities to EDDL for extension beyond the traditional control system environment.
30
FIELDBUS LAYERS
Figure 1-12 — Radar level gauge
Figure 1-13 — Valve signature with best fit line
FIELDBUS LAYERS
31
1.3.2 FDT/DTM
Enterprise automation requires two main data flows: a “vertical” data flow
from enterprise level down to the field devices, including signals and configuration data, and a “horizontal” data flow between field devices operating with the same or different communication technologies.
With the integration of fieldbuses into control and engineering systems, the
vertical information flow, such as that associated with the maintenance and
set-up devices in automation systems, is increasing.
Although Fieldbus and device-specific software tools exist, there is no unified way to integrate those tools into higher-level, system-wide tools for
planning, control, engineering or asset management. FDT (Field Device
Tool) has the objective of doing so, and toward that end a series of standards has been prepared under the project IEC 62453/ISA-62453-n-2011
(103.00.nn), where n refers to the various individual parts (mostly different
protocols) within the series of standards with this number.
To ensure the consistent management of a plant-wide control and automation technology, it is necessary to fully integrate fieldbuses, devices and subsystems as seamless parts of a wide range of automation tasks covering the
whole automation life cycle.
FDT provides an interface specification for developers of components to
support function control and data access within a client/server architecture.
The availability of this standard interface facilitates development of servers
and clients by multiple manufacturers and supports open interoperation.
A device or module-specific software component called a DTM (Device
Type Manager) is supplied by a manufacturer with the related device type or
software entity type. Each DTM can be integrated into engineering tools via
defined FDT interfaces. This approach to integration is, in general, open for
all fieldbuses and thus supports the integration of different devices and software modules into heterogeneous control systems.
32
FIELDBUS LAYERS
As a result of this common interface defined by the standard and Layout
Guides, significant savings are available in operating, engineering and maintaining the control systems.
The objectives of the FDT standards are to support:
•
Universal plant-wide tools for life-cycle management of heterogeneous
Fieldbus environments, multi-manufacturer devices, Function Blocks
and modular sub-systems for all automation domains (e.g., process
automation, factory automation, and similar monitoring and control
applications).
•
Integrated and consistent life-cycle data exchange within a control system, including its fieldbuses, devices, function blocks, and modular
sub-systems.
•
Simple and powerful manufacturer-independent integration of different
automation devices, Function Blocks, and modular sub-systems into the
life-cycle management tools of a control system.
The FDT concept is made of three components; two FDTs and a DTM, as
shown in Figure 1-14.
The purpose of the first FDT, the Frame Application, is to provide the following: Common Environment, Network Configuration, Navigation, User
Management, Device Management, and Database Storage. The second
FDT, the interface, is like a printer interface and provides the gateway or
“interpreter” between the Frame Application and its associated WindowsTM
drivers and the tunnel through the appropriate digital communications protocol, such as Fieldbus, HART, or DeviceNet.
The DTM is like a printer driver, and just like a printer driver, it tells a computer how to print characters on a page. The DTM works by defining all the
parameters related to a specific field device and how they should appear on
the appropriate HMI and associated software. In the cases of HART, Profibus PA, and FOUNDATION Fieldbus, the DTM, at its core, is based on interpreting and expanding on the device’s DD file.
FIELDBUS LAYERS
33
Figure 1-14 — Field device interface communications
Define characteristics of the
field device itself
EDDL
Define interface of how the
information is to be
presented Device Driver
DTM - Device Driver
Gateway between field
communications protocol
and Windows environment.
Standard “connection”
between protocol and
Frame
FDT Interface (Communications DTM)
Frame Application
Implementation and human
interface DSC System,
Engineering Tool, etc.
All three of the standards used to support FDT developments are now part
of the IEC TC65E standards. OPC UA (Universal Architecture) is going
forward as IEC 62541, EDDL is part of IEC 61804, and as indicated above,
FDT is IEC 62423.
The Frame Application provides the following capabilities to FDT technology:
•
A common environment for presentation of the device data.
•
Network Configuration to establish the communication links between
the HMI and the field device through one or more protocols between
the two end points.
•
Navigation between the various windows, devices, and protocol communication messages.
•
User Management to control who has access to what portions of the
network and data.
•
Device Management by linking the appropriate DTMs together to
enable communication and viewing of the data.
34
FIELDBUS LAYERS
•
Database Storage to archive and retrieve information.
The DTM, which is provided by the device manufacturer, is the driver representing the actual device. The DTM, which has a standardized Graphical
User Interface to the Frame Application, can be loaded on any Frame
Application and includes the complete parameters of the device.
There are two types of DTMs: the COMM DTM represents communication to the devices like PC communication cards, couplers, gateways, and
linking devices while the Device DTM represents field devices like valves,
sensors, actuators, transmitters, motors, and pumps.
The FDT organization, a not-for-profit association of international companies dedicated to establishing the FDT Technology as an international standard, has adopted a number of key life-cycle policy elements, including a
requirement that all DTMs and FDT Frame Applications must be maintained for at least 10 years after initial release. In addition, a maintenance
release (update) must not require an update of the operating system or any
of the hardware components. FDT components must also be interoperable
within a major FDT version while remaining compatible to application
(instance) data of a former version of the component.
Individual vendors certify their FDT components according to the certification procedures of the FDT Group before they are released to the market,
and all FDT Group members must apply the rules and guidelines specified
in the Life Cycle Policy for their FDT-based products.
1.3.3 Field Device Interface (FDI)
The FDI cooperation project was announced at Interkama 2007 as a joint
effort between the Fieldbus Foundation and the FDT organization. The
intent of this agreement is to develop a single common interface for the
complete range of data communications in a modern control system, from
configuration through commissioning and maintenance.
FDI, the Field Device Integration project represents the next evolutionary
step in Device Description language on which the three predominant field
device protocols (HART, FOUNDATION Fieldbus, and Profibus PA) are based.
FIELDBUS LAYERS
35
Consequently, FDI will have a significant impact on the future look and
feel of digital field sensors, especially after the formation in 2011 of a separately incorporated entity, the FDI Corporation.
At the NAMUR (a not-for profit German end-user trade association)
Annual General Meeting in late 2011, the FDI Corporation demonstrated a
multi-vendor Host system prototype, using FDI device packages for FOUNDATION Fieldbus, HART, and PROFIBUS device integration. The purpose
of the prototype was to verify the FDI concepts and apply the standard
Host components in a system context to demonstrate FDI functionality.
Publication of the first draft of the FDI specifications was expected by the
end of 2011; completion of conformance test concepts, along with validation and release of the specifications for member review within the foundations, will follow in mid-2012; and the complete FDI standard Host
components, such as EDD Engine and User Interface (UI) Engine, by the
FDI Corporation should be done by the end of 2012.
So what is FDI? In simple terms, when completed, FDI will be the replacement for all EDDL (IEC 61804-3) based languages⎯HART, FOUNDATION
Fieldbus, and Profibus PA.
While EDDL is a common, text-based description of a device, the text
description is normally converted to a “binary DD” through a tokenizer
before being shipped with the device. Unfortunately, the format of the
binary DD is different for each process Fieldbus, even though they originate
from the common EDDL language. The manufacturing company members
of the FDI Corporation have made it a high priority to harmonize the
binary DD through secondary standards and tools so that the result will be
a single binary format file, regardless of the protocol used by the device.
Figure 1-15 illustrates how all these systems will work together. The FDI
developer for each of the protocols to be supported will develop a tokenizer
to “convert” its original version of the EDD language to the new common
interface so that the end result to the FDI server will be a single file format
that can be read by the common interpreter.
36
FIELDBUS LAYERS
Figure 1-15 — FDI Flow Device and Host
FDI – Data Flow Device and Host
Host Application : System A
FDI Server
Depackager
Device vendor
source
EDD
EDD
Test Tool
Device
Package
Packager **
Bridge *
For UIP
User
interface
FDI
Bin.
EDD
Tokenizer **
For EDD
UIP
FDI Server
Reference
Implement.
OPC UA
Binary Reader
Development Cycle
Common
Interpreter
Tested
FDI
Device
Package
Data
OPC UA
UIP
FDI Client
Reference
Implement.
FDI Client
User Interface
Plug-in (optional)
FDI Server
Design Tool
Depackager
Binary Reader
* Bridge ensuring interoperability
Common
Interpreter
Data
** Tokenizer and Packager could be integrated into designtool.
All tools are supporting HCF, FF, PROFIBUS
OPC UA
UIP
System B
FDI Client
FDI Server
Depackager
Binary Reader
Common
Interpreter
Data
OPC UA
UIP
System C
FDI Client
The EDDL file for each protocol will be processed through a tokenizer
much like is done today; this also ensures backwards compatibility. Because
each protocol is not exactly the same, but rather closer to 90% the same, it
will be necessary to develop an FDI Developer environment for each of the
three EDDL-based protocols to assist the protocols in defining how to map
the various parameters of each protocol to the appropriate FDI parameters.
The resulting binary file from the tokenizer will then be passed to a “packager,” where it will be converted to an FDI file. Note that at their discretion,
a device manufacturer will be able to define a User Interface Plug-in that is
integrated into the FDI file by the “packager” to create the single common
file. FOUNDATION Fieldbus device manufacturers may do this today by creating “Extended” Function Blocks that contain information beyond what has
been fully defined by the Fieldbus Foundation.
What will be important to end users, of course, is the interoperability of
these devices, which will be ensured through the appropriately colored
green “test tool” box to verify the “raw” FDI Device Package is compliant
FIELDBUS LAYERS
37
with the standard so that after testing, the “Tested FDI Device Package” will
have the necessary “check mark” from the appropriate organization that the
devices are not only compliant with FDI but also backwards compatible
with existing equipment. This is obviously important for the instances when
a new device needs to be added to an existing network, either for expansion
or because of failure of an installed device, and everything will have to continue to work together seamlessly as they did prior to the change.
When the new device is connected and communicating on the network, the
process needs to be reversed, with the DCS/host converting the FDI information into a format usable by the internal system databases. This is not
any different than is done today, where each system needs to interpret the
information from the field to the appropriate database register within the
Host. (If you have used Modbus, you have had to do this manually when
cross-referencing a specific terminal input from an I/O card to the appropriate memory register.)
Since all the components, such as the Device Definition and User Interface,
are based on either existing or proposed IEC standards, the IEC documents
shown in Figure 1-16 will also need to be updated.
Figure 1-16 — Device Package showing documents to be updated
38
FIELDBUS LAYERS
Note that the User Interface Plug-In, which will be used to provide
improved access to the maintenance, diagnostic, and related parameters in
the field devices, will use the knowledge gained from the use of FDT technology, and combine that with the open interoperable communications
capabilities of OPC UA to provide a platform independent access to the
rich data set contained in a modern digital field device.
FIELDBUS LAYERS
39
2 — Fieldbus Cabling
With an understanding of what all the “pieces” are, it is time to start “putting them together.” Chapter 2 summarizes information on FOUNDATIONTM
Fieldbus physical layers, H1 and HSE, including information on sizing the
network and connecting all the pieces together to build a network.
The FOUNDATION Fieldbus H1 Network uses twisted-pair wires. A twisted
pair is used, rather than a pair of parallel wires, to minimize externally
induced electromagnetic noise in the wires. A shield over the twisted pairs
further reduces the noise.
For new installations or to get maximum performance for a FOUNDATION
Fieldbus Network, twisted-pair cable designed especially for FOUNDATION
Fieldbus should be used. The important twisted-pair cable characteristics are
listed in Table 2-1.
Table 2-1 — Fieldbus cable characteristics
Type
A
B
18 AWG (0.8 mm)
22 AWG (0.32 mm)
90%
90%
3 dB/km
5 dB/km
100 Ω ± 20%
100 Ω ± 30%
Capacitance
2 nF/km
2 nF/km
Resistance
44 Ω /km
112 Ω /km
Maximum propagation delay
between 0.25 fr and 1.25 fr
1.7 μs/km
1.7 μs/km
Wire size
Shield coverage
Attenuation at 39 kHz
Characteristic impedance at
31.25 kHz
In new installations, a type A twisted-pair shielded cable should be specified. Provided that the limits presented in Table 2-2 are observed, other
types of cables may be used, such as twisted-pair multicables with an overall
FIELDBUS CABLING
41
shielding (this is called type B cable). Manufacturers are now also manufacturing multiconductor type A Fieldbus cables.
Table 2-2 — Cable type specifications
Parameters
Conditions
Type
A
Type
B
Type
C
Type
D
Characteristic impedance, Z0,
Ω
fr (31.25 kHz)
100 ±
20
100 ±
30
**
**
Maximum DC resistance, Ω/
km
per conductor
22
56
132
20
Maximum attenuation, dB/km
1.25 fr (39
kHz)
3.0
5.0
8.0
8.0
0.8
(#18
AWG)
0.32
(#22
AWG)
0.13
(#26
AWG)
1.25
(#16
AWG)
2
2
**
**
Maximum length, m
1900
1200
400
200
Capacitance, pF/m
150
150
150
Attenuation, dB/km
3
5
5
Nominal cross-sectional area
of the conductor (gauge), mm2
Max. non-balanced capacitance, pF
1 m long
** not specified.
A type of cable that is not recommended is the single or multiple twisted
pair without shielding; this is called type C cable.
The least used type is the multipair cable without twisted pairs and with
overall shielding, called type D cable.
Types C and D cables are listed in the specification for completeness, so
users are aware of the cables’ limitations if they choose to use them as part
of a retrofit project.
The maximum lengths indicated in the specifications and shown in Table 22 are recommendations that include a safety factor to satisfactorily minimize communication problems. As a general rule, the maximum cable run
is related to the cable type and its characteristics, the chosen topology, and
the quantity and type of devices used.
42
FIELDBUS CABLING
The Fieldbus Foundation has recently released specification FF-844, which
describes the requirements for cable to be used to connect the various components in a system. What is most interesting about the specification is
what is not specified. The parameters not specified include signal propagation velocity, wire insulation colors (though they do mention as an option
that it should be brown for positive and blue for negative), cable jacket colors, and characteristics such as cold temperature pull strength and cable wire
size/diameter.
In addition to the above requirements for the Fieldbus-related aspects of the
cable, it is also necessary to consider other characteristics of the cable for a
proper installation, including:
•
FF-844 specifies that FF-certified cable shield types shall be metalized
polyester tape with a minimum of 90% coverage. In many Intrinsically
Safe or equivalent installations, this polyester tape has a light blue
rather than silver color to indicate the change in service.
•
Ultraviolet protection must be a consideration for the outer cable
jacket. Black is still the best color for UV resistance.
•
The cable itself must also be suitable for the environment in which it is
to be installed; for example, armored for use in trays, direct burial,
marine shipboard, oil resistant, abrasion resistant, weld flash resistant
and cutting fluid resistant.
•
Overall cable roundness is important, as it can affect the effectiveness of
gland sealing and hence the integrity of a junction box used for environmental and electrical isolation.
As mentioned above, FF-844 does not specify or test signal propagation
velocity, wire insulation or cable jacket colors or special cable characteristics, such as cold temperature pull strength. Other than signal propagation
velocity, these properties are often site-specific, and some projects include
the requirement that the cable jacket colors be different for trunk versus
spur cables.
Besides specifying the key cable properties, the specification also standardizes how connectors are to be mated to the cable for M12 and 7/8”-16 UN2A Thd connectors.
FIELDBUS CABLING
43
Other key parameters covered in the specification include:
•
Characteristic Impedance (Zo) – 100 ± 20 Ohms
•
Attenuation – 3db/km at 39 kHz
•
Wire – both 18 AWG for trunks and 22 AWG for spurs are included.
•
Shield Construction – each pair must be individually shielded using
metalized polyester tape as the preferred choice, though other equivalent options are allowed.
•
Wire-to-Wire Capacitance – a minimum of 12 picoFarads/meter based
on a minimum 30 m cable length.
•
Wire Twists per Meter – a fairly typical 18 to 22 twists per meter.
•
Jacket Resistance – 1 MegaOhm/330 Meters (1000 feet) minimum resistance between the cable shield and the metal structure the cable may be
in or on.
End users should note that the Characteristic Impedance can vary by ±
20% from what is considered the norm of 100 Ohms; resistance can change
considerably with temperature. Be sure you know how Zo changes as a function of temperature, and at what temperature the cable’s Characteristic
Impedance has been determined. Otherwise, if you purchase a cable with an
impedance of 80 Ohms, still within the specification, you may have an “out
of specification” cable at operating temperatures.
The end result of this specification is that customers will now be able to
specify that the cable to be used for their FF project be compliant with FF844 and have the associated FF “check mark.” Of course, an offshoot of this
is that adventurous end users now have a clear picture of what characteristics are needed to use any other type of cable for their design—provided it
meets all the identified criteria.
The maximum overall length of cable when mixing cable types is determined according to the formula
Lx
Ly
------------- + -------------≤1
L max L max
x
44
y
FIELDBUS CABLING
where:
= length of cable x
= length of cable y
Lx
Ly
L max
x
L max
y
= maximum length of cable type x alone
= maximum length of cable type y alone
A FOUNDATION Fieldbus Network consists of the total wire pair length, trunk
length, and spur lengths. Total wire pair length is the sum of the trunk cable
plus all the spurs. A trunk is the longest cable run between any two devices
on a network. A spur can vary in length from 1 to 120 m. A spur that is
<1m is considered a splice.
The same ratio calculation can be performed if more than two cable types
are present in the system, as long as the sum of the ratios is <1.
In addition to the physical limitations described earlier, which are generic
guidelines based on voltage and capacitance limitations, the following equation can be used to calculate the maximum trunk length on a system with
approximately equal spur lengths and devices with nearly equivalent current, voltage, and capacitance needs.
6
( ( V PS – V Min ) × 10 – I D × 2 × R S × L S )
L TMax < ---------------------------------------------------------------------------------------------------ID × 2 × RT

where:
LTMax
VPS
VMin
ID
=
=
=
=
RS
LS
ΣI D
RT
=
=
=
=
maximum length of trunk cable, meters
power supply voltage, volts
largest minimum voltage of all the field devices, volts
DC current draw of the field device with the largest minimum
voltage, milliamps
manufacturer-specified resistance of spur cable, Ω /km
length of spur cable, meters
total (sum) of DC current draw of ALL field devices, milliamps
manufacturer-specified resistance of trunk cable, Ω /km
FIELDBUS CABLING
45
If the installation is a Chickenfoot arrangement, or if each of the field
devices has very different minimum voltage (e.g., a temperature transmitter
and a valve positioner) and current specifications, then the voltage available
at each device on the segment should be calculated using the following formula:
V D = V PS – 

 ID × 2 × RT × LT + ID × 2 × RS × LS × 10
–6
> V Min
where:
VMin
VD
VPS
ΣI D
RT
LT
ID
RS
LS
=
=
=
=
=
=
=
=
=
minimum voltage of the field devices, volts
DC voltage available at the field device, volts
power supply voltage, volts
total (sum) of DC current draw of ALL field devices, milliamps
manufacturer-specified resistance of trunk cable, Ω/km
length of trunk cable, meters
DC current draw of the field device, milliamps
manufacturer-specified resistance of spur cable, Ω /km
length of spur cable, meters
Capacitance constraints must also be considered since the effect of capacitance on the signal of a spur <300 m long is very similar to that of a capacitor. In the absence of actual data from the manufacturer, a value of
0.15 nF/m can be used for Fieldbus cables.
CT =
 ( LS × CS ) + CD
where:
CT
LS
CS
CD
46
= total capacitance of network, nF
= length of spur cable, meters
= capacitance of wire for segment, nF/m (use 0.15 if no other
number is available)
= capacitance of device, nF
FIELDBUS CABLING
The attenuation associated with this capacitance is 0.035 dB/nF. To estimate
the attenuation associated with the installation, the following formula provides a useful guideline:
dB
A = C T × LT × 0.035 ------- < 14dB
nF
where A is attenuation, dB.
Cables have attenuation ratings for a given frequency. The frequency of
interest for FOUNDATION Fieldbus cable is 39 kHz. Standard FOUNDATION
Fieldbus cable has an attenuation of 3 dB/Km at 39 kHz or about 70% of
the original signal after 1 Km. If a shorter cable is used, the attenuation
would be less. For example, a 500 meter standard Fieldbus cable would have
an attenuation of 1.5 dB.
A Fieldbus transmitter can have a signal as low as 0.75 volts peak-to-peak. A
Fieldbus receiver must be able to detect a signal as low as 0.15 volts peak-topeak.
Tip 6 — Total attenuation must be <14 dB, which is the minimum signal that can be detected between the lowest level transmitter and the least sensitive receiver.
2.1 Segment Protection
Because Fieldbus circuits are wired in parallel, it is important to be able to
isolate a failed device due to a short circuit from the balance of the network;
otherwise, the entire segment will fail. It is for this reason that practically all
field device couplers contain short circuit protection. The use of fuses is the
simplest way to implement short circuit protection and to ensure that at the
time of the short circuit, communication on the rest of the network will not
be interrupted. However, fuses provide “one time” protection and require
manual replacement. For this reason, electronic circuits that work by detecting an increasing DC current to the instruments are used in modern junction boxes. This action happens quite quickly, before the occurrence of the
short circuit, thus preventing the interruption of H1 communications.
FIELDBUS CABLING
47
The short circuit protection electronics are typically connected to the individual spurs within the field device coupler base, so that the occurrence of a
failure (short circuit) on the cable connection (spur) or in the Fieldbus
instrument does not affect the other instruments of the segment, thus maintaining perfect digital communication.
We can understand how to determine the rating of a short circuit protection
circuit by looking at Figure 2-1, where we must consider a “worst-case”
Fieldbus instrument of 28 mA, then add margins for other network effects
as indicated. 28 mA is high for today’s typical device consumption; however, we must also consider inrush current and the 10 mA oscillation of the
signal itself. In the event of an increase in the current caused by a short circuit at the instrument, or even on the cable connection (spur), the current
value will be limited to the range of 50 mA, without causing any interruption in digital communication and isolating this spur from the other devices
in the segment.
Current
Figure 2-1 — Short circuit protection “sizing”
53+ mA
~ 5 mA margin for
Background noise
10 mA
Handhelds
10 mA “in rush” and FF Signal
oscillation
28 mA
Base instrument current (worst-case device)
Time
Tip 7 — When sizing a Fieldbus segment, the difference in current demand between the device with the lowest current consumption and the short circuit rating of the field device coupler
48
FIELDBUS CABLING
must be included in the overall current load. If this margin is not included
and there is a short circuit, the entire segment will be overloaded (more current than the power conditioner can provide), and as a result, the complete
segment will fail.
Figure 2-2 is a field device coupler with an embedded Terminator (represented by the large “T”), which shows the difference when a short circuit
occurs, as an LED will indicate the failed spur.
Figure 2-2 — Spur overcurrent failure indication
Spurs must
be чϭϮϬŵ
Short Circuit LED
Indicator
To Host System
> 9V Power
Indicator
Because field device couplers are the most likely component in a Fieldbus
network to be worked on (for example, for the addition/removal of a field
device, field device maintenance, etc.), and because the couplers are typically mounted inside enclosures, most manufacturers use some form of
connector device that is removable and then secured to the base of the
device coupler in some way – typically with screws.
Connector blocks are devices used in installations where the devices must
be periodically disconnected and/or displaced. They can also be very conve-
FIELDBUS CABLING
49
nient for connecting temporary equipment to a certain point. Figure 2-3
shows a number of these connections.
Figure 2-3 — Fieldbus connector blocks
Terminator
Cap
Gland
The same terminal blocks used in 4-20 mA applications can be used for
Fieldbus if connected in parallel.
Terminal blocks make multiple trunk connections possible so that equipment can be connected to any terminal set in the trunk.
Figure 2-4 shows how a Fieldbus segment termination and connection to
several field devices in a junction box could be done with conventional terminal blocks.
50
FIELDBUS CABLING
Figure 2-4 — Fieldbus wiring with conventional terminal blocks
Fieldbus Junction Box
Home Run Trunk
To Host
100 :
1 PF
Tip 8 — The terminator should be placed in the junction box
(Figure 2-5) so that accidental removal of a field device does not
result in the loss of network communications.
Figure 2-5 — Terminator inside junction box
Field connector block
with internal Terminator
Standalone
Terminator
Block
Terminator
Circuitry
100 :
1 PF
Most projects today do not use built-in terminators but rather have external
terminators that attach to the continuous trunk line. The reason for taking
this approach is so that if in the future you wish to expand or extend the
FIELDBUS CABLING
51
segment, you can do so by adding additional cable and device coupler(s) to
the line (bearing in mind the maximum total cable length limitation of
1900 m) while maintaining the terminator at the device coupler at the
remote end of the trunk, thus keeping all spurs within the required maximum length of 120 meters.
Tip 9 — If a multiple home-run cable goes to a field junction box,
do not attach the cable shield wires from different networks
together. Doing this creates ground loops and introduces noise
into the network.
Do not connect the cable shield of a field instrument to the instrument
ground or chassis.
Figure 2-6 illustrates the correct way to terminate a field device. Note that
special care must be taken to ensure isolation of the shield and signal cables
since failure to do this can introduce extraneous, sporadic noise into the
network.
Figure 2-6 — Termination guidelines
52
FIELDBUS CABLING
The illustrated termination can be achieved by adhering to the following
good installation practices:
•
Use proper wire strippers that do not nick the wire as they strip the insulation.
•
Use color-coded crimp ferrules to prevent stranded wires from getting
loose and shorting to other wires. Preferably the ferrule material should
be the same metallurgy as the wire material on the wiring blocks since
similar metals are more corrosion-resistant than a bare wire in a wire terminal.
The following represents a series of guidelines to consider when designing
FOUNDATION Fieldbus segments. In some cases, the guidelines may conflict
with each other (such as shorter spurs are better versus distributing measurements on separate networks / H1 cards) or with an economic design decision made by the project team.
•
Each segment will normally have 12 or fewer devices and not more than
two control valves.
•
A general rule to remember is that “shorter is better” when it comes to
designing spurs.
•
To allow for devices to be added to the network in the future if
required, networks should be designed for 25% spare capacity. Remaining capacity should be calculated on the basis of the lesser of the power
or bandwidth constraint.
•
A Fieldbus segment in a hazardous area should consist only of the type
and number of devices that will not cause the segment current draw to
exceed the rating of the barrier or power supply. For intrinsically safe
circuits, this is typically 80 mA.
•
For vessels with multiple temperature and/or pressure measurements,
the multiple measurements should be distributed to reside on separate
H1 cards.
FIELDBUS CABLING
53
2.2 High-Speed Ethernet
High-Speed Ethernet (HSE) has been designed to fully incorporate all the
functionality of Fieldbus H1 and supports both the Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP) Transport Layer protocols, but UDP is the default transport protocol. In addition, HSE supports
full redundancy of devices and the Data Link Layer.
Figure 2-7 shows how the Fieldbus-specific and Commercial-Off-the-Shelf
(COTS) parts of the specification combine to provide this integrated solution. In this figure, the shaded areas are defined by other than the Fieldbus
Foundation and are common to many of the other Industrial Ethernet protocols. The balance of the “blocks” in the diagram is defined in the Fieldbus
protocols prepared as part of the HSE specifications and should only be of
interest to a device manufacturer. The key blocks that allow encapsulation
of the H1 protocol within HSE are the H1 Interface blocks 1 through N for
each H1 function block to be connected to the HSE network.
Figure 2-7 — HSE profile functional areas
All Ethernet devices are built on the same platform; therefore, they can
coexist on the same network with different Application Layer protocols
without conflict. Since both Ethernet and Internet Protocol (IP) standards
54
FIELDBUS CABLING
have addressing schemes called Ethernet Medium Access Control (MAC)
address, which is both a hardware address and an IP address, it is possible to
mix devices from Profibus and Fieldbus on the same physical wires. The
MAC address is set in the hardware by the device manufacturer, and it is
unique to each type of device.
Although possible, it is not recommended to share the physical layer
among different protocols and to place the Industrial Ethernet protocol,
corporate LAN traffic, and even VoIP phones on a single wire or fiber. If
you decide to try this, the network (as a minimum) must use IP V6 switches
to not only minimize the impact of any data storms but also to monitor
each packet for its priority so that the control-related information always
has the highest priority. However, since the most significant cost of installing cable is not the cable itself but the associated labor, there is no reason to
mix traffic on a single wire or fiber.
When preparing the HSE specifications, the Foundation made an effort to
determine the types of functionality that would be required of devices connected to the network and came up with the following set of requirements
or profile classes:
•
Four types of Field Devices identified as Class 41
•
Four types of Linking Devices identified as Class 42
•
An I/O Gateway Device, Class 43
•
Two classes of Host Devices, Classes 44 and 45
•
Two levels of redundancy, Classes 46 and 47 and Classes 48 and 49,
which are used for timing, synchronization, and network connectivity
Table 2-3 summarizes the differences between the various classes of device.
A Class 43 I/O Gateway Device supports Fieldbus Messaging Services
(FMS) that are supported by Class 42c.
A Class 44 Simple Host is an HSE host of some kind; for example, it might
be a Process Operator Workstation that supports subscription to Function
Block data such as AI, AO, PID etc. and can be a report sink or receiver of
data from the network.
FIELDBUS CABLING
55
Table 2-3 — HSE class summary
Class
Main Feature
41a
Function Blocks, sources events, publishes data, responds to client
requests
41b
41a plus Subscribes
41c
41b plus basic FFB support
41d
41c plus extended FFB support
42a
Linking Device - Client access HSE  H1, server response H1  HSE
(e.g., configuration)
42b
42a plus sources events from H1  HSE
42c
42b plus publisher/subscriber H1  H1, H1  HSE
42d
42c plus extended services for Flexible Function Blocks in H1 devices
43
I/O Gateway device - allows interoperation with other instrument systems
44
Simple Host
45
Configurator Host Reserved
46-D1
Does not support redundancy, but sends required redundancy diagnostic
messages
46-D2
46-D1 plus redundant device - manual switchover
46-D3
46-D2 plus automatic switchover
47-L1
Single HSE communication port
47-L2
Dual HSE communication ports
48-TC
HSE Time Synchronization - can request time from server
48-TS
48-TC plus can be the time server
48-TN
Has no time synchronization capabilities
49-AY
Automatic IP Address - using DHCP
49-AN
IP Address is obtained using local means
The Class 45 Configurator Host is as the name implies—an HSE host with
configuration capabilities. It is capable of configuring HSE and H1 devices
and configuring HSE LAN redundancy information in HSE devices.
Every HSE device is also required to support, as a minimum, Classes 48 and
49 and one of Classes 46 and 47.
56
FIELDBUS CABLING
For 10 Mbps Ethernet, there can be no more than four repeaters or shared
hubs between any two devices on the network. The hubs can typically be
located at a maximum distance of 100 m from each other. FOUNDATION
Fieldbus uses 100 Mbps full-duplex standard Ethernet as its basis, so some
of the limits of 10 Mbps Ethernet are overcome by the increased bandwidth
available.
Ethernet can be made “deterministic,” meaning the message will be delivered within a specified time frame by using 100Base-TX and switched hubs
with a maximum 50% loading, and passing the message through the minimum number of nodes on the network. This way, the chance of a message
getting delayed is less than the chance of a message being lost due to noise,
and the network limitation resides in the switch, not in the other system
components.
To make networking easier to administer, it is a good idea to use patch panels at the hub to facilitate interconnections between various networks and
sub-networks. Patch cords are then used to make the desired final connections.
Two types of twisted-pair cables are used for Ethernet:
1.
UTP, which has four unshielded twisted pairs, sometimes with an
overall shield. This is the preferred cable for industrial Ethernet installations. CAT 5E or CAT 6 cable is what is normally used in today's
Ethernet installations, and industrial-grade versions of this cable type
are on the market.
2.
STP, which has four individually shielded twisted pairs with an overall
shield, called Category 7 cable (rarely used.).
Only two of these pairs are used for 10Base-T and 100Base-TX. Twisted-pair
cables use RJ45-style connectors, and industry is working to develop a ruggedized form of this termination assembly.
To prevent degraded performance, UTP cable should never be placed in a
metal conduit. If it is necessary to run the cable in a harsh environment, it
is preferable to use a special cable with higher pull strength and a suitable
jacket for the conditions.
FIELDBUS CABLING
57
Figure 2-8 shows how a twisted pair is used for Ethernet connectivity, while
Figure 2-9 shows how HSE is connected to the H1 network to create a full
control network.
Figure 2-8 — Ethernet wiring
Physical Media
Name
10Base5
10Base2
10Base-T
10Base-F
Cable
Thick Coax
Thin Coax
Twisted pair
Fiber
TX+
TX-
Max Segment
500 m
200 m
100 m
2000 m
Nodes/ Segment
100
30
1024
1024
RJ45
RX+
RX-
Signal
Power
+
Figure 2-9 — Wiring H1 devices to a linking device
HSE
Linking Device
Terminator
Terminator
H1
Power
Supply
Power
Cond.
Spur
Spur
May be part of link
device hardware
58
FIELDBUS CABLING
Because Ethernet standards do not directly support ring topology, if it is
decided that a ring topology is to be used for the Ethernet backbone, all the
hubs/switches/repeaters that are part of the ring must be from the same
manufacturer.
Fieldbus HSE is another link in the integrated digital enterprise of the
future, as shown in Figure 2-10. H1—and in some cases, HSE field devices—
will be used in the field, HSE will be used with linking devices to gather
multiple field level signals onto the Control network, and then conventional Ethernet with appropriate security can be used to connect to the
remainder of the control system.
Figure 2-10 — Integrated fieldbus system
The Fieldbus Foundation released the wireless input/output (WIO) specifications in December 2010 to standardize the remote I/O (RIO) capabilities
of its protocol as a means of bringing high-speed data collection of analog
and discrete I/O into the control system with a consistent configuration
and communication protocol. Once the data is in the HSE format, it is possible to integrate this information directly into the control and asset manFIELDBUS CABLING
59
agement/maintenance systems at a facility using e-EDDL, the upcoming
OPC-UA and FDI enhancements. More information on the HSE RIO/
WIO project is included in Chapter 9.
2.3 Grounding/Earthing
Proper grounding—or earthing, depending on where you are located in the
world—is critical to the success of any instrumentation and control project,
as ground loops are a common cause of signal degradation and failure.
An industrial plant has the most significant generators of electrical noise in
its electrical components, such as high-powered motors, variable frequency
drives generating signals from such items as their frequency inverters, electric welding systems, and induction furnaces. The levels of this electrical
noise are often much higher than the levels of voltage of the frequencies
used by most digital protocols, which are less than 1-2 volts peak-to-peak. In
general, grounding systems are scaled so that all these induced voltages and
their associated current frequencies can be drained to ground.
The preferred method of installing instrumentation systems, including
Fieldbus systems, is to have a single ground point at the host system. This is
shown in Figure 2-11.
Figure 2-11 — Continuity of ground
Final field termination is
isolated from case or
equipment ground/earth
60
FIELDBUS CABLING
The instrument ground is then connected to the plant ground at a single
location per control room or system as shown in Figure 2-12. Note that the
cable trays, conduits, enclosures (including transmitter housings), and
device supports must all be connected to the plant ground/earth grid.
Figure 2-12 — Plant ground and instrument ground
Plant Earth/Ground
For grounding the shield of a signal cable, it is recommended that the shield
be grounded to a single point, preferably in the cabinet of the control system. The grounding of the cabinet and its components must also take into
account the grounding of the metal enclosure of the cabinet to the general
ground grid of the plant. Typically, the instrument signal grounding system
is separate from the general plant grounding system. The two systems are
interconnected to ensure that the important condition of the same potential exists between all parts of the facility, as this is the basis of the electrical
safety of the staff and facilities.
The American National Electrical Code allows a grounding resistance value
of up to 25 Ohms between each of the grounding points shown in Figure 212.
Improper connection of the communication cable shield to the ground circuit, other than as shown in figures 2-11 and 2-12 above, can easily cause a
short circuit between adjacent segments. In part, this is why it is critical to
use good work practices in properly terminating all connections, preferably
with ferrules so that any stray wire strands do not inadvertently cause a
short circuit, as shown in Figure 2-13.
FIELDBUS CABLING
61
Figure 2-13 — Cable shield grounding
Power
Conditioner
24 V DC +
Bulk
Power Supply
Segment
A
Short Circuit
Power
Conditioner
Segment
B
Figures 2-14, 2-15 and 2-16 show the various recommended ways of installing a Fieldbus grounding system.
Figure 2-14 is the most commonly practiced method around the world of
single-point grounding at the master reference ground typically located in
the interface room and shown in Figure 2-15 as it would be applied to a
Fieldbus installation.
Figure 2-14 — Recommended fieldbus grounding
Fieldbus
Junction Box
Host
H1
Port
Wiring
Block
A capacitively grounded system (Figure 2-15) has been found to be a useful
way of reducing high frequency noise.
62
FIELDBUS CABLING
Figure 2-15 — High frequency capacitive ground
Fieldbus
Junction Box
Host
H1
Port
Wiring
Block
Figure 2-16 shows equipotential bonding, or “uniform” grounding, which is
frequently used in Northern Europe, where the facilities are designed with a
common-ground grid throughout the facility.
Figure 2-16 — Equipotential bond
Fieldbus
Junction Box
Host
H1
Port
Wiring
Block
The control cabinets normally accommodate the racks with interface modules and other components of the system, such as power supplies and segment couplers, control system interface modules, intrinsic safe barriers, etc.
The cabinets need to be designed to take into account that 4-20 mA systems
within the same cabinet or system could “hide” ground loops as undetected
incorrect signals or drift/noise in the analog measurement that is not easily
found with conventional techniques. This noise can then potentially affect
the Fieldbus signal as well because of the connection to the common mas-
FIELDBUS CABLING
63
ter reference ground. It is therefore important to remember that faults can
arise from outside the H1 network itself when troubleshooting systems.
With Fieldbus as shown in Table 2-2, there are maximum distances of interconnection between the control cabinets and signal conditioners to the segments.
2.4 Surge Protection
Because we are dealing with low-level digital signals, these signals and the
associated microprocessors used in today’s modern electronics are susceptible to damage by electrical surges. For example, lightning-generated voltages
can run to hundreds of kV. Signal isolation is good up to a point, perhaps to
a level of 8-10kV, but it is prone to break down, and the surge protection
commonly included in field components typically provides the same level
or degree of protection as signal isolation.
The most commonly protected component of a Fieldbus system is the
trunk since the loss of this component of the network will result in the loss
of multiple control signals. As a rule of thumb, if the two-wire trunk cable is
longer than 50 meters in the horizontal plane or 10 meters in the vertical
plane (for example, when a sensor/transmitter is positioned on a column,
stack, pole or pipe), then surge protection may warrant consideration.
When considering surge suppression for the local field wiring, the requirement is to look for more obvious situations, such as:
a) The spur cables connecting to a single instrument. Anything longer
than 50 meters should be considered for surge protection; cables longer than 100 meters are at higher risk.
b) Particular installations where lightning currents would preferentially flow; for example, instruments mounted along a pipeline that
crosses a non-conducting surface, such as dry sand.
c) Installations that involve considerable vertical distances on structures that may be struck by lightning. A pressure sensor on top of a
distillation tower is a classic example.
64
FIELDBUS CABLING
d) Installations where sensors are associated with high-voltage or highpower electrical equipment, for example, temperature sensors embedded in the windings of high voltage motors.
Suitable surge protection devices should be used for hazardous and nonhazardous areas. The surge protectors should not cause any signal attenuation.
The load on a circuit introduced by surge protection is approximately equal
to 30 cm of cable.
Surge protection is available in two formats: terminals and “bull plugs” that
thread into a field mounted transmitter, thus giving you the capability to
install surge protection where and when it is required.
2.5 Cable Installation
Industry practice has always been to separate different types of electrical
cables, not only for safety reasons, so that a worker might not inadvertently
come in contact with a high-voltage cable while expecting a low-voltage
environment, but also to minimize the affects of electromagnetic coupling
(EMC) between conductors.
In 1997, the IEC released standard IEC 61000-5-2:1997 Electromagnetic
Compatibility (EMC) Part 5: Installation and Mitigation Guidelines, in
which Section 2, Earthing and Cabling, addresses this issue by assigning different Classes to different types of cables based on the energy levels they
contain.
The classes defined in the standard are as follows:
•
Class 1 is for cables carrying very sensitive signals. Low-level analog signals such as millivolt output transducers and radio receiver antenna
feeds are in Class 1A. High-rate digital communications such as Ethernet are in Class 1B. Classes 1A and 1B should not be bundled together,
although their bundles may be run adjacent to each other.
•
Class 2 is for cables carrying slightly sensitive signals, such as ordinary
analog (e.g., 4-20mA, 0-10V, and signals under 1MHz), low-rate digital
FIELDBUS CABLING
65
communications (e.g., RS422, RS485), and digital (i.e., on/off) inputs
and outputs (e.g., limit switches, encoders, control signals).
•
Class 3 is for cables carrying slightly interfering signals, such as low voltage AC distribution (< 1kV) or DC power (e.g., 48V telecommunication power), where these do not also power “noisy” equipment. Power
distribution that also feeds noisy equipment may be converted from
class 4 to class 3 by the correct application of filtering
•
Class 4 is reserved for cables carrying strongly interfering signals. This
includes all the power inputs or outputs (to or from) adjustable speed
motor drives and power converters and their DC links. Class 4 also
applies to the cables associated with electrical welders, RF equipment
(e.g., plastic welders, wood gluers, diathermic apparatus, microwave dryers and ovens), DC motors or sliprings, and similar “noisy” apparatus.
•
Class 5 and 6 are reserved for medium-voltage and high-voltage supply
distribution cables, respectively.
Associated with these classifications are recommendations on the spacing of
the different types of cables within a single cable tray as per Figures 2-17
and 2-18.
Figure 2-17 — Segregation of cable classes
ůĂƐƐϭ
Cables
ϭϱϬŵŵ
ůĂƐƐϮ
Cables
ϯϬϬŵŵ
Class 3
Cables
ϭϱϬŵŵ
Class 4
Cables
ϰϱϬŵŵ
ϰϱϬŵŵ
ϲϬϬŵŵ
Parallel Earth Conductor (PEC) e.g., cable tray
66
FIELDBUS CABLING
These are minimum spacings for cables run close to PEC for 30 meters distance. For longer runs, multiply by length in meters divided by 30.
Figure 2-18 — Segregating cables in trays
Figure 2-19 illustrates the recommended spacing between trays of different
voltages and Classes.
Note: If there are variable frequency drive motors or other potentially “high
noise” sources with cables in the adjacent tray, the minimum distances
between trays should be double the minimum shown. When installing
cable tray, vertical spacing needs to be considered to ensure access to the
rear part of the tray for installation and maintenance.
FIELDBUS CABLING
67
Figure 2-19 — General cable installation guideline
Cables for : 4-ϮϬŵн,Ăƌƚ͕WƌŽĨŝďƵƐ
WͬW͕DeviceNet, Ethernet, Modbus
or FKhEd/KE Fieldbus
ϯϬϱŵŵ
ϯϬϱŵŵ
(min)
ĂďůĞƐĨŽƌϭϮϬͬϮϮϬs
ϯϬϱŵŵ
ϯϬϱŵŵ
(min)
ĂďůĞƐĨŽƌDŽƚŽƌƐϯϴϬͬϰϰϬs
68
FIELDBUS CABLING
3 — Fieldbus Power Supplies
As seen in Chapter 2, FOUNDATIONTM Fieldbus was designed to use as much
of the existing infrastructure as possible. This means that it can also be
installed in the same environments. However, because Fieldbus is a digital
communication network, it has unique power conditioning requirements.
This chapter discusses those power requirements for three different conditions: Intrinsically Safe (IS), Non-Intrinsically Safe (NIS) and the Fieldbus
Intrinsically Safe Concept (FISCO)/Fieldbus Non-Incendive Concept
(FNICO) systems.
Fieldbus devices may receive their power supply either via the same communication cables or separately.
Table 3-1 defines the characteristics required of network- or loop-energized
devices, while Table 3-2 lists the requirements for the associated power supply for these devices.
Table 3-1 — Characteristics of network-energized devices
Characteristics of network-powered devices
Limits for 31.25 Kbps
Operating voltage
9.0 to 32.0 VDC
Maximum voltage
35 V
Maximum idle current rate of exchange (not transmitting); this is not applicable to the first 10 ms after the
device is connected to an operating network, nor in the
first 10 ms after the network is energized.
1.0 mA/ms
Maximum current: This is adjustable in the interval of
100 μs to 10 ms after the device is connected to an
operating network, or 100 μs to 10 ms after the network
is energized.
Idle current plus 10 mA
FIELDBUS POWER SUPPLIES
69
Table 3-2 — Networks’ power supply requirements
Networks power supply requirements
Limits for 31.25 Kbps
Output voltage, non-intrinsically safe
≤32 VDC
Output voltage, intrinsically safe (IS)
Depends on the barrier
Output impedance, non-intrinsically safe, measured
within the frequency range of 0.25 to 1.25 fr
≥3 kΩ
Output impedance, intrinsically safe, measured within
the frequency range of 0.25 to 1.25 fr
≥400 kΩ (An intrinsically
safe power supply
includes an intrinsically
safe barrier.)
If an ordinary power supply were to be used to power the Fieldbus, it would
absorb the signals on the cable because it would try to maintain a constant
voltage level. Fieldbus power supplies are conditioned by putting an inductor in parallel between the power supply and the field cable and adding a
resistor to the inductor to prevent “ringing” or unwanted oscillation of a
signal. The power conditioner consists of the equivalent of a 50 Ω resistor
and a 5 millihenry inductor in series.
The Fieldbus Foundation released a specification, FF-831 Fieldbus Power
Supply Specification, in 2004. All Fieldbus Foundation power supplies/conditioners used in a project should be tested against this specification and
should have received the Fieldbus Foundation certification “check mark.”
These parallel RC circuit shunts result in a 50 Ω impedance, and as the
transmitter’s Manchester current passes through, this impedance creates a
0.5 VAC voltage drop. All Fieldbus devices transmit their signal as a current
and receive as a voltage. Therefore a device will generate an AC signal (1520 mA) going through the 50 Ω resultant impedance, which results in a 1
volt peak-to-peak signal at the receiving end. The 50 Ω resultant impedance
is calculated by adding the resistance of the two 100 Ω resistors at the terminators at either end of the network and is another reason to have two terminators on each segment. All the devices on the network are sensitive enough
to pick up the attenuation of the signal along the wires. The net effect is
that the transmission is done as current and the reception as voltage.
Because the signals are received as a voltage, polarity might be considered
an issue, however all Fieldbus systems may or may not be sensitive to polar70
FIELDBUS POWER SUPPLIES
ity. There is no requirement for polarity insensitivity in the Physical Layer
standards. The use of nonpolarized equipment, however, simplifies design
and installation.
Fieldbus devices connect to the network in parallel, and to be truly specification-compatible, they should be polarity-insensitive. Despite the fact that
Fieldbus devices, according to the specifications, are supposed to be polarity-insensitive, in many cases, for older transmitters this is not true, so when
installing devices, take care to match the polarity indicated on the device
terminals to that of the wire pair from the host and power supply. For this
reason, unless you are using the specified Fieldbus cable, it is a good idea to
continue to use the wire pair colors that the on-site maintenance people are
familiar with having, for example black as negative and white as positive.
Tip 10 — Not all manufacturers’ devices are polarity-insensitive,
so the user must be cautious when specifying and purchasing
devices.
It is good practice to continue to use one wire color for positive and the
other for negative. The FF Cable specification FF-844 “H1 Cable Test Specification” recommends the colors brown for positive and blue for negative.
In the case of shielded cables, as stated in the standard, the impedance measured between the Fieldbus cable shield and the Fieldbus device ground
shall be >250 kΩ for all frequencies below 63 Hz.
The maximum nonbalanced capacitance for the ground of both input terminals of a device shall not exceed 250 pF.
A terminator shall be placed on both ends of a trunk cable, connecting one
signal conductor to another. No connection should be made between the
terminator and the cable shield.
The terminator impedance shall be 100 ? ±20% within the frequency range
of 0.25 to 1.25 fr (7.8 to 39 kHz). This is approximately the mean impedance
of the cable in the operating frequencies and has been chosen to minimize
reflections in the transmission line.
FIELDBUS POWER SUPPLIES
71
Direct current leakage by the terminator shall not exceed 100 µA. The terminator shall be nonpolarized.
To comply with noise immunity requirements, it is necessary to make sure
there is shielding continuity along the cabling, connectors, and couplers.
A Fieldbus cable shield, by usual practice, is grounded at one end only, and
it may not be used as a power conductor.
Fieldbus devices are specified to work with voltages of 9–32 VDC.
Tip 11 — The 9 VDC specified is a minimum; it is highly
desirable that a margin of at least 1 VDC (i.e., a minimum of
10 VDC) be maintained. Any segment designed to operate below
11 VDC normally should carry a warning in the segment documentation
about additional loads. Minimum segment voltage should always be shown
in the segment documentation.
3.1 Intrinsic Safety
Intrinsic Safety (IS) is an instrumentation design methodology for flammable atmosphere areas. Safety is obtained by limiting the power and current
values that could create sparks or cause surface heating as a result of their
normal operating conditions or specified fault conditions, or by limiting
electrical charges that could cause ignition. Flammable gas may ignite
because of two unrelated parameters: the minimum quantity of energy
required to create sparks capable of igniting a given flammable gas, and the
minimum temperature at which a heated surface will cause the same effect.
To be sure that energy on network is below the explosion threshold while
still supporting and FF signal, the impedance of intrinsic safety barriers
needs to be higher than 400Ω for all frequencies in the interval between 7.8
and 39 kHz. An intrinsic safety barrier cannot be more than 100 m away
from one of the terminators, and for this reason, the terminator is normally
integrated into the FF power supply. The terminator resistance must also be
sufficiently low so that when it is in parallel with the barrier’s impedance,
the equivalent impedance is an entirely resistive one and thus does not
adversely affect the resistance on which the intrinsically safe circuit is based.
72
FIELDBUS POWER SUPPLIES
These requirements are valid for separate intrinsic safety barriers and for
those internally integrated into the affected power supplies.
Within the operating voltage range of an intrinsic safety barrier (in the 7.8–
39 kHz interval), the capacitance measured between the positive terminal
(hazardous side) and the ground shall be less than 250 pF higher than the
capacitance measured between the negative terminal (hazardous side) and
the ground.
Tip 12 — In the case of intrinsically safe systems, the operating
voltage may be limited to comply with the certification requirements. In this case, the power supply will be located within the
safe area, and its output voltage will be attenuated by a safety barrier or by
an equivalent component.
In Europe and the majority of the rest of the world, equipment is classified
on the basis of its design and construction characteristics; in North America, equipment is classified considering its possible area of installation. In
practical terms, both systems are equivalent if the differences are smaller
than those presented in Table 3-3.
Table 3-3 — Equipment classification guide
Explosive
substance
Methane
Equipment classification
Europe
North America
Group I
Non-classified
Ignition energy
Acetylene
Hydrogen
Ethylene
Propane
Group II, C
Group II, C
Group II, B
Group II, A
Class I, group A
Class I, group B
Class I, group C
Class I, group D
>20 μJoules
>20 μJoules
>60 μJoules
>180 μJoules
Metal dust
Coal dust
Grain dust
Under elaboration
Class II, group E
Class II, group F
Class II, group G
Easier
Ignition
Fibers
FIELDBUS POWER SUPPLIES
Class III
73
As a rule, if the device characteristics are not known, a good starting point
for designs appears in Table 3-4.
Table 3-4 — Default function block information for all designs
Function Block
Execution time (ms)
Current (mA)
AI
30–50
15–20
AO
60–75
25–30
PID
30
N/A
3.2 Fieldbus Intrinsically Safe Concept
The Fieldbus Intrinsically Safe Concept (FISCO) was developed to provide
a way to supply additional power to a Fieldbus segment while still keeping
the energy level below that which could cause an explosion. Unlike the IS
Entity concept. which is derived from theoretical calculations, FISCO is
based on actual field trials and experiments by the Physikalisch-TechnischeBundesaanstalt (PTB) research center in Germany. Because this IEC 6007915 standard is based on experimentation, all installations must operate
within the maximum limits within which the experiments were conducted.
The total cable length of the system is limited to a maximum of 1,000 m in
IIC/Groups A, B gases and 1,900 m in IIB/Groups C, D (limited by FF-831
and therefore identical to any Fieldbus system). The maximum spur length
for any FISCO installation is 60 m per spur. If these length restrictions are
adhered to, the IEC 60079-27 standard for FISCO permits the cable parameter calculations (normally associated with IS circuits) to be omitted. Longer spurs of up to 120 m can be used where required by carrying out the
cable parameter calculations for the entire segment.
Figure 3-1 shows how the ignition curve (point above which the combination of energy, oxygen and fuel will result in an explosion) changes for various forms of electrical protection and Gas Groups. This figure demonstrates
why the energy available to FISCO and FNICO power conditioners is a
function of the gas group in which the field devices will be installed.
74
FIELDBUS POWER SUPPLIES
Short Circuit Protection Current (mA)
Figure 3-1 — Protective systems incendive limits
500
400
300
IIB non-incendive
200
IIB intrinsically safe
IIC non-incendive
IIC intrinsically safe
100
12
14
16
18
Open Circuit Voltage (V)
The intrinsic safety of Fieldbus installations based on the Physical Layer
standard IEC 61158–2 requires special consideration. Because of the limited DC power to be shared by a number of field devices, long cable runs,
and terminators storing capacitive energy, the traditional intrinsic safety
installation and interconnection rules restrict the application of such systems. FISCO allows the interconnection of an intrinsically safe apparatus
to an associated apparatus not specifically examined in such combinations.
The criterion for such interconnection is that the voltage (Vi), the current
(Ii), and the power (Pi) that the intrinsically safe apparatus can receive and
remain intrinsically safe, considering faults, must be equal to or greater than
the voltage (Vo), the current (Io), and the power (Po) that can be delivered
by the associated apparatus (supply unit). In addition, the maximum unprotected residual capacitance (Ci) and inductance (Li) of each apparatus (other
than the terminators) connected to the Fieldbus must be not greater than 5
nF and 10 µH, respectively.
In each IS Fieldbus segment, only one active device, normally the associated apparatus, is allowed to provide the necessary power for the Fieldbus
system. The allowed voltage (Vo) of the associated apparatus used to supply
the bus is limited to a range of 14 VDC to 24 VDC.
FIELDBUS POWER SUPPLIES
75
All the equipment connected to a FISCO network must comply with the
constraints of FISCO design by not introducing energy into the system
greater than a leakage current of 50 µA for each connected device. Compliance with this directive is obtained by manufacturers having their devices
FISCO certified in much the same way as they have them IS certified. Fortunately, this requirement is not a constraint as most manufacturers and
hence practically all field devices are both IS and FISCO certified. In the
event that a field device is not FISCO certified but only IS certified, there
exist on the market devices similar to a terminal block to make IS devices
FISCO compliant.
Figure 3-2 — Typical FISCO network
With 16 mA of current consumption
of each FISCO device and an IIC system
FISCO
Cable A
Junction
Box
Barrier
FISCO
Barrier
FISCO
Cable A
Junction
Box
FISCO Devices
(Max. 6 devices)
FISCO Devices
(Max. 6 devices)
Cable A
Junction
Box
Barrier
76
FIELDBUS POWER SUPPLIES
Table 3-5 summarizes the FISCO parameter range as defined in the Physikalisch-Technische Bundesanstalt (PTB) Report PTB W39 and in IEC
60079.
Table 3-5 — FISCO parameters
Area Classification/Level of protection
Supply unit
US (IS maximum value)
Eex ia IIC
Eex ib IIC / IIB
Trapezoidal output
characteristic
Approximately rectangular
output characteristic
14–24 V
Uo ≥ 2 × US
14–24 V
Ik
According to PTB report W-39
Cable parameters/km
R'
15–150 Ω
L'
0.4–1 mH
C'
80–200 nF (incl. screen, if existing)
C' = C'conductor/conductor + 0.5 C'conductor/screen
if the bus circuit is potential-free (balanced)
C' = C'conductor/conductor + C'conductor/screen
if the screen is connected with one pole of the supply unit.
Maximum cable length if
there are no observable
safety-related
restrictions.
1000 m
1900 m
Terminator RC elements
R
90–100 Ω
C
90–100 μF
Note: One terminator at each end of the trunk cable is required. According to EN
50020, the resistor must be infallible.
FIELDBUS POWER SUPPLIES
77
Table 3-6 — Characteristics of FISCO and FNICL networks
Type of Power Supply
FISCO
Apparatus Group
Safety Description
FISCO Ex ic
IIC
IIB
IIC
IIB
Voltage
V
14.0
14.8
14.0
14.8
Current
mA
180
359
233
380
Voltage
V
12.4
13.1
12.4
13.1
Current
mA
120
265
180
320
7
16
12
21
Maximum length of trunk for 7 devices.
Assume 50 Ω/km and
15 mA/device.
552 m
1800 ft
742 m
2430 ft
552 m
1800 ft
742 m
2430 ft
Maximum trunk length for maximum number of devices at 44 Ω/km and 15 mA/
device.
627 m
2050 ft
369 m
1210 ft
454 m
1490 ft
259 m
850 ft
Usable Output
Maximum number of field devices.
Assumes 15 mA per device.
Group IIC = North America Gas Group A, B
Group IIB = North America Gas Group C, D
Table 3-6 note: The FNICO standard has now been merged into the FISCO standard
as FISCO ic, although previously installed FNICO installations will be “grandfathered”
to the new FISCO ic standard. See 3.3 below.
As with any design, one of the key considerations in good design is risk
management. Since Fieldbus places multiple devices on a single, highly reliable segment, there is always the small risk that the segment and all the
devices on it could fail. It is for this reason that many systems use redundant power supplies, and of course, as long as there is power to the field
devices and a LinkMaster device on the segment, Fieldbus is designed to
continue to operate at the last setpoint until communications with the Host
are reestablished.
3.2.1 Architecture with FISCO installed in the DCS
cabinet
As can be seen in Figure 3-2, FISCO power conditioners are typically
installed in a cabinet in the safe area and the home run cable connection
runs to the classified area. As shown in Figure 3-1 and the restriction on
energy to remain below the ignition curve, the limitations of energy to the
field result in a maximum physical distance with Type A fieldbus cable of
78
FIELDBUS POWER SUPPLIES
552 m (IIC) and 690 m (IIB). However, should the user choose to install a
larger diameter cable, within the resistance, inductance and capacitance
constraints shown in Table 3-5, longer overall distances as constrained by
the voltage loss due to the cable resistance as calculated by Ohm’s law may
be used.
It should be noted that because FISCO power conditioners are also repeaters, if desired, it is possible to install up to four FISCO power conditioners
in a single Fieldbus network, thus making it possible to achieve a total overall length of over 2,700 meters or 8,400 feet. Each of the repeater power
conditioners requires a 24 VDC bulk power supply and is restricted to
being mounted in Zone 2 (Class 1 Division 2) areas or other area classification constraints as specified by the power conditioner manufacturer.
Figure 3-3 shows a diagrammed example of the calculations summarized in
Table 3-6 of the voltage drop with the FISCO power conditioner located in
the control room cabinet to a Fieldbus junction box with spurs for
instruments in a IIC classified area. Two cases are shown to demonstrate the
effect on the available overall trunk length of using cable with lower
resistance.
IIC
Figure 3-3 — FISCO calculation for area classification IIC
FISCO IIC
Rtrunk = Us – UJB = 12.4 V – 9.5 V = 27.6 ȍ
ȈI
Rcable = 50 ȍ/km
12.4 V
120 mA
105 mA
Case 1 – 50 ȍ/km
552 m
50 ȍ = 1 km
27.6 ȍ = D km
Case 2 – 44 ȍ/km
627 m
D = Rtrunk = 27.6 = 552 m
Rcable
50
9.5 V
Rcable = 44 ȍ/km
Field
JB
D = Rtrunk = 27.6 = 627 m
Spurs
44
15
m
A
I=
15
I=
m
A
15
m
A
A
FIELDBUS POWER SUPPLIES
m
m
15
I=
15
A
A
I=
m
15
I = 15 mA
I=
I=
Rcable
79
Figure 3-4 is an example of the calculation of the voltage drop using a control room mounted FISCO power conditioner to the Fieldbus junction box
for instruments located in a IIB classified area.
g
IIB
Figure 3-4 — FISCO calculation for area classification IIB
FISCO IIB
Rtrunk = Us – UJB = 13.1 V – 9.5 V = 37.1 ȍ
ȈI
Rcable = 50 ȍ/km
13.1 V
265 mA
105 mA
7 devices
Case 1 – 50 ȍ/km Case 3 – 44 ȍ/km,
742 m
240 mA
Case 2 – 44 ȍ/km 369 m
843 m
50 ȍ = 1 km
37.1 ȍ = D km
D = Rtrunk = 37.1 = 742 m
Rcable
50
9.5 V
Rcable = 44 ȍ/km
D = Rtrunk = 37.1 = 843 m
Rcable = 44 ȍ/km Ȉ I = 240 mA
16 devices
Spurs
D = Rtrunk = 16.25 = 369 m
44
Rcable
15
m
m
m
15
15
I=
I=
15
44
I=
m
A
15
m
A
A
I=
A
A
A
I = 15 mA
I
5m
=1
I=
Rcable
Field
JB
Because FISCO power conditioners can be used as repeaters, it is also possible to install up to four units in parallel and connect them to a single H1
Host port as per Fieldbus Foundation AG-163 and as shown in Figure 3-5.
In applications where the number of field devices per segment is limited
due to intrinsic safety constraints (such as in IIC/Groups A, B, where the
output voltage and current of the FISCO power supply are more limited),
combining several segments can take full advantage of the logical capacity
of the Host system’s H1 interface card. This yields hardware cost savings in
the host system. For example, if four field devices are supported on each of
three multi-dropped hazardous area trunks, then the Host H1 card sees 12
devices on a single port. One benefit of this configuration is that, since
other supplies on the same segment will continue to operate, the scope of
loss is reduced in the event of the failure of any single non-redundant
FISCO power conditioner.
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FIELDBUS POWER SUPPLIES
Figure 3-5 — FISCO repeater wiring to field
Figure 3-5 - FISCO repeater wiring to field
H1 Host Card
24Vcc
24Vdc
Multiconductor
cable to Field
JB
JB
3.2.2 Redundant FISCO
The IEC 60079 standards specify that only one FISCO power conditioner
may be connected to a segment at any one time; however, with the evolution of technology, it is now possible, through the use of monitoring and
switching electronics, to guarantee this condition without loss of H1 communications. The result is the availability of redundant FISCO solutions
and the elimination of the risk of network failure on the loss of a single electronic network component, while allowing for live working on the full
Fieldbus network.
3.3 Fieldbus Non-Incendive Concept (FNICO/
FISCO Ex ic)
FNICO was a derivative of FISCO, specifically intended for Fieldbus
installations in Zone 2 and Division 2 hazardous areas; however, the fifth
edition of the IEC 60079-11 standard changed the classification of non-
FIELDBUS POWER SUPPLIES
81
incendive FNICO to FISCO standard Ex ic. As a result of these changes
the new Ex nL classifications have been changed as follows:
The former Ex nL classification IEC 60079-15 (FNICO) for installation in
Zone 2 doesn’t exist anymore, and IEC 60079-11 (FISCO) introduces a new
classification Ex ic.
As a result, projects are now designed to the following “Ex i_” classifications:
•
Ex ia for work in Zone 0, 1 and 2
•
Ex ib for work in Zone 1 and 2
•
Ex ic for work in Zone 2 only
Because of the reduced risk of a hazardous gas being present, FISCO ic
takes advantage of the relaxed design requirements of a lower safety margin
for non-incendive (energy-limited) circuits compared with those for intrinsic safety. FISCO ic enjoys the same benefits as FISCO designs in general
in terms of simple safety documentation and the elimination of cable
parameter calculations, while retaining the ability to connect and disconnect the field wiring in the hazardous area under power and without “gas
clearance” procedures.
FISCO ic has the following additional benefits compared with “traditional”
FISCO Ex ia and Ex ib installations:
•
Higher levels of bus current, allowing more devices to be connected to
the hazardous area trunk.
•
Easier selection of approved field devices. Suitable devices include EEx
nL, non-incendive, IS (Entity) and IS (FISCO).
•
Installation rules for non-incendive wiring are less onerous than those
for intrinsic safety.
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FIELDBUS POWER SUPPLIES
3.4 High Energy Trunk – Fieldbus Barrier
In its simplest form, the Fieldbus Barrier Architecture is simply the FISCO
concept installed in the field. The Fieldbus Power Conditioner circuitry of
inductor and isolator is installed in one location while the FISCO circuitry
is mounted in an enclosure in the field, with the intervening cable operating
at a higher energy level and therefore Ex “e” rated. Figure 3-6 shows diagrammatically how the signal conditioning and safety circuitry of both
FISCO and Fieldbus Barrier are similar.
Figure 3-6 — Typical fieldbus power conditioner
Non-classified signal
5 mH
50 ɏ
IS, FISCO signal
Bulk
Power
Supply
H1 Fieldbus
Conditioner
H1 Fieldbus
Isolation /
Transformer
IS/FISCO Power Conditioner
Power
Conditioner
Fieldbus
Barrier
With the High Energy Trunk concept and the Fieldbus Power Conditioner
able to operate at high voltage and current while in the safe area (normally
the control room cabinet), the home run cable connection from the cabinet
to the dangerous area up to the Fieldbus barrier in Zone 1 is due to the high
energy levels on the trunk not live workable. For this reason the Ex “e” portion of each Fieldbus barrier must be kept physically isolated and separate
from the intrinsically safe side of the installation. This is typically done
FIELDBUS POWER SUPPLIES
83
using a plastic/polycarbonate shield over the high energy cables as well as a
small shelf/barrier between the sets of terminals.
Because of the high voltage available, it is possible to install a Fieldbus network to the maximum overall cable length of 1900 m.
Tip 13 — Some field devices in compliance with EN standards
have a maximum input voltage of 24 V. Care must be taken when
using high-voltage power conditioners that this voltage level is
not exceeded.
Manufacturers have launched various models of Fieldbus barriers, and most
of these barriers can be installed in Zone 1. The FISCO equivalent spurs
from the barriers can connect to devices with spurs of up to 120 m. Figure
3-7 shows how Fieldbus barriers are typically installed with the possibility of
mounting the enclosures in Zone 1 and the spurs suitable for connection to
field devices such as valves and transmitters in Zone 0.
g
Figure
yp3-7 — Typical fieldbus barrier installation
FF Power Conditioner
28 V
500 mA
Ex ‘e’
Zone 1
Zone 0
Ex ‘i’
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FIELDBUS POWER SUPPLIES
Typical installations can have up to four Fieldbus barriers on a single segment, and most Fieldbus barriers have four spurs per barrier, though there
are also eight spur Fieldbus barriers on the market.
Figure 3-8 shows an example of a typical Fieldbus barrier calculation. There
is a voltage drop across the Fieldbus barrier to accommodate the need for
Intrinsic Safety/FISCO, though the instruments can be installed in a Zone
0 Area Classification.
Figure 3-8 — High-energy trunk calculation
Figure 3-8 - High energy trunk calculation
FISCO IIB
Rtrunk = Us – UJB = 28 V – 16.5 V = 63.9 ȍ
ȈI
Rcable = 50 ȍ/km
28 V
500 mA
180 mA
12 devices
Case 1 – 50 ȍ/km
1278 m
50 ȍ = 1 km
63.9 ȍ = D km
D = Rtrunk = 63.9 = 1278 m
Rcable
50
16.5 V
I = 15 mA
10.5 V
Fieldbus
Barriers
I = 15 mA
I = 15
5 mA
I=1
15
m
Spurs
I=
15
I=
m
A
I=
15 m
A
15
mA
A
I=
15
A
m
15
I=
m
A
A
I=
m
15
I = 15 mA
I=
mA
3.5 DART (Dynamic Arc Recognition and
Termination)
DART is the next generation development from PTB and is a way to obtain
higher energy and hence allow larger loads at higher voltage than is available via FISCO ic while still making live working anywhere on the segment
possible.
FIELDBUS POWER SUPPLIES
85
A DART system detects a rapid change in the current that would occur if a
hazardous area circuit is shorted or open-circuited. The source of power is
then shut down within a few microseconds, before the energy in the spark
becomes high enough to cause ignition. If the fault is momentary, the
power is then quickly re-applied, before the operation of the field circuit is
affected.
When a spark is about to occur, the voltage increases and the circuit (relatively) gradually heats up. The energy level remains non-incendive during
its initial phase, only reaching incendive temperatures during the critical
phase. Figure 3-9 shows the timing of current (IF), voltage (UF) and power
(PF) of a typical break spark. Note the characteristic current change (di/dt)
in the diagram when the circuit is opened, as this is the indicator of an
incipient or potential spark. Power now escapes the electric circuit, and
power over time is energy.
U, I, P
Figure 3-9 — Typical spark behavior
Initial
Phase
Critical Phase
IF
di/dt
UF
PF
Spark duration tF : 5 ʅs to 2 ms
t
DART detects the current change di/dt and extinguishes the spark before
the initial phase is over. The energy is effectively limited, and an incendive
spark is prevented.
Figure 3-10 shows the timing of a break spark interrupted by a DART power
supply.
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FIELDBUS POWER SUPPLIES
U, I, P
Figure 3-10 — DART extinguished spark
Initial
Phase
Critical Phase
IF
UF
PF
t
Spark duration with DART: tF чϱʅ
In circuits with long cable lengths, such as in Fieldbus applications, DART
has to take into account the time required for the change in current caused
by an intermittent fault to travel along the cable. Given that the interruption travels at approximately half the speed of light, the longer the cable,
the shorter the available time for the monitoring circuitry to detect the
incipient spark. As a result there is a relationship between available power
and cable length for DART. DART is based on a trunk cable of up to
1000 m with spur lengths to 120 m and uses the same topology as a general
purpose high-power trunk, with the field devices similar in form factor to a
passive device coupler that can be mounted in a Zone 1 gas environment.
The output power to the trunk of a DART power supply is 22 V and 360
mA, and the spur output power is a minimum of 10.5 V at 34 mA.
3.6 Selecting the Right Power Supply
Selection of the correct power supply requires that you have the following
information on hand:
•
Area Classification – The area classification and method of protection
determine the type of power supply you will be able to use and still
ensure a safe installation. The power supply selected (general purpose,
FIELDBUS POWER SUPPLIES
87
FISCO, IS) will determine the maximum current and voltage available
on the segment and hence the maximum number of devices that can be
connected to a single power supply.
•
Layout – The number of devices on a single segment determines the
maximum current requirements for the segment. The decision on
redundancy impacts the space requirements and which power supplies
can provide redundancy. Live working also impacts the choices, as
Explosion Proof and High Energy Trunk cannot be worked on without
gas testing and associated additional safety requirements over Intrinsically Safe, and FISCO.
•
Total number of devices per segment – The total number of devices to be
installed in the plant, divided by the number of devices per segment as
verified by the power supply selection, determines the number of segments and corresponding H1 port cards required.
•
Plot plan – This determines the maximum segment length, which has an
impact on the required voltage level at the Fieldbus Power Conditioner
to allow for the voltage drop in the system under load.
The following pieces of information are then used to complete the segment
design:
•
Closed loop control – If you are planning to implement control in the
field, it will be necessary to have the input and output devices on the
same segment. This may result in having to run the spur or home run
cable in a different location than originally thought.
•
Loop critical level – Some facilities develop rules and guidelines based on
risk management principles to limit the number of a certain type of
device on a single segment. This can also impact network design by
requiring the reallocation of devices from one segment to another or a
change in the routing of the field cable.
Figure 3-11 is a flow chart that will assist in selecting the correct power supply, based on the area classification and redundancy requirements of the
project.
To confirm the selection process illustrated above, let’s take a look at what
might be considered a reasonable current load for a Fieldbus system. As a
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FIELDBUS POWER SUPPLIES
Figure 3-11 — Power supply selection
Hazardous
Location?
Intrinsic Safe
No
Explosion
Proof?
Yes
No
Single G.P
Supply
Yes
Complete
Live Working
System?
Intrinsically
Safe?
No
No
No
No
Zone 2?
Redundant?
Fieldbus
Barrier with
H. P Supply
FISCO ic
FISCO
Yes
Redundant?
Yes
Yes
Yes
No
Redundant?
Yes
Yes
Long Trunk
or High mA
Load?
No
No
(formerly FNICO)
High Power
Supply
DART
Redundant
FISCO
rule, when designing a Fieldbus system, use a conservative value of 20 mA
for each device. Most installations do not use more than 12 devices on a
single network, not because of physical constraints, but rather to manage
the risks of losing so many signals in the event of a single point of failure.
Therefore, the maximum current load on a network will be on the order of
240 mA, plus additional capacity for the connection of handheld diagnostic
tools (10 mA) and approximately a 50 mA short circuit protection load. The
short circuit protection load is based on the following: 60 mA is the highest
load introduced to the network by the short circuit protection circuitry and
13 mA is the power draw of the field device on the segment with the lowest
energy requirements, so (60-13) or approximately 50 mA is the load in the
event of a short circuit. The maximum current required in a fully loaded
segment of 12 devices at 20 mA/device + (60-13 mA) is 300 mA (12 × 20 +
60). A reasonable maximum current demand for most systems is therefore
around 300 mA.
FIELDBUS POWER SUPPLIES
89
The other half of the power calculation is voltage, for which a minimum of
9 V is required at any point in the network. The worst case scenario is one
in which the maximum length of 1,900 meters of cable is installed. Fieldbus
Type A cable has a nominal resistance of 50 W/km, so to keep the calculation simple, we can assume an overall system resistance of 100 W.
Using Ohm’s law that V = IR, we can calculate the worst-case voltage drop:
ΔV= 0.3 A × 100 = 3 Volts
The main reason to specify a power conditioner with more than the above
voltage output is either to ensure more voltage at the end device, or to have
a larger margin for the voltage drop due to cable resistance. Cable resistance
will change with temperature, cable resistivity (a function of manufacturing
and cable diameter) and length.
A complete example calculation using the sizing equations from earlier in
this chapter and the above criteria can be found in Appendix E.
Because Fieldbus barrier systems include transformers and a number of
components to convert the high energy trunk to the low energy of the
Intrinsically Safe [IS] spurs, the barriers themselves consume some power
and also have an associated voltage drop.
It is these Fieldbus barriers that drive the need for larger power conditioners. Each Fieldbus barrier consumes a maximum of approximately 250 mA
per unit (depending on the manufacturer), supporting a maximum of four
IS spurs and devices. Again, to refer to our 12-device network, the maximum sized power conditioner should not exceed 750 mA. This is approximately 2.5 times what is required when installing a more traditional
Fieldbus system.
If you purchase the largest power conditioner as the “easy way out,” you as
an engineer or designer are not doing your job. Doing this results in an
overspecified system that will not be the most economic choice initially or
over the facility’s lifecycle. Remember that the power conditioner is part of
the system and must therefore match the demands that it will experience
once installed.
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FIELDBUS POWER SUPPLIES
4 — Documentation
To be able to actually build any control system requires documentation:
telling others what is wanted, how to connect it, how to size it, and how to
represent this information on drawings. Chapter 4 contains samples of how
to do this, though not how to use the forms or worked examples. Worked
examples of a Fieldbus design are included in Appendix E.
The traditional loop diagram in which a typically single loop (input – controller – output) is shown so that a single drawing shows the relationships
between these various devices, for Fieldbus projects must be expanded to
show all the field devices on the segment. The loop diagram is therefore
replaced with a network or segment diagram. To be sure that maintenance
workers are aware of how they could be affecting operation of the segment
when working on a field device, field devices containing a control algorithm
shall be shown on the appropriate network drawing. Figure 4-1 shows a
sample network drawing, including a field-based repeater and intrinsic safe
barrier.
Figure 4-1 — Network diagram with repeater
Notes:
1. Back-up LAS device marked
with a “B.”
Field
Field Junction
Box
23V
LT-5
+
-
23 V
Terminator
Block
+
-
“Marshalling”
And
Power Conditioning
Host I/O
Pair 1 of 24
IS
Barrier
+
-
+
+
-
T
TT-6
23V
+
-
PT-4
23V
+
-
+
-
+
-
+
-
FF Power
Supply
2. PID control device indicated by
a “P.”
3. Master LAS device marked
with “M” (if not in the Host).
25 V
4. PD control device shown by a “D.”
5. Bias control device indicated by
an “A.”
+
+
-
Ref Dwg
+
-
TT-4
Description
P&ID
Location Drawing
Cable Schedule
23V
+
-
+
-
+
-
B
LCV-5
P
23V
+
-
PD-7
24V
25 V
9V
+
-
Rev. Date
Description By Chk App
Repeater
To DC
Power
Supply
Network Connection Diagram
Drawing Number
Sheet
Rev.
CADD File:
DOCUMENTATION
91
A Fieldbus device differs from a conventional instrument only in the way it
communicates with the remainder of the system; therefore, the sensor specification will not change, and it will be possible to continue using the ISA20-1981 – Specification Forms for Process Measurement and Control
Instruments, Primary Elements, and Control Valves, for this purpose. What
will be different is the addition of a second page to capture the information
needed for Fieldbus network design.
The key information to be derived from the Fieldbus data sheet used to purchase a device includes:
•
Function Blocks in the device and their associated execution time – this
information is used for configuring and scheduling.
•
Current requirements – needed for network design.
•
Device capacitance – needed for network design.
•
Network assignment – used for troubleshooting and maintenance so
that once the data sheet is located, all other associated documentation
can be found.
•
Software/DD (Device Description) revision number – used for commissioning and maintenance.
•
LinkMaster or Basic device – necessary to fully support control-in-field”
capabilities of Fieldbus.
A complete list of all Fieldbus-certified devices is always available and is
maintained by the Fieldbus Foundation at www.fieldbus.org.
Figures 4-2 and 4-3 show two different ways in which the information
needed to specify the Fieldbus aspect of a device may be captured on a data
sheet. Figure 4-2 assumes a single device per page, while Figure 4-3 is more
generic and is intended for multiple devices per page.
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DOCUMENTATION
Figure 4-2 — Fieldbus data sheet: individual device
Instrument Tag Number:
Basic Fieldbus Function Blocks
Segment Information
 Analog Input (AI)
___Number
___Execution Time (msec)
 Arithmetic (A)
___Execution Time (msec)
 Digital Alarm (DA)
___Execution Time (msec)
 Discrete Input (DI)
___Number
___Execution Time (msec)
 Calculate (C)
___Execution Time (msec)
 Analog Alarm (AA)
___Execution Time (msec)
 Bias/Gain Station (BG)
___Execution Time (msec)
 Deadtime (D)
___Execution Time (msec)
 Manual Loader (ML)
___Execution Time (msec)
 Complex Analog Output (CAO)
___Execution Time (msec)
 Proportional/Integral/
Derivative (PID)
___Execution Time (msec)
 Step Output PID (SOPID)
___Execution Time (msec)
 Analog Output (AO)
___Number
___Execution Time (msec)
 Set Point Ramp Generator (SRG)
___Execution Time (msec)
 Discrete Output (DO)
___Number
___Execution Time (msec)
 Signal Characterizer (SC)
___Execution Time (msec)
 Control Selector (CS)
___Execution Time (msec)
 Digital Human Interface (DHI)
___Execution Time (msec)
 Proportional/Derivative (PD)
___Execution Time (msec)
 ______________________
___Execution Time (msec)
 Ratio (R)
___Execution Time (msec)
Device:
Advanced Function Blocks
Segment #:
 Pulse Input (PI)
___Execution Time (msec)
LAS Capable: YES  NO 
 Input Selector (IS)
___Execution Time (msec)
Device current draw (mA):
Device In-rush Current (mA):
 Input Selector (IS)
___Execution Time (msec)
Device Lift-off (Minimum)
Voltage:
Device capacitance:
 Integrator (I)
___Execution Time (msec)
Polarity Sensitive:
YES  NO 
 Input Selector (IS)
___Execution Time (msec)
Segment terminator location:
VCR’s:
 Input Selector (IS)
___Execution Time (msec)
DD Revision:
 Input Selector (IS)
___Execution Time (msec)
CFF Revision:
ITK Revision that Device was tested with:
 Signal Splitter (SS)
___Execution Time (msec)
Notes
Vendor to enter here all
Non-Standard or Enhanced function block data:
 Timer (T)
___Execution Time (msec)
Vendor to enter here all unique Vendor Diagnostic/
Advance Diagnostics capabilities:
DOCUMENTATION
93
Figure 4-3 — Fieldbus data sheet for multiple devices
2
3
4
5
6
7
8
9
1
0
1
1
1
2
TAG NUMBER
1
Number of AI’s
AI Execution Time (msec)
Number of AO’s
AO Execution Time (msec)
Number of SS’s
SS Execution Time (msec)
Number of TOT’s
TOT Execution Time (msec)
Number of AR’s
AR Execution Time (msec)
Number of PID’s
PID Execution Time (msec)
------------------------Number of _____
Execution Time
Number of _____
Execution Time
Number of _____
Execution Time
Number of _____
Execution Time
Number of _____
Execution Time
Number of _____
Execution Time
94
DOCUMENTATION
Figure 4-3 — continued
Number of _____
Execution Time
Number of _____
Execution Time
------------------------Channel
I.S. Segment
(If Applicable)
LAS Capable
(Yes / No)
DD Revision
ITK Revision
Polarity Sensitive (Yes / No)
CFF Revision
Notes:
For each project, it is necessary to consider whether it is necessary to show
Fieldbus device connections differently than signal connections for conventional instrumentation on piping and instrumentation diagrams (P&IDs).
The basis for the decision is whether the people using the P&ID are concerned with how the signals are interconnected other than as a physical wire
in the field or a software connection in a microprocessor, beyond needing
to understand how the various input signals (PV) and output signals (MV)
relate to each other and to the process.
Figure 4-4 shows the symbology recommended in ISA-5.1 to differentiate
Fieldbus communications from other forms of serial communications.
DOCUMENTATION
95
Figure 4-4 — Digital communication signal symbols
Electrical lead
Bus serial link
Internal DCS serial/Software
communications link
Fieldbus serial link
The other potential change on P&IDs as a result of Fieldbus is how to represent a multivariable transmitter. The following is one suggested and commonly used way of representing multivariable transmitters on the P&ID.
The different signals/ Function Blocks are represented as individual “bubbles” within a single rectangle. The rectangle represents the single enclosure
(physical instrument) while the circles/bubbles represent the individual
functionalities in the device. Figure 4-5 shows two examples of how this
information might be presented.
Figure 4-5 — Multivariable Device Representation on P&ID
PT
101
FIT
101
PT
101
FIT
101
FC
101
The first example is an indicating differential flow transmitter that also
transmits the bulk line pressure via a second Analog Input (AI) Function
Block. The second example shows the same transmitter with a PID Function Block as well.
96
DOCUMENTATION
Tip 14 – It is generally not recommended to place a control
Function Block in a multivariable transmitter, simply because
you are placing many functions in a single device and thus
increasing both the load on the device’s microprocessor and the
risk, should this single device fail.
As far as possible, when multifunction transmitters are used, the transmitter
number should be the same, with the function of the block (second AI
block or control/PID block) being shown by the letters.
In the example above the flow (FIT) and bulk pressure (PT) have the same
instrument number.
A number of tables are shown on the following pages to help with the
design and installation of a Fieldbus system.
Table 4-1 — System decision analysis
Issue
Concern
Impact
Weight
Weight
Frequency
Weight
Combined
Weight
Comment
Weight
1. Control in Field
Sum
2. Control in Host
Sum
3. Multivariable
Transmitters
Sum
4. Etc.
Sum
Other factors that may be worth considering include: location of power
supply, Macrocycle rate, etc. The weighting factors above are similar to
DOCUMENTATION
97
those used in a Risk Analysis process, and the weightings should be determined for each site based on their level of willingness to accept risk. Consistent with the practices used in Risk Analysis the ‘Impact” and “Frequency”
analysis should be done independently of each other and then the overall
weighting summed to provide a combined weight. Using a tool such as this
will provide the project with a documented basis for selecting the basis for
their fieldbus design decisions.
The Network Decision Analysis (Table 4-2) is a two-stage process in that the
team must agree on a Criticality Matrix and “hurdle rate” or level of risk
that is acceptable for the analysis proper. Some facilities may find a financial risk of $100K to be high while other larger facilities may find this same
value as low; hence the “hurdle rate” will vary from company to company.
Table 4-3 illustrates how the Criticality Matrix may appear.
Table 4-2 — Network decision analysis
Criticality
Weighting
System Trip (Criticality 1 valve or similar)
10
Criticality 2 Final Control Element
Criticality 3 Final Control Element
Final Control Element with Control in Device
Final Control Element without Control in Device
Multivariable device, part of control loop, with LAS capability
Multivariable device, part of control loop, without LAS
capability
Multivariable device, view only, with LAS capability
Multivariable device, view only, without LAS capability
Single variable device, part of control loop, with LAS
capability
Single variable device, part of control loop, without LAS
capability
Single variable device, view only, with LAS capability
Single variable device, view only, without LAS capability
0
Weighting is assigned on the basis that 10 is most critical and 0 has no impact. No
individual loop shall have an overall criticality rating of greater than x* without specification deviation from the project owner/operator. (* x is to be determined by the project
owner/operator.)
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DOCUMENTATION
Table 4-3 — Device criticality decision matrix
Device Criticality
Tag
Level
Function Current
Criticality Loop
Consumption
Weighting Number Block
Execution (mA)
Sum
Inrush
Current
(mA)
Time
The above table also captures the critical information required for design of the balance of the Fieldbus network on the right-hand side of the double border.
Table 4-4 recommends some items that should enter into the discussion on
the type of connections and features to be specified in their purchase of
field device couplers. As with the above table 4.1, the impact and frequency
of the risks associated with using one type of connection or the other are
analyzed, and then the selection should be made to minimize the overall
risk to the project.
Table 4-4 — Connector decision analysis
Issue
Concern
Impact
Weight
Frequency
Combined Comment
Weight
Weight
1. Screw Terminals
Sum
2. Quick Connections
Sum
3. Spur short circuit
protectors
Sum
4. Etc.
Sum
The project site may already have a standard termination option they prefer. If not, this
table and process, which are similar to those of the System Decision Analysis table,
can be used to make an informed decision.
DOCUMENTATION
99
4.1 Segment Loading Calculation
There are two different forms of loading calculation: one for projects using
the FISCO installation method (Table 4-5) and another for an IS or NIS
installation (Table 4-6).
Table 4-5 — FISCO installation
Trunk
Spur Cable
Okay
Okay
R’
R’
L’
L’
C’
C’
Lmax
Isup
Power Supply
Category
VPS
Uo
IPS (Amps)
Ik (Amps)
Power Supply Location Host or Field
JB
Pmax (Watts)
Spur protection short
circuit load (mA)
Output
Characteristic
Device
Tag
100
CD
ID (mA)
Trapezoid,
Rectangular
Segment
ΣID
Okay
ΣCT
DOCUMENTATION
Table 4-6 — IS/NIS installation
Trunk
R
C
L
Voltage
Drop:
Spur Cable
Ω/km R
nF/km C
Meters
Volts
LTMAX
Ω/km
nF/km
V I(mA) C (nF) Spur
(m)
VPS
IPS (mA)
Power Supply Location Host or Field JB
Spur protection short
circuit load (mA)
meters
Device
Tag
Power Supply
Vd
Check
Segment
Vd>Vmin CS
CT
IN
A
Table 4-7 is a partial worksheet/checklist that can be used for configuring a
Fieldbus system. Depending on the nature of the installation, additional
items may need to be added to this list, which is intended to provide an
indication of the types of items requiring consideration.
Table 4-7 — Configuration worksheet/checklist
Parameter
Device
Complete
Initials
Tag match
DD Revision
Transducer Block Mode
Resource Block Mode
Function Blocks and mode
Network
Cyclic/Acyclic time ratio
Schedule published/downloaded
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101
5 —System Integration
Now that everything is built, it is time to configure the system so that all the
parts work together. Chapter 5 discusses configuration and scheduling, or
who “talks” directly to whom, and when.
When a message is transferred, it goes down through a channel called Virtual Communication Relationship (VCR) to a Physical Device (PD) before
it goes to the wire. At the destination, it goes up through the partner VCR
to the receiving application. Process-control information packets are
appended and removed when a message goes through VCRs to allow layers
to perform their specific function.
In effect the VCR creates a virtual map in the memory of the two devices of
the parameters and their associated locations in the memory of each device.
This mapping makes it possible to link VCRs via parameter name rather
than via memory register offset or address, which will be different for each
device manufacturer. Much like how a dictionary provides definitions, the
Device Description (DD) file is used to create the map between the device
memory register or index and the parameter name by defining where these
various pointers should be linked.
Figure 5-1 represents how data is transmitted through the layers and across
the Data Link Layer via VCRs.
Before configuration is started, the following files will be required:
•
Device commissioning files – Device Description files *.sym and *.ffo
•
Network configuration file – Capabilities file *.cff
Capabilities file is a common text-format file that can be read and interpreted by a Host so that it can create the internal files and database links it
requires to use the Device Description files.
The Fieldbus Foundation Web site maintains the most recent version of all
these files for every certified device. It is important to manage the revisions
of these files and hence the software version of the devices themselves since
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103
Figure 5-1 — Fieldbus VCR communications
they are not always compatible between different revisions, especially if the
Host and field device are at different revision levels.
5.1 Configuration
Network Configuration is responsible for giving the correct information of
the index and Data Link (DL) address, as well as other operating information, to VCRs through Network Management.
There are different types of VCRs: Client-Server, Report (Sink-Source) Distribution, and Publisher-Subscriber.
5.1.1 Client-Server VCR Type
The Client-Server VCR type is used for queued, unscheduled, user-initiated,
and one-to-one communication between devices on the Fieldbus.
Queued means that messages are sent and received in the order submitted for
transmission, according to their priority, without overwriting previous messages.
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The Client-Server VCR type is used for operator-initiated requests, such as
setpoint changes, tuning parameter access and change, alarm acknowledge,
and device upload and download.
5.1.2 Report Distribution VCR Type
The Report Distribution VCR type is used for queued, unscheduled, or
user-initiated one-to-many communications.
Tip 15 — Fieldbus devices that send alarm notifications to the
operator consoles typically use the Report Distribution VCR
type.
5.1.3 Publisher–Subscriber VCR Type
The Publisher-Subscriber VCR type is used for buffered, one-to-many communications.
Buffered means that only the latest version of the data is maintained within
the network. New data completely overwrites previous data.
The Publisher-Subscriber VCR type is used by the field devices for cyclic,
scheduled publishing of User Application Function Block input and output
such as process variable (PV) and primary output (OUT) on the Fieldbus.
Table 5-1 summarizes the different VCR types and their uses.
Each device requires a minimum number of VCRs to communicate with
each other and with a Host. Below are suggested minimum guidelines to be
used when selecting devices on a segment.
Each device requires the following five basic Device Blocks:
1.
One Client-Server for each Management Information Base (MIB).
2.
One Client-Server for the primary Host.
3.
One Client-Server for the secondary Host or maintenance tool.
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Table 5-1 — VCR types and their uses
Client–Server VCR Report Distribution Publisher–Subscriber VCR
type
VCR type
type
Used for operator
messages
Used for event notifi- Used for publishing data
cation and trend
reports
Setpoint changes
Mode changes
Tuning changes
Upload/download
Alarm management
Access display
views
Remote diagnostics
Send process alarms Send transmitter PV to PID
to operator consoles. Control Block and operator
Send trend reports to console.
data historians.
4.
One Report Distribution for Alerts.
5.
One Report Distribution for Trends.
Tip 16 — Not all Hosts support trends and alerts, so these two
blocks may not be used, though they must be included in the calculations in case this capability, as specified in the Fieldbus documents, is added in the future.
Each Function Block requires the following:
•
One Publisher-Subscriber for each I/O.
•
A VCR (unless it is used internally).
Any input parameter may be linked to another parameter but only in the
same device, otherwise it will require the use of a VCR and associated I/O
channel on the network. The device DD file and VCR are what make it possible to link parameters between devices without the need to refer or map to
memory addresses manually.
Depending on the type of actuator being used, valve action can be set in
both the PID and AO Blocks, so there is a risk of inconsistency and confusion if “100%” sometimes means open and other times means closed.
Therefore, it is best to be consistent and implement a control strategy. For
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example, a PID output of 100% means a valve is fully open, independent of
the actuator being air-to-open or air-to-close. This can be done by configuring the AO Block so that the I/O option is “increase to close” for air-toclose actuators.
As illustrated in Table 5-4, Fieldbus has various levels of alarms, which
allows interlocks to be suited to the process. For example, for an important
shutdown interlock, action should be taken on either BAD or UNCERTAIN data quality. Lost communication is always indicated as BAD status,
with resulting loop shutdown.
The result is that to improve system safety, the status should not only be
displayed to the operator but should also be included as part of the system
interlock logic.
Internal Fieldbus and device diagnostics are far better at identifying errors
than identifying external discrepancy logic. For high availability, which
often contradicts high safety, the loop should be configured to shut down
only when the status is BAD; an UNCERTAIN status will only alert the
operator and, if appropriate, maintenance personnel.
Table 5-2, from Jonas Berge’s Fieldbus for Process Control, summarizes suggestions on how to configure a network for safety versus availability.
Tip 17 — A backup Link Active Scheduler (LAS) should be configured for all control loops and should normally reside in the
device with the minimal processing load, for example, a temperature transmitter. This is so the loop can continue to operate, in a controlled
manner, as long as it has power.
Systems should be configured for control bumpless “fail over” to the
backup LAS if control or communication is lost with the primary LAS,
which is often in the Host control system.
Where the transmitter and control valve in a loop cannot be conveniently
wired to the same segment, control should normally reside in the Host controller. Because the network communications must go through the Host
control, it is normally done there to minimize the demands on the field
devices.
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Table 5-2 — Configuring a network for safety vs. availability
Parameter
Safety
Availability
STATUS_OPTS
Don’t (i.e., as BAD)
Use UNCERTAIN as GOOD
STATUS_OPTS
Target to Manual if
BAD IN
Don’t (i.e., return to normal at
once)
STATUS_OPTS
Initiate fault state if
IN is BAD
Don’t (go to manual)
STATUS_OPTS
Initiate fault state if
CAS_IN is BAD
Don’t (go to automatic)
STATUS_OPTS
Set Target mode to
manual if IN is BAD
Don’t (return to normal once
okay)
STATUS_OPTS
Set quality as
UNCERTAIN if limited
Don’t (consider GOOD)
STATUS_OPTS
Set quality as BAD if Don’t (consider GOOD)
limited
STATUS_OPTS
Set quality as
Don’t (consider GOOD)
UNCERTAIN if block
is manual mode
IO_OPTS
Fault state to value
IO_OPTS
Use Fault State value Don’t (restart from present
on restart
position if available)
Function block link
state count limit
Few
Many
SHED_OPTS
No return
Normal return
FSAFE_TIME
Short
Long
FEATURE_SEL
Fault state supported Don’t (disabled)
Don’t (freeze output)
Backup LAS Function Disabled
Enabled
SHED_RCAS
Short
Long
SHED_ROUT
Short
Long
For cascade loops the primary measurement should reside on a separate H1
card from the secondary measurement since doing this reduces the potential for a common mode failure to affect loop control.
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Fieldbus devices provide support for trend function and alert blocks to
reduce segment traffic by transmitting multiple signals as one block.
A shorted segment or segment power supply failure should send valves to
their failure position, regardless of the device hosting the PID algorithm.
5.1.4 “Fail Over” Strategies and Design Considerations
The control narrative and project risk assessment(s) will determine how each
segment and device should operate under normal operating and failure conditions. With Fieldbus, it is possible to specify different failure strategies,
depending on the type of failure. For instance:
•
Loss of Communication
• With Host – continue operations with control in the field, fail
open, fail closed, or fail last position, meaning the output device
will remain at the same open/closed position as when the communications fail.
• With segment – fail open, fail closed, or fail last.
•
Loss of Power – fail open, fail closed, or fail last.
•
Bad AI Signal – fail open, fail closed, or fail last.
•
Cascade Failure – PID control only, fail open, fail closed, or fail last.
•
Loss of Motive Fluid (instrument air) – fail open, fail closed, or fail last.
Provided that power is still available on the network, automatic switchover
of the process automation network should not interrupt other system operations. FOUNDATIONTM Fieldbus is designed so that automatic switchover
from the process automation network to backup LAS device on the individual segment shall not interrupt other segment devices, such as the backup
PID or backup LAS.
Scenarios where the loop would go to its fail position could be a broken
segment, a shorted H1 segment, a failed H1 power supply, or a failed H1 filter-isolator.
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A system can be configured for control class blocks to use the status option
USE UNCERTAIN as GOOD so they will treat UNCERTAIN as either
GOOD or BAD. The default value is to treat UNCERTAIN as BAD.
The process variable filter time parameter, PV_FTIME, is used to set the
time constant (63% of steady-state value) in seconds of a first order lag. If
the time is set to zero seconds, the damping is disabled.
Figure 5-2 is indicative of the final result of a configuration screen on a typical Host.
Figure 5-2 — Host configuration screen
Table 5-3 shows the relative priority assigned to each of the Fieldbus operating modes. A device in Out Of Service (OOS) mode will cause the loop to
move to this state, even if the other components of the network remain in
automatic mode.
Fieldbus has 15 levels of alarms, as well as, depending on the device type,
several orders of magnitude, typically x 1000 more information than is
available with a conventional distributed control system or analog control
system. Configuration must not only handle and assign actions to each of
these 15 alarm levels, it must also route the various signals to the correct
output device or system. Table 5-4 summarizes the alarm levels of a Fieldbus system.
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Table 5-3 — Fieldbus operating mode priorities
Mode
Description
Setpoint source Output source Priority
OOS
Out of Service Operator (SP)
Operator (OUT) 7
IMan
Initialization
Manual
Operator (SP)
Lower Block
(BKCAL_IN)
6
LO
Local Override Operator (SP)
Other Block
(TRK_VAL)
5
Safe Value
(FSAFE_VAL)
Highest
Control
Class
Output
Class
Man
Manual
Operator (SP)
Operator (OUT) 4
Auto
Automatic
Operator (SP)
This block
3
Cas
Cascade
Higher Block
(CAS_IN)
This block
2
RCas
Remote
Cascade
Other Application This block
(RCAS_IN)
1
ROut
Remote Output Operator (SP)
Other
Application
(ROUT_IN)
0
Lowest
Priority
Table 5-4 — Fieldbus alarm levels
Alarm
FF Alert Levels
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Critical
Critical
Critical
Critical
Critical
Critical
Critical
Critical
Advisory
Advisory
Advisory
Advisory
Advisory
Low – fixed
No notification
No indication
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When devices or communications fail, the associated control loops cannot
continue to operate. Fieldbus has a “Fault State” function to monitor the
status of every loop. When the Fault State is activated, the output class
block goes into local override mode. However, for the Fault State function
to work, it must be enabled by using the “Fault State-supported option in
the Feature Selection (FEATURE_SEL) parameter in the Resource Block.
The Fault State function can be activated in one of four ways:
1.
Initiate Fault State on the cascade setpoint input.
2.
Cascade setpoint input communication watchdog times out.
3.
Power on.
4.
Forced from the Resource Block.
As with the devices, the HMI (Human Machine Interface) of a Fieldbus system will not be significantly different from the one the process operators
see today. The major enhancement that is possible with Fieldbus is that signal and loop status can be communicated to them in real time. As a result,
the panel operators will have more confidence in the control system since
they will always know how reliable any given signal is.
5.2 Scheduling
As with any network, the more information transmitted in a given time
frame, the faster the network and all its components must operate to keep
pace with this information. Since Fieldbus H1 is constrained to a rate of
31.25 Kbps, the only way to transmit additional information is to increase
the cycle time.
A link can carry about 30 scheduled messages per second. This means the
network could have three devices, each sending 10 messages per second, or
120 devices, connected by repeaters, each sending one message every four
seconds.
Tip 18 — A good rule to use for the initial estimate related to
bandwidth requirements is to assume each device requires 50 ms
to execute its Function Block.
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The total bandwidth required can then be estimated, using the following
formula:
t LOAD = ( N P + N C ) × 50 ms
where:
tLOAD = time to execute all Function Blocks in loop
NP
= number of Publishers (devices on the network)
= number of communications with the HMI
NC
Based on the experience from the many projects in the world and the fact
that a significant portion of the communications on an H1 network rely on
other than the Publisher-Subscriber communications used for closed-loop
control, the minimum unscheduled/acyclic time should be 70–80% for a
newly commissioned segment. Some Host system suppliers have this coded
into their systems as a default minimum of 60%, which cannot be lowered.
This allows room for future growth, if required.
For example, assume that a 1s macro LAS cycle (macrocycle) gives 150 ms
for scheduled data transmission, with 70% of the available 500 ms available
for acyclic communications. The time available for future use in this case
would be 350 ms.
Cyclic traffic time may be determined by the summation of the individual
Function Block execution times plus the publish time on the network.
Now, sum the run time of each device’s Function Blocks. For our example,
an AI Block runs in 50 ms, a PID Block in 150 ms, and an AO Block in 100
ms; add to this the required number of Compel Data or Publish commands
to determine the minimum macrocycle time.
Tip 19 — As a rule, each external link (through the Fieldbus channel) uses about 15 to 17 ms.
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For the loop shown in Figure 5-3 in which the PID and AO Blocks are
located at the valve’s positioner, the macrocycle is estimated as 325 ms.
g
p
g
Figure 5-3 — Loop configuration
PID Block
AI Block
AI Block
AO Block
If a second configuration identical to this one were on the same network,
the macrocycle would not double because as the blocks run in parallel, the
new macrocycle time would be 350 ms because only the time corresponding to the extra link included in the network must be added. This is shown
in Figure 5-4 with two separate loops on the same segment: an analog control loop A-110 and a discrete loop D-101.
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Figure 5-4 — Multiple loop function block scheduling
Scheduling
Scheduled Function
Block Execution
(SM)
Scheduled Cyclic
Communication
(DLL)
Unscheduled
Communication
AI110
PID110
AO110
DI101
DO101
loop 110 period of execution
Cyclic
Function Block Execution
Cyclic Communication - Publish
Acyclic Communication
Acyclic
Alarms/Events
Maintenance/Diagnostic Information
Program Invocation
Permissives/Interlocks
Display Information
Trend Information
Configuration
As can be seen, since each loop is independent, it is not necessary for the
devices in one loop to subscribe to the publications of the other loop. As a
result, the Compel Data command for one loop can be made while one of
the Function Blocks in the other is doing its internal calculations.
As a general guideline in the hydrocarbons industry, the number of devices/
Compel Data messages for segment execution time should be as follows:
•
For loops requiring 1 s execution time, limit segment to eight devices
with a maximum of three valves (four valves with client approval where
all loops are simple loops with control in the valve positioner).
•
For loops requiring 0.5 s execution time, limit segment to four to eight
devices.
•
For loops requiring 0.25 s execution time, limit segment to fewer than
three devices with a maximum of one valve.
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The above are suggested strategies to manage the risk associated with a single component failure in a Fieldbus system. Since every facility has a different risk management strategy and levels of “acceptable risk” they can
economically manage, the project team needs to determine a set of guidelines, such as these at the start of the project.
Tip 20 — Do not mix devices with different macrocycle times (1 s
versus 0.25 s) on the same segment. The mixing of macrocycles
can lead to schedules that may not be within the capability of
some LinkMasters.
Mixing macrocycles requires diligent design practices, with particular attention to the possibility of “periodicity” and hence conflicts after a significant
number of cycles. A problem of this type is difficult to diagnose and hence
the recommendation to “keep it simple.”
The cardinal rule for scheduling networks is that they must have a minimum amount of acyclic time in each cycle and they must operate at a
higher frequency than the process itself. The process frequency is the period
or response time of the control loop from input through output. The acyclic time is required to communicate information other than the Function
Block parameters, including alarms and configuration information, while
the cycle proper must be sufficiently fast to ensure that it is representative
of the process changes. The minimum recommendation is that the network
cycle time be one-third that of the process, though a sampling frequency of
six is preferred.
For example, if the residence time of an inlet separator is 2 min, then the
minimum cycle time for loop LIC-1 is 120/3 = 40 s.
Tip 21 — A good rule to start a design is to use the typical cycle
time for a traditional control system, which is 2 s per I/O scan.
If a control loop must communicate through the Host because AO devices
such as a variable-speed drive pump are on a different control network, the
loop response time will have to be calculated as not only the response time
of the H1 network, but also the time it takes to have (1) the H1 network
scanned by the Host, (2) the Host to act on this information, (3) another
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SYSTEM INTEGRATION
cycle to complete, and then (4) at least two more cycles to complete in the
other network, such as the ControlNet™ system, for the change to occur at
the pump.
Figure 5-5 shows a possible format for calculating the cycle time and number of VCRs required for a Fieldbus network.
Figure 5-5 — Segment bandwidth calculation
Network: ______________
Target Macrocycle (sec):
___________
% Acyclic Time: ___
Host Limitations
Network
System
Maximum Number of
Function Blocks:
Maximum Number of
Function Blocks:
Min. Macrocycle time:
msec Min. Macrocycle
time:
Target Macrocycle
time:
sec
Cycle
Tag Function
Time Publish
Block
(msec)
msec
% Acyclic Time
Subscribe
Internal
Cum. Cycle
Time
Check
Determining the most efficient use of bandwidth is a critical factor in the
success of your project. The worked example in Appendix E shows how
bandwidth and voltage drop need to be considered as part of the network
design process.
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Figure 5-6 — Typical fieldbus architecture
As has been described, the typical architecture of a network using the Fieldbus Foundation protocol (Figure 5-6) is built from a Host, a 24 VDC bulk
power supply, an FF Power Conditioner, cables, device couplers, terminators and Fieldbus field instruments connecting to the FF interface module
of the Host. The field instruments can be of many categories, such as pressure, temperature, flow, and level transmitters; process analyzers including
pH, conductivity, silica, and oxygen; and control valves with their positioners as the most common final control elements.
A significant number of DI (Discrete Input), DO (Discrete Output) devices
are also available for use on H1 networks; however, because response time is
related to the macrocycle rate, which is effectively the update or scan rate
for the segment, the macrocycle rate must be considered if a DI and DO
signal must have a faster response time than the macrocycle time period.
FOUNDATION Fieldbus does support the use of interrupts on the network;
however, few systems support this capability.
One of the unique advantages of FOUNDATION Fieldbus is that it supports
Control in the Field (CIF) so regulatory control can be placed in the field or
at the Host. Provided you have specified at least one LinkMaster capable
device on the network, there is also the possibility of having a backup of
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SYSTEM INTEGRATION
these loops in the field, depending on the configuration in the Host. When
redundant H1 interface cards are used (redundant I/O), the most common
backup location for the LinkMaster device is the second H1 card. As we will
see, the location of the regulatory control function has an impact on macrocycle timing.
FOUNDATION Fieldbus technology allows the control designer to choose the
location of the control function from three options: in the Host, in the
field transmitter, or in the field positioner mounted to the final control element/control valve.
Figure 5-7 — Fieldbus Foundation Network with control in the field
DCS
Field
FT-1
FT-1_AI_1
Out
FCV-1
In
BKCAL_IN
FCV-1_PID_1
Out
Fluid
CAS_IN
FCV-1_AO_1
BKCAL_Out
Figure 5-7 shows the most common Control-in-Field configuration, with
the control PID block in the valve with the AO Function Block. Once all
the instruments are connected to the segment and the Host, all devices
(transmitter, valve and Host) for that loop will start to receive the information that is being published⎯in this case, the AI variable, published by the
flow transmitter. An alternate Control-in-Field configuration would place
the PID Block in the transmitter with the AI Block. The macrocycle diagram for the above is shown in Figure 5-8.
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119
AI
Compel Data
Figure 5-8 — Macrocycle – control in valve
AI
PID
AO
Macrocycle
Repeats
Acyclic Communications
FT-1
FT-1_AI_1
Out
FCV-1
In
FCV-1_PID_1
BKCAL_IN
Out
CAS_IN
FCV-1_AO_1
BKCAL_Out
Note that there is one Compel Data command for every network communication; in this case, the single communication between the AI and PID
Function Blocks. Because the PID and AO Function Blocks reside in the
control valve positioner they do not consume network bandwidth to talk
with each other.
The decision about the location of the control function depends on several
factors including the basic project control philosophy, the availability of a
control block in the transmitter or positioner, and the requirement that the
input and output device must reside on the same physical network/segment. The decision to implement Control-in-Field should be made as early
as possible in the project as it can affect many other subsequent design decisions.
Tip 22 — Regardless of whether you choose to implement Control-in-Field in your original design, you should always (as far as
possible) design your segments to have the input and output
devices on the same segment so that should you wish to implement Control-in-Field at a future date you can do so without having to physically
relocate any devices.
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Because Fieldbus devices are becoming more efficient and requiring less
power, in many cases the limiting factor in a design is now becoming the
number of messages that can be sent during a single macrocycle. We therefore need to know how to calculate the loading of a macrocycle scan time
because it is becoming very important for the distribution of the instruments in FF segments.
Figure 5-9 — Fieldbus Foundation Network with control in the DCS
DCS
FCV-1_PID_1
Field
FT-1_AI_1
FCV-1_AO_1
Fluid
In this example (Figure 5-9), the control function is running in the DCS
(the Host). Once they are connected to the same segment, all field instruments in this loop and the Host are subscribers to the Publications of the
other Function Blocks that form the control loop (AI – PID – AO) and
receive the information that is being published. After receiving the AI block
information, the PID control block located in the Host completes its calculation and then publishes the output of its PID control block to the positioner, where the AO Block causes the positioner to move the valve as
required (Figure 5-10).
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121
AI
Compel Data
PID
Compel Data
AI
Compel Data
Figure 5-10 — Macrocycle – control in host
Macrocycle
Repeats
AO
Acyclic Communications
FCV-1_PID_1
FT-1_AI_1
FCV-1_AO_1
The updating of the links between Function Blocks is done each macrocycle. Macrocycle time can vary depending on the kind of control system in
use, the execution time of the Function Blocks in the instruments, and the
number of parameters for publication.
The macrocycle time must be compatible with the response time of the process to ensure that, not only will we be able to control the process, but that
the segment macrocycle time doesn’t jeopardize the dynamics of the process. In most applications, a default macrocycle time is used uniformly
across a Host system and is normally either 1 or 2 s, though it can be as
short as 100 ms.
The cycle time in a Fieldbus Foundation segment is split into two parts:
1.
122
Cyclical Period: when control information is published that has a
cyclical function and must be deterministic. Cyclic communications
use a Publisher-Subscriber communications model in which information is shared only between the Host and those devices that are configured as part of the same control loop. Other devices on the same
network that are not part of the control loop do not subscribe or lisSYSTEM INTEGRATION
ten to these communications and can therefore continue with their
own internal calculations during this time.
2.
Non-cyclical (Acyclic) Period: when there is data exchanged that is not
for control, such as data for monitoring, set point changes, alarms, or
other data transfers between devices or nodes on the network; this is
done in the acyclic period of the macrocycle. Acyclic communications include both higher priority Report Distribution communications for alarms and Client/Server communications for all other
forms of data transfer. Acyclic communications use a token passing
mechanism to share the network bandwidth.
Because so much information is communicated using non-cyclical/acyclic
communications, at least 50% of the total macrocycle time must be
reserved for these messages. Consequently, for new system design, the recommendation is to plan for closer to 70% acyclic communications time per
macrocycle. Now that we understand how the macrocycle is affected by
where we locate control for a single loop, the next step is to have a look at
how we might optimize the configuration and network communications for
a more typical segment with multiple control loops.
Table 5-5 calculates the macrocycle requirements for different configurations of the same H1 segment with three control loops (two analog and one
discrete) and the following characteristics:
It is assumed that there are three loops on the segment (T-1 and P-2 plus H1); other AI signals that might exist to populate the segment to reach the
required minimum/maximum number of devices for physical loading are
used for monitoring only and will access the process data using views.
Starting with the recommended method of implementing Control-in-Field,
where control resides in the AO devices, we can see (Figure 5-11) how a
macrocycle with Control in the Analog Output devices TV-1 and PV-2
makes efficient use of the available bandwidth.
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Table 5-5 — Macrocycle requirements for different configurations
Device
Tag
Execution time
Host System (PID
calculation time)
15 milliseconds
Compel Data
20 milliseconds
Device 1 – AI
TT-1
25 milliseconds
Device 2 – AI
PT-2
40 milliseconds
Device 3 – AO
TV-1
70 milliseconds
Device 4 – AI
FT-1
30 milliseconds
Device 5 – AO
PV-2
75 milliseconds
Device 6 – AI
LT-1
50 milliseconds
Device 7 – DI
HS-1
30 milliseconds
Device 8 – DO
HV-1
40 milliseconds
Device 1 – PID
TC-1
90 milliseconds
Device 2 – PID
PC-2
80 milliseconds
Device 3 – PID
TCV-1
40 milliseconds
Device 5 – PID
PCV-2
60 milliseconds
Figure 5-11 — Control in output device
AI
PID
AO
AI
PID
Device 1
Device 3
Device 2
Device 5
TT-1
TCV-1
PT-2
PCV-2
TT-1
CD -1
AI
CD -2
TC-1
CD - 3
PID
TV-1
PT-2
AO
AI
PC-2
PID
PV-2
HS-1
HV-1
124
AO
AO
DI
6
DO
SYSTEM INTEGRATION
All the Compel Data commands are executed in sequence, so we do not
need to alternate between the three different types of communications on
the network. Because the Compel Data command is deterministic and must
always happen at the same time, the network traffic is managed so that if
the schedule approaches the time for the next Compel Data command, the
network is idled. You can also see that the individual Function Blocks are
scheduled to complete their internal execution or calculations just prior to
publishing this data to the other members of the control loop to ensure that
the latest process data is being used.
The second Control-in-Field scenario is with one control loop in the transmitter and the other in the output device, as shown in Figure 5-12.
Figure 5-12 — Control in input and output device
AI
AO
AI
PID
Device 1
Device 3
Device 2
Device 5
TIC-1
TV-1
PT-2
PCV-2
TT-1
PID
CD - 2
AI
TC-1
CD - 3
CD -1
CD -1
PID
AO
TV-1
PT-2
AI
PC-2
PID
PV-2
HS-1
AO
AO
DI
HV-1
DO
Because of the extra bandwidth needed for the additional Compel Data
commands for the BKCAL_OUT and BKCAL_IN information, this configuration would only be used for a Control-in-Field application where the
Analog Output device did not have the required control Function Block.
Figure 5-13 shows how the macrocycle for the above three instrument loops
with a Control in the Host system might look if it were optimized
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125
Figure 5-13 — Control in Host
AI
PID
AO
AI
PID
AO
Device 1
Host
Device 3
Device 2
Host
Device 5
TT-1
TC-1
TV-1
PT-2
PC-2
PV-2
TT-1
CD -1
AI
TC-1
CD - 2
CD -1
CD -2
PC-2
HV-1
CD -2
AO
AI
PID
PV-2
HS-1
CD -1
PID
TV-1
PT-2
CD - 3
AO
DI
DO
Note that this configuration requires significantly more Compel Data commands and hence uses a larger amount of time for cyclic communications.
One other challenge with implementing Control in the Host was identified
in an ISA paper by Marcos Peluso and Mona Cognata, “Control in the
Field: Reliability, Performance, and Industrial Application,” that shows
how, if the timing of the Fieldbus network and Host are not perfectly synchronized, the control could potentially be updating on information that is
one or more macrocycles old, as shown in Figure 5-14.
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SYSTEM INTEGRATION
Figure 5-14 — Control in the DCS – when there are delays
CD
CD
DAT
A
PID
0
CD
PID
0
250
DAT
A
AI2
CD
250
DAT
A
AI2
CD
DATA
Macrocycle
CD
250
0
AO1
0
DAT
A
DATA
250
AO1
250
0
Macrocycle
250
As can be seen in Figures 5-11 through 5-13, the number of steps representing Publisher-Subscriber messages on the network for the various configuration options, keeping the PID in the Host does have an impact on the
amount of time it takes for a PID algorithm to be completed.
Despite this, many end users prefer to implement Control in the Host with
the mistaken belief that it is more reliable than the Control-in-Field option,
asking “What would happen if the control valve of the H1 network failed?”
The answer is that in such an event, you would not be able to execute control in either situation. If you are unable to communicate with your field
devices, you cannot receive or send control commands. If a control valve
should fail, it will not be possible to control the process.
SYSTEM INTEGRATION
127
6 — Commissioning
This is it, time to turn things on and see them work as a digital control system. Chapter 6 presents a number of procedures, tips, and tricks to help
make this process as smooth as possible.
6.1 Physical Layer Checks
Make sure that all final checks have been completed before you start up
your measuring point.
Tip 23 — The technical function data of the FOUNDATIONTM Fieldbus interface to IEC 61158-2 must be maintained.
The bus voltage of 9–32 V and the current consumption at the measuring
device can be checked using a conventional multimeter.
Check that the network has the correct number of terminators—only one at
each end.
Cable integrity and signal strength can be tested with handheld meters from
Relcom Inc. and Pepperl+Fuchs. Both handheld meters have USB cable
connections to allow uploading of stored information to a computer. These
meters, as well as the Relcom FBT-5 Cable Validator, are shown in
Figure 6-1.
The measuring device must have a default node address in the range 248–
251. The Fieldbus system will assign the device a final operational address
in the range 16–247.
The files required for commissioning and network configuration can be
obtained from the manufacturer or at the Fieldbus Foundation Web site
(www.fieldbus.org).
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129
g
Figure 6-1 — H1 network analysis tools
y
Wire and cable validator
Handheld diagnostic/network monitoring tools
In addition to the handheld diagnostic tools specifically designed for H1
networks shown in Figure 6-1, two other tools are required for testing of the
integrity of the cable itself. The first tool is a Megohmmeter (more commonly referred to as a “Megger”) to test cable isolation and the second is a
capacitance meter to confirm that signal attenuation will be within acceptable limits, specified by the appropriate FF and IEC standards.
The following general specifications apply to a digital Megohmmeter or
Megger. The unit must meet the following minimum requirements:
• Local display with bar graph and backlight logarithmic
• Four user-selectable ranges: 125 V/200 MW, 250 V/200 MW, 500
V/2000 MW, 1000 V/2000 MW
• Automatic tension discharge function
• AC voltage measurement of 600 V across 45-400 Hz
Additional useful features include:
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• Measurement of continuity; > 400Ω starts an indicating buzzer
• Annunciation if the insulation value is below a reference value predetermined by the user
• Memory function: 20 positions for recording measurements
Similarly, a generic specification for a capacitance meter includes as a minimum:
• Local indicator/display with scaling
• Selectable measurement ranges: 200 pF, 2000 pF, 20 nF, 200 nF, 2
µF, 20 µF, 200 µF, 2000 µF, 20 mF
• Accuracy: ± 0.5% (+1 digit)
• Resolution: 0.1 pF, 1 pF, 10 pF, 100 pF, 1 nF, 10 nF, 100 nF, 1 uF, 10
uF
• Frequency: 800 Hz, 80 Hz, 8 Hz
• Sampling rate: 2 to 3 seconds
• Low battery indication
• Indication of scale
• Manual zero adjustment
Figure 6-2 shows images of typical Megger (left) and Capacitance meters.
6.1.1 Cable Testing
According to the IEC standard 60227, cables for an operational voltage of
300 V must be tested with an AC voltage of 1000 V for 1 minute, between
the conductors and also between the conductors and ground. Cables with
an operational voltage of 500 V must be tested with AC voltage of 1500 V
for 5 minutes, between the conductors and also between the conductors
and ground.
Prior to inspecting any coil of cable (Figure 6-3), check to ensure that the
ends are protected against contact with water. Make the isolation measure-
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131
Figure 6-2 — Electrical cable test meters
Digital Meg Meter
Digital Capacitance Meter
Figure 6-3 — Reel of Fieldbus cable
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COMMISSIONING
ments to the levels above to confirm that they meet the minimum specifications.
To confirm the cable integrity of installed cable systems, start by checking
the trunk between the Host system and the first field junction box.
Prior to connecting the cable to the terminals or field equipment, make the
isolation and capacitance measurements for the various installed cables and
record the resulting measurements in the commissioning release document.
Repeat the testing procedure for each part of the trunk and for all the spurs
documenting the results in the release/precommissioning document, verifying the installed cable integrity. This precommissioning documentation
should be complete prior to connecting any devices to the segments.
6.1.2 Electronic Commissioning
After the above cable tests have been passed, connect the trunk to the
device couplers in all field boxes and then to the FF power supply installed
in the cabinet of the control system, ensuring the continuity of the conductor wires and of the cable shield.
Check the voltages at each input and output of the trunk in the field boxes,
and record the values in the precommissioning document.
Connect the instruments to the field device couplers in the field junction
boxes, taking the precaution of isolating the shield at the instrument.
Check the voltages of each input and output of the field box and at the connections of the instrument. Record the voltage values in the precommissioning document because they form part of the system baseline readings.
6.1.3 Configuration Commissioning
As each device is connected to the H1 network, it will appear on the “Live
List” in the Host. Confirm with the team responsible for device configuration, typically the application engineers, that each device that appears on
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133
the live list matches the one connected in the field so that the application
engineers can then start configuration of the instruments on the segment.
6.1.4 FOUNDATION Fieldbus Digital Communication
Certification
Once all the devices on the segment have been connected and communications established, the integrity of the H1 signals can be confirmed. To certify the digital communications, one of the available advanced diagnostic
tools should be used to record the integrity of the segment communications. Results should be stored, printed, and archived.
An oscilloscope capture of the packets is encouraged as it provides a visual
indication of the shape of the waveforms/packets themselves and when
reviewed by an expert can provide an indication of potential future problems on a network. As a minimum, gathering the waveform and other information while the network is new will provide an excellent baseline should
you need to investigate communications problems in the future.
The digital and paper file of the commissioning releases for each segment
should be part of the segment check-out and certification.
Figure 6-4 is an image of a correct H1 packet waveform. Notice that there is
little noise on the peaks. Although they are slightly sloped, this is normal
due to attenuation on the cable.
Figure 6-5 shows a change of base frequency and also a change of amplitude. The dashed line also shows an unbalanced signal. The most likely
cause of this kind of disturbance is high-frequency noise in the earthing/
ground system.
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Figure 6-4 — Correct H1 packet waveform
16 ʅsecond transition,
required for two “1’s” or
“0’s” in a row
Typical 32 ʅsecond
transition
Figure 6-5 — Change in base frequency and amplitude
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135
The waveform in Figure 6-6 is caused by electrical current/disturbances
resulting from inductive components being derived from the earthing/
grounding system.
Figure 6-6 — Effects of inductive components on waveform
The FF waveform in Figure 6-7 shows complete signal distortion. In this
case an inductive load was caused in a segment with instruments connected
by another segment of a length of circa 850 meters with no instruments
connected but left open without the connection of a terminator. The result
shown is known as an antenna effect, which is most evident in segments
without a terminator. A similar antenna effect occurs in spurs of lengths
longer than the IEC 61158 standard recommends.
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COMMISSIONING
Figure 6-7 — Complete signal distortion
This check sheet (Figure 6-8) can also be a useful tool to confirm that your
design and installation meet the minimum conditions as described by the
FF requirements and your project specifications.
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Figure 6-8 — Check sheet
FIELD TEST REPORT
Nº
Page:
USER:
RTC-03123-FF
1 of 1
FACILITY: Hydrotreater
UNIT U-2211
SEGMENT COMMISSIONING REPORT
FF Power Supply
PANEL: UC-1102-R
CONTROLLER: UC-1102
RULE: LB
POSITION: 13
C HANNEL: H2
SEGMENT LABEL: FF-2311207
MANUFACTURER: PEPPERL-FUCHS
MODEL: HD2-FBPS-1.25.360
REDUNDANT: Yes
X No
VOLTAGE MEASUREMENT AT POWER SUPPLY:
DESIGN VALUE (Vdc)
DESIGN VALUE (Vdc)
DC VOLTAGE (POWER)
> 18,6
25,45
POWER SUPPLY VOLTAGE OUTPUT
25,0 ~ 28,0
26,73
FIELDBUS JUNCTION BOX / DEVICE COUPLER
TAG: JBF-2311207/1
NUMBER OF SPURS: 8
VOLTAGE IN BOXES:
MANUFACTURER: PEPPERL-FUCHS
INTEGRATED TERMINATOR? NO
VOLTAGE TRUNK IN
VOLTAGE TRUNK OUT
SPUR 1
SPUR 2
SPUR 3
SPUR 4
SPUR 5
SPUR 6
SPUR 7
SPUR 8
MODEL: R2-SP-N8
TRUNK OUT CONNECTED? YES
DESIGN VALUE (Vdc))
23,2
23,2
23,2
23,2
23,2
23,2
23,2
23,2
23,2
-
SERIES N°:
MEASURED VALUE (Vdc)
25,65
25,64
25,35
25,36
25,24
25,36
25,36
25,35
25,47
-
Instrument Data
Voltage Values
Design
Measured
TAG
SPUR
Manufacturer
Model
Serial Number
TT-019
01
23,20
25,35
EMERSON
644HFE
248007
TT-020A
02
23,20
25,36
EMERSON
644HFE
TT-020C
03
23,00
25,24
EMERSON
644HFE
TT-051
04
23,00
25,36
EMERSON
644HFE
PT-232
05
23,00
25,36
ROSEMOUNT
3051CG1
14338
TT-207
06
23,00
25,35
EMERSON
644HFE
248006
07
23,00
25,47
YOKOGAWA
ZR402G
08
-
-
AT-001A
248005
NOTES:
Meter Used
Certificate Nº
Validated
FLUKE 175 (FNK 0171)
NE 2660/2009
28/09/2011
138
EXECUTED BY
A. Pereira
I.Verhappen
RESPONSIBLE
DATE 01/03/2011
DATE 02/03/2011
Notes:
CLIENT WITNESS
DATE
/
/ ___
CLIENT ACCEPTANCE
DATE
/
/ ___
COMMISSIONING
6.1.5 Typical Installation Problems
The following figures and text describe some of the typical installation
problems that can be found during commissioning and identify the fault
that needs to be corrected.
Figure 6-9 shows the correct installation of a transmitter. The connector was
correctly specified, complete with metal locking with metal before reaching
the end of the threads.
Figure 6-9 — Correct transmitter installation
Figure 6-10 illustrates the correct installation of several Fieldbus junction
boxes in which the trunks and the spurs are installed in a safe area. As can
be seen by the highlighted earthing or grounding cable, the boxes are correctly grounded. Note that this ground is for the enclosures and remains
isolated from the H1 signal cable ground, which is only connected at a single point.
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Figure 6-10 — Correct installation of Fieldbus junction boxes
Figure 6-11 — Field device grounding error
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The transmitter shown in Figure 6-11 is close to being a correct installation
because the input of the Fieldbus cable via cable gland is properly connected. However, the angle of the photo does not allow us to see the housing grounding to determine if that has been done correctly.
Standard IEC 60079-14 edition 02/20/2009 in section 9.4 – Conduit, the
North American Electrical Code, and Canadian Electrical Code, specifies
that when the conduit contains three or more cables, the area of the crosssection of the cables, including the insulation, cannot be more than 40% of
the area of the conduit. The installation shown in Figure 6-12 does not meet
this standard.
Figure 6-12 — Cable cross-section exceeds 40% of conduit area
Continuing with good design practices, the same standard (IEC 60079-14 –
edition 02/20/2009), in clause 9.3.7 “Prevention of Damage” recommends
that cable systems and accessories be installed, whenever possible, in places
that prevent the cables from being exposed to mechanical damage or chemical influences.
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Figure 6-13 (two photos) shows another incorrect installation: There are two
different types of segment cables (orange) together with power cables (blue)
after the intrinsic safety barrier installed in the same tray without mechanical separation as a minimum. This mistake puts the plant in great danger
because if there is a short circuit between the cables in this cable tray, the
intrinsically safe instruments will be fed with high energy, which could
potentially lead to an explosion or as a minimum will void the intrinsically
safe designation for the instruments affected.
Figure 6-13 — Failure to maintain required mechanical separation
Another very common error in project assembly practice with 4-20 mA signals is to coil the signal cable (Figure 6-14; two photos) to prevent stress on
the cable. However, for projects with digital communication protocols, the
coiling of cable is strongly discouraged because it can provoke a change in
the communication due to the increase of the inductive component of the
segment impedance, which results in increased signal attenuation. What
makes this sort of error difficult to troubleshoot is that this kind of mistake
doesn’t continually provoke the interruption of communication, but can
cause intermittent communication problems, just the kind of defect that is
very difficult to discover with a plant in operation. A single 360 degree coil
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of cable is acceptable, though a partial loop of no more than 270 degrees is
recommended.
Figure 6-14 — Coiled signal cables
In Figure 6-15, we can see how the entry of water or another liquid into the
instrument box has resulted in significant corrosion. This is a common
problem that arises from not taking proper precautions during installation.
The problem exhibits itself by starting with increased difficulty in communication between the Host and the instrument, which can be observed in
some DCSs by an increase in the number of retransmissions. With the passing of time and with the increase in moisture, the communication continues to deteriorate, with a continued increase in the number of
retransmissions and intermittent total loss of communication. This intermittent communication is also very difficult to diagnose because it is sometimes interpreted as a defective instrument rather than a corrosion problem.
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Figure 6-15 — Corrosion caused by liquid entry
Figure 6-16 — Corrosion in a junction box
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Due to installation errors, the junction box shown in Figure 6-16 was
exposed to corrosive liquid. The high corrosion rate at the lower terminals
is the result of the exposure. This kind of defect is also difficult to discover
because, in this case, we can see that only the spurs not yet affected by the
corrosion could be working with a rate of errors much smaller than the
spurs in the lower part of the box.
Another very common installation error is shown in Figure 6-17. In this
panel, rather than cut the cables to length, installers left them coiled in the
cabinet (see Figure 6-14). The correct installation method is to keep a short
length of cable for flexibility without doing a complete loop, and then to
cut the cable to length.
Figure 6-17 — Excess cable length
There are at least two errors shown in Figure 6-18. In the upper part of the
photo, the sensor cable is installed without extra length, and because of
that, the cable is under tension and is suffering very high stress, while in the
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145
lation of the earthing (grounding) cable together with the Fieldbus cable.
One consequence of putting two cables through a single gland is that the
gland cannot properly seal around them, thus allowing a path for moisture
and gases into the enclosure.
Figure 6-18 — Two installation errors
6.2 Device Configuration
Once the device has been physically connected to the network it will appear
on the Host system “live list” with either its factory default setting
(DEVICE_ID) or if specified in the purchase document, the tag number.
The default DEVICE_ID is a combination of the manufacturer ID, device
type, and device serial number. It is unique and can never be duplicated.
The following description allows step-by-step commissioning of the measuring device and all the necessary configuration for the FOUNDATION Fieldbus:
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1.
Switch on the measuring device. Note the DEVICE_ID on the device
nameplate
2.
Open the configuration program.
3.
Load the device description file or CFF file into the host system or
into the configuration program. The first time it is connected, the
device will report something similar to Figure 6-19.
Figure 6-19 — Device display on first connection
Display text (XXX = serial number)
Description
DEVICE_NAME xxxxxx
Field device tag name (PDTAG)
nnnannnnn-
Device ID
RESOURCE_xxxxxxxxx
Block name - Resource Block
TRANSDUCER_ xxxxxxxxx
Block name - Transducer Block
ANALOG INPUT_1_xxxxxxxxxxx
Block name - Analog Input
Function Block 1
PID_xxxxxxxxxxx
Block name - PID Block
Tip 24 — Be sure the device is supplied with a bus address that is
in the address range reserved for the readdressing of field devices,
between 248 and 251. This means that the LAS (Link Active
Scheduler) automatically assigns the device a free bus address in
the initialization phase.
Tip 25 — Each DEVICE_NAME in the system must be unique
and can be up to 32 characters in length. It is recommended that
suffixes be used to identify the signal and block type.
Table 6-1 provides examples of how this may be applied.
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Table 6-1 — Examples of identifying signal and block type
Analog Input signal 1
PT-1_AI1
Analog Input Signal 2
PT-1_AI2
Transducer Block
PT-1_TB
Resource Block
PT-1_RB
PID Block
PT-1_PID
4.
Identify the field device using the DEVICE_ID that you wrote down
and assign the desired field device tag name (PD_TAG) to the Fieldbus device in question.
6.2.1 Configuration of the Resource Block
5.
Open the Resource Block.
6.
On delivery, write protection is disabled so that you can access all the
write parameters. Check this status via the parameter WRITE_LOCK:
• Write protection activated = LOCKED
• Write protection deactivated = NOT LOCKED
• Deactivate the write protection if necessary.
7.
Enter the desired block name. Factory setting: RESOURCE_
xxxxxxxxxxx
8.
Set the operating mode in the parameter group MODE_BLK (parameter TARGET) to AUTO.
6.2.2 Configuration of the Transducer Block
9.
Enter the desired block name. Factory setting: TRANSDUCER_
xxxxxxxxxxx
10.
Open the transducer block.
11.
Set the operating mode in the parameter group MODE_BLK (parameter TARGET) to OOS, that is, block Out Of Service.
12.
Now configure all the device-specific parameters for the measurement.
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Tip 26 — Changes to the device parameters can be made only
after entering a valid access code in the Access Code parameter.
13.
Set the operating mode in the parameter group MODE_BLK (parameter TARGET) to AUTO.
6.2.3 Configuration of the Analog Input Function Block
Some Fieldbus devices have multiple Analog Input Function Blocks that
can be assigned optionally to different process variables. The following
description provides an example for Analog Input Function Block 1.
14.
Enter the desired name for the Analog Input Function Block. Factory
setting: ANALOG_INPUT_1_ xxxxxxxxxxx
15.
Open the Analog Input Function Block.
16.
Set the operating mode in the parameter group MODE_BLK (parameter TARGET) to OOS, that is, block Out Of Service.
17.
Using the parameter CHANNEL, select the process variable that is to
be used as the input variable for the Function Block algorithm (scaling
and limit value monitoring functions).
18.
In parameter group XD_SCALE, select the desired unit of measure
and the block input range (measurement range of the flow application) for the process variable in question.
Caution: Make sure that the selected unit of measure is suitable for the
measurement variable of the selected process variable. Otherwise, the
parameter BLOCK_ERROR will display the error message “Block
Configuration Error.”
19.
In the parameter L_TYPE, select the type of linearization for the input
value (Direct, Indirect, Indirect Sq Root).
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Caution: Note that with the type of linearization “Direct,” the
configuration of the parameter group OUT_SCALE must agree with the
configuration of the parameter group XD_SCALE. Otherwise, the blockoperating mode cannot be set to AUTO. Such incorrect configuration is indicated via
the parameter BLOCK_ERROR (Block Configuration Error).
20.
Use the following parameters to define the limit values for alarm and
warning messages:
HI_HI_LIM
limit value for the upper alarm
HI_LIM
limit value for the upper warning
LO_LIM
limit value for the lower warning
LO_LO_LIM
limit value for the lower alarm
The limit values entered must be within the value range specified in the
parameter group OUT_SCALE.
21.
In addition to the actual limit values, you must also specify the action
taken if a limit value is exceeded using so-called “alarm priorities”
(parameters HI_HI_PRI, HI_PRI, LO_PRI, LO_LO_PRI).
Tip 28 — Reporting to the Fieldbus Host system takes place only
if the alarm priority is higher than 2.
22.
System configuration/connection of Function Blocks as shown in Figure 5-2.
A concluding “overall system configuration” is essential so that the operating mode of the Analog Input Function Block can be set to AUTO and so
that the field device is integrated into the system application.
To do this, configuration software is used to connect the Function Blocks to
the desired control strategy—generally graphically—and then the sequence of
the individual process-control functions is specified.
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23.
After specifying the active LAS, download all the data and parameters
into the field device.
24.
Set the operating mode in the parameter group MODE_BLK (parameter TARGET) to AUTO. However, this is possible only if two conditions are met:
• The Function Blocks are correctly connected with each other.
• The Resource Block is in operating mode AUTO.
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7 — Troubleshooting
Congratulations, you are up and running. Time now to maintain, “fine
tune,” and optimize this system. This chapter offers a few things to consider
and tools available to do just that, that is keep a Fieldbus system in “top
shape.”
7.1 Optimization Tools
A control system is only as good as its network, since without reliable signals it is not possible to do any control. It is therefore important to keep
your network in good shape, and fortunately now there are a number of
tools (both intermittent and permanent) on the market to make this possible.
As discussed in Chapter 6, “intermittent” (handheld) tools are available to
analyze a Fieldbus network. One of these is Relcom Inc.’s FBT-6 Network
Monitor. This handheld device can be directly connected to the H1 network by the three leads (+, -, Shield) where and when required, and uses a
simple two-button interface to check key traffic and signal level characteristics. The navigation menu, default alarm limits, and suggested sources of
any alarms can be checked against the included manual. The other product
on the market is the Pepperl+Fuchs unit that uses its USB connection to a
laptop as its interface. Both units are shown in Figure 6-1.
Another network analysis tool that has been available for some time,
though it is much more complex, is the BusMonitor software from National
Instruments. The BusMonitor combination hardware and software package
resides on a laptop or personal computer connected to the network as a Visitor/Guest device to minimize its impact on network communications and
is capable of monitoring all aspects of network traffic packets. It typically
requires a knowledgeable user to be able to interpret the information provided by the BusMonitor tool. Data captures by the BusMonitor tool can
be archived and sent to other users/experts for interpretation.
TROUBLESHOOTING
153
Tip 27 — The National Instruments equipment can be configured
as a Host or Guest. If it is configured as a Host, it will conflict
with the already existing Host and will likely cause the entire network and/or system to crash.
There are now four manufacturers offering diagnostic tools that are permanently connected to the network and are capable of transmitting diagnostic
information to the host or asset management system and associated maintenance software. Below are a short summary and an image of each system.
Tip 28 — Because Fieldbus barriers (active device couplers) have
isolation between the trunk and spurs, on-line diagnostic tools
will only be able to view Physical Layer information for the trunk
in this type of installation. A handheld unit will need to be connected to individual spurs if that information is required.
MTL has developed their diagnostic module as a FOUNDATIONTM Fieldbus
H1 device. The module monitors up to eight segments and a maximum of
32 devices per segment. The unit can be mounted with or without an associated power conditioner and is shown in Figure 7-1 as the stand-alone unit.
Because this unit is an FF device, it uses a DD (Device Description) file and
appears on the network, segment 1, 8 or a dedicated network without any
additions to the system or network. This unit is closely related to the Relcom FBT-6 handheld unit.
The second unit, and the one on the market longest, is from Pepperl+Fuchs. It monitors four segments and is integrated into their FF power
conditioner backplane system. The output from this system works in a similar fashion to a HART multiplexer, with a serial output that can either be
connected directly to the computer with the diagnostic software or (as
shown in Figure 7-2) converted to Ethernet and then transmitted that way.
Diagnostic software can reside in the same computer as the predictive maintenance or asset management system. Like the MTL solution, this product
is similar to the P+F handheld module.
A third solution, shown in Figure 7-3, is offered by Turck and uses HSE
(High-Speed Ethernet) as the communications backbone from the diagnostics module to the host system. At the time of printing, Turck did not have
a handheld version of their diagnostic solution.
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TROUBLESHOOTING
Figure 7-1 — MTL diagnostic system
FF H1 Connection
Module on backplane
F809
Figure 7-2 — P+F on-line diagnostics solution
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155
Figure 7-3 — Turck on-line diagnostic solution
Since the last edition of this book was printed, R. Stahl has released a diagnostic module as well that can communicate either via RS-232 or an H1
interface module. Figure 7-4 shows the values recorded with the RS-232 and
Hyperterminal interface.
7.1.1 Physical Fault Symptoms
All the above diagnostic tools provide a similar set of physical layer measurements, interpret those measurements based on the following changes to
the base level signals, and are often caused by the reasons below.
An extra terminator on a segment can normally be seen as an approximately
30% reduction in signal level, which translates to about 300 mV in most
cases. A missing terminator typically results in a similar increase in absolute
mV reading, which is a 70% increase in signal level.
Fieldbus devices are required to reject signals within the Fieldbus frequency
band that are less than 75 mV peak-to-peak.
Low frequency noise is often caused by the coupling of signals from 50 or
60 Hz power lines with the Fieldbus cable. High frequency noise is often
caused by variable frequency drives, so it is important to practice proper
cable spacing and installation techniques as described in Chapter 2.
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TROUBLESHOOTING
Figure 7-4 — R. Stahl diagnostic module
The number of retransmissions is a good indicator of overall network
health. When the number of retransmissions increases, likely causes include
high bus noise levels, low signal levels, or a low bus voltage level. You can
always expect a few retransmissions on a segment; however, if the rate of
retransmissions does change, it is a likely precursor of more difficulties to
follow.
7.2 Communications and Configuration
Dedicated network analysis tools provide information on the physical layer
of a Fieldbus system. In addition there are a number of items affecting system reliability that are a function of the way in which the network communications (Layers 2 and above) themselves are configured.
The quality GOOD (NONCASCADE) means the value may still be used
for control, despite the fact that signal quality is deteriorating.
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157
Tip 29 — The stale rate, which is a measure of how many cycles in
a row have missed a scheduled communication, on PID blocks
must be set to a minimum of 3.
If the PID stale rate is left at the default state of 1, the loop can periodically
cycle between in and out of service if there is a sporadic communication
problem on the segment, which causes the AI block to be missed. A side
effect of cycling in and out of service is the inadvertent changing of setpoints when setpoint tracking is specified.
When the output from the secondary control block of a control cascade
reaches the limit for the setpoint in the primary control block, the primary
will set limited status in its back calculation output to the secondary so as to
prevent the upper block from moving its output further in that direction.
The secondary will not change mode.
If the LAS fails, the backup LAS assumes control of the network. The result
is that the loops may be running on outdated information of a few seconds.
To prevent this from accelerating to cause a problem, limits should be
placed on the time a loop can operate in this alternating LAS primary node
before having the segment shut down in a controlled and configured way.
7.3 Tuning
Figure 7-5 represents the internal functions that reside in the PID Function
Block. To make the tuning constants for the PID block dimensionless, the
PV_SCALE parameter must be configured.
Three tuning parameters are in a PID loop: Proportional Gain (GAIN),
Integral Reset Time (RESET) in seconds per repetition, and Derivative Time
Constant (RATE) in seconds. To disable the integral function, set RESET to
+Inf. Derivative action (RATE) is set to zero to disable this action.
Caution: Many different forms of the PID algorithm are implemented in
the various PID Blocks of different manufacturers. To ensure the bumpless
transfer of a loop, be sure of the form implemented in the PID locks of the
loop.
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TROUBLESHOOTING
Figure 7-5 — PID Function Block internal functions
The CONTROL_OPTS parameter can be used to configure the output as
direct or reverse action, with the default being reverse action.
Caution: When the controlled variable input to a primary PID Block in
a cascade loop is “BAD,” the primary’s PID algorithm cannot function.
It is possible to let the secondary PID control analog output block pass
directly to the primary PID. Caution is necessary since not all control schemes are
stable in bypass. To minimize the risk, enable the bypass feature by setting the
CONTROL_OPTS parameter option to “Bypass Enable.” Bypass itself is activated
with the BYPASS parameter, which can only be set in manual or OOS (Out Of
Service) mode.
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159
8 — Operations & Maintenance
Fieldbus introduces a number of changes to the traditional ways of doing
business in a processing facility. Fully deploying and using Fieldbus technology in a facility, with associated changes in operating procedures, can
result in savings of up to 50% of the automation maintenance budget
alone. This chapter highlights just a few of the changes required to be able
to realize the benefits of a fully digital control system.
8.1 Operations
Digital communications technology provides operations the potential for
several orders of magnitude, typically 1000 times, more operational data
than is available from a traditional analog field device/system with its single
4–20 mA signal. Key among the additional information is the “status” signal sent along with every process variable transmission.
This status information can be used to inform the panel operator that
despite what they think, the device is in fact reading correctly. In addition,
rather than have the maintenance technician reference or calibrate the
device, the status information should instead be included in the decisionmaking process. Similarly, application engineers can make use of the status
information in their control algorithms/applications so that they are only
run when the signals on which they rely are known to be operating
properly.
Since Fieldbus has multiple levels of “failure” and alarming, it is also possible to configure the control system to respond in different ways, depending
on the nature of the status being transmitted. Obviously, if the device status
is “BAD” or “OOS – Out Of Service,” meaning it has failed, the signal
should not be used for control. If, however, the device status is “UNCERTAIN,” the decision is not as clear, and it needs to be determined by the
control and process staff as to how this could impact the operation; they
must then have the control system respond accordingly.
OPERATIONS & MAINTENANCE
161
The key to successfully implementing Fieldbus is to only send information
to process operators that they can use to improve the reliability and profitability of your operation. Doing otherwise is counterproductive and results
in “information overload,” where the important information the process
operator requires cannot be found among all the other data with which the
operator is being bombarded.
8.2 Maintenance
Maintenance departments are the people in a facility to be most impacted
by the introduction of Fieldbus technology, not only because of the technology itself but also because of the associated changes that will result in
traditional work practices. The technicians will have to learn how to work
with a number of new tools, most important of which will be a laptop or
another computer with associated specialized software for in-depth system
analysis.
The two most significant changes with Fieldbus versus analog control systems are:
1.
Fieldbus devices MUST be connected to a network for them to be
“live” and worked on for such things as calibrating1/referencing, reranging or any other work that needs to be performed on a “live”
device.
2.
Fieldbus devices are part of the control system network, and therefore,
any changes made to the device are propagated through the system
and will likely change the configuration in the DCS once they have
been connected. If they do not change the host system configuration,
they will (at a minimum) raise an alarm, so change management procedures need to be established for all work done on the host or field
devices, including who (application engineer or maintenance technician) is responsible for what.
1.
Modern digital transmitters are HIGHLY accurate devices for which field calibration
will not be nearly as accurate as what can be achieved in a factory setting. Some manufacturers are quoting stability guarantees of ten years on their devices at levels better
than were possible with earlier generations of equipment. Therefore, the term calibrate
should be replaced with the term reference or reference check, as the bench test is simply to
confirm that the device is configured for the range specified on the data sheet.
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OPERATIONS & MAINTENANCE
In addition to using the permanently connected host system, be it an engineering station, operator console, or maintenance console, additional
classes of host system, handheld maintenance tools are also available to
make changes to device configurations. One unit from Emerson Process
Management, shown in Figure 8-1, is the replacement for the HART handheld communicator device. The Emerson 375 is able to access and alter all
the parameters in a Fieldbus device.
Figure 8-1 — Emerson handheld communicator
(Courtesy of Emerson Process Management)
The second handheld unit on the market is from Beamex, and because this
unit (as shown in Figure 8-2) is a calibration device, the manufacturer has
restricted the device’s access to those parameters related to calibration
only—predominantly in the Transducer Block.
Note: As mentioned in Footnote 1, with all digital instruments, it is better
to do a “reference check” in the field because the calibration in the factory
OPERATIONS & MAINTENANCE
163
Figure 8-2 — Beamex Fieldbus calibrator
will be more accurate and fully traceable than can be obtained in most shop
facilities. Because of this, “calibration” in the field is actually a check against
an introduced reference signal and not a change in true device calibration.
Because the sensing technology on which Fieldbus devices are based is
unchanged from the past, though it is now more accurate, the majority of
new problems with Fieldbus systems arise from something in the Physical
Layer. Maintenance technicians must therefore learn how to gather information on the Physical Layer so they can determine the most likely cause of
network communication problems.
Fortunately, the traditional multimeter is still useful for measuring the voltage at each juncture of the network:
• Power Conditioner inlet and outlet
• Field junction boxes
• Field devices
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OPERATIONS & MAINTENANCE
The multimeter should also be able to measure the capacitance of a segment since this parameter also has an impact on communication signal
losses.
A complete set of these measurements should be taken as a baseline measurement as soon as the field cabling is installed; again once the entire segment is commissioned; and finally, after every change (addition or removal)
of a device on the segment. Doing this will identify grounding problems
and provide a baseline to work from in the future if problems should occur.
The most widely used handheld network analysis tool is the Relcom FBT-6,
which “polls” a number of key physical layer parameters.
The following list is a representation of parameters (data points) that provide an indication of changes in the stability of your network:
Volts – DC voltage on the network at the point the device is connected should be at least 9 volts.
LAS – Address and Signal level of the Probe Node should be greater
than 150 mV.
Device – The number of devices on the segment as well as an indication of any changes in the number of devices since the last cycle.
Noise Av – Average noise on the network in the low (50 Hz – 4 kHZ),
FF (9 – 40 kHz), and high (90 – 350 kHz) frequency bands. This
should be less than 75 mV.
Noise Pk – Peak noise recorded in the low-, FF, and high-frequency
bands since the device has been connected to the network.
Short Circuits – Indication of potential deterioration in the integrity
of the network.
Retransmissions – Packets are not being sent or have been corrupted
in some way. The number of retransmissions provides an indication of
overall communications deterioration.
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165
Jitter – The deviation from the ideal “zero crossing point” with a maximum change in the expected transition time of 16 or 32 µ seconds of
±10%. Figure 8-3 shows jitter in a single Fieldbus signal. When the signal crosses outside the time window, the result is a bit error and the
variation in the time of crossing is known as jitter. The FBT-6 does not
make a jitter measurement, rather, it infers this information from the
change in the number of retransmissions.
Figure 8-3 — Fieldbus signal jitter
time window
100%
bit error
overshooting
50%
jitter
0%
Time
These data points are useful in determining the possible cause of problems
and serve as a starting point to work from with the more sophisticated tools
available on a laptop or other computer.
As shown in Figure 6-1, other manufacturers make similar devices, and new
product is regularly being released that may have similar or additional functionality to that described above. The Relcom FBT devices have been
widely deployed in industry as a Fieldbus technician’s “multimeter” to provide for a preliminary inspection of the health of a Fieldbus network.
Many new power conditioners support the transmission of their operational
status to the control system, via either an Ethernet or another connection,
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OPERATIONS & MAINTENANCE
or as a minimum with a solid-state contact to raise an alarm that potential
problems with the Physical Layer are (or soon will) occur.
Most important to the lifecycle operation of a facility is the diagnostic data
available from Fieldbus devices when connected to a computerized maintenance system. This data makes it possible to do prognostic maintenance of
the control system, with the associated possibility of reducing maintenance
costs versus a traditional analog-only system by anywhere from 25% to
more than 50%.
OPERATIONS & MAINTENANCE
167
9 — New Developments
9.1 Fieldbus Safety
A successful demonstration of the Fieldbus Foundation safety Fieldbus
implementation SIF (Safety in Field) technology occurred at Shell in the
Netherlands in May 2008. At the time of printing this fourth edition (early
2012), the Fieldbus Foundation and suppliers are releasing their first Fieldbus safety products. The reason for the delay between proving the technology works and being able to purchase devices that receive the Fieldbus
“check mark” (indicating a device is compliant with the new standard and is
developed to be consistent with IEC 61508 and verified by TÜV) is that
getting the FF “check mark” does not mean the device itself is safety-certified to a certain SIL (Safety Integrity Level). The manufacturer must still
verify the SIL rating of the device after the “check mark” has been received.
The Fieldbus specification has been designed so that the device communications will meet a SIL 3 rating. Because the device SIL rating is independent of the safety bus rating, the actual SIL rating for a device will continue
to be the responsibility of the manufacturer and associated safety certifying
organizations to have the device certified to the appropriate SIL rating.
The Fieldbus safety bus, as with most other safety buses, uses a “black channel” model as its basis. This means that rather than develop a new communications protocol from scratch, the safety protocol adds a number of
protections and other features to ensure the timing/transmission, as well as
the integrity, of communications between devices. Consequently, a number
of new standards (see 9.2) needed to be developed, and several of the existing standards needed to be updated to include the required new functionality. In addition, a number of new safety specific Function Blocks including
a Voting Function Block have been defined to allow for safety logic control
in the field.
Figure 9-1 shows how the IEC 61508 requirements have been added to
either end of the H1 communications channel.
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169
Figure 9-1 — SIS (Safety Instrumented System) user layer extensions
The following diagnostics are indicative of the communication diagnostics
added to the H1 “black channel” to allow the detection of errors not found
with H1 CRC (Cyclic Redundancy Check) and related mechanisms. Potential errors not detected by H1, as summarized in Table 9-1, can include single or multiple bit errors, message insertion/omission/retransmission,
disordered messages, and false message addressing.
The Foundation SIF protocol uses several methods to control the effects of
transmission problems as per Table 9-1, such as:
• Duplicated data and CRCs.
• Comparison of data and related CRCs.
• Connection key and object index to identify safety-related devices
and objects.
• A sequence counter that checks for correct message sequence,
allows Function Blocks to implement a stale counter to indicate
data transfer timeout, and detects queuing delays.
• A time synchronization monitor to detect failures of black channel
time synchronization.
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Table 9-1 — IF communication errors
Corruption
Late
Early
Excessive Jitter
Masquerade
Inconsistency
Ɣ
Ɣ
„
Ɣ
Ɣ
„
Ɣ
„
Ɣ
Ɣ
„
„
„
„
Ɣ
„
„
„
„
Ɣ
Hamming distance applied
to node addresses or
message identifiers
„
Inhibit times
„
Atomic broadcast
„
„
Membership control
„
„
„
Prioritization of messages
Redundancy (replication)
Cryptographic techniques
Safety code (CRC)
Identification procedure
Time out
Ɣ
Apply bus guardian
Ɣ
„
Ɣ
„
Apply time-triggered
architecture
Ɣ
Feedback message
(acknowledgements)
Ɣ
Ɣ
Ɣ
Source and destination
Identifier
Repetition
Deletion
Insertion
Incorrect Sequence
Time stamp
Defense
Sequence number
Threat
„
Ɣ
„
„
„
„
„
„
„
Notes:
„ are not supplied by EN 50159-2
Valid, if the CRC calculation includes data that are not in the message itself, but are known by the transmitters and
receiver(s) a priori (i.e., a message key and expected send time).
One of the other major advantages of using a “black channel” model is that
the physical layer for the safety network will be the same as for the standard
protocol, and if desired, a single network may contain both safety-related
and non-safety-related devices.
Figure 9-2 shows how a 2oo3 (two out of three) voting scheme could be
implemented in the field using the Fieldbus safety bus. The individual
physical devices are shown inside the rectangles, and like Control-in-Field,
they must all reside on the same network.
All devices will require a “Write Lock” Block to allow them to be changed as
required. The Safety Discrete Input Block (SIS-DI) serves the function of
enabling and disabling the SIS-AI Block in each of the three transmitters.
Of course, the Write Lock Block has the necessary security features to
ensure that it is not inadvertently changed by someone in error. Since the
Resource Block is the “switch” that turns a device on or off, it is activated or
changed from OOS (Out Of Service) to AUTO (Automatic) or Cascade as
required to turn the device on once any changes have been made. The SISAI Blocks then record the process condition and communicate their results
to a new SIS-AVTR (Analog voter) Block that compares the readings against
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171
Figure 9-2 — Example SIS application analog 2 out of 3 voter
configured limits, and when two of the three devices are outside those
bounds, activates the SIS-DO (Discrete Output) Block to change the state
of the final control element.
Implementation of Fieldbus safety systems will likely follow the same adoption cycle as the original H1 devices, with devices being available some time
before the appearance of associated host systems that are capable of taking
full advantage of their new functionality and features.
In addition, not all Function Blocks will be available in the initial products.
The initial two blocks to be released will be the SIS_AI and SIS_DO Blocks.
Partial stroke testing has also been defined as part of the Advanced Positioner Transducer Block specification.
9.2 Wireless & Remote I/O (WIO)
The Wireless and Remote I/O initiative Wireless I/O (WIO), which was
demonstrated in December 2011, defined the first in a series of interoperable gateways (a smart RTU − Remote Terminal Unit) to bring control I/O
(both analog and discrete) back to plant automation systems over an HSE
(High Speed Ethernet) interface. WIO systems are intended to function
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NEW DEVELOPMENTS
much like an RTU, making it easy to bring discrete-in, discrete-out, analogin, analog-out (including the associated wired or wireless HART information and Foundation H1 devices) to a DCS via a single protocol and interface.
To make this happen, as shown below, a number of new specifications
needed to be written, and several existing specifications needed to modified.
FF - 633 Function Block Application Process – Part 6 (HSE RIO)
FF - 913 Transducer Block for Wired and Wireless HART
FF - 915 Transducer Block for HSE RIO Module
FF - 061 Addendum to System Architecture for WIO
FF - 634 Addendum to HSE System Management for WIO
Transducer Blocks for Wired & Wireless HART, Fieldbus Specification FF913, defines a Fieldbus Foundation Transducer Block used to represent both
wired HART and WirelessHART™ devices within Fieldbus Foundation
gateways. The specification also describes the method expected to allow
configuration of the HART device through DCS or an asset managing Host
to use the native HART command protocol transported through the HSE
network and field-mounted WIO module.
A special set of HART Transducer Blocks was developed as FF Application
objects to allow mapping of HART variables to appear as FF devices and
thus be directly interoperable with other FF devices on the network.
As defined in Transducer Block (TB) for HSE RIO Module (FF-915), there
is usually one HSE-RIO-TB for every HSE RIO device internal physical
location (slot). This TB handles the data transfer between the physical I/O
and the FF Function Block architecture. WIO provides for optional redundant elements within an HSE unit. Redundancy is not limited and may
include, for example, two elements, three elements, or four elements, as
applications require. Tags of redundant elements must be identical; only the
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173
positions or reference to the associate physical connection terminals will be
different.
The FF-061 System Architecture for WIO specification describes the link
between the HSE_RIO-TB and a Function Block as being by channel tags.
As shown in Figure 9-3, each I/O Transducer Block contains VAR_NAME*
parameters with which the channel tags align.
Figure 9-3 — Device Mapping Diagram (Channel Mapping of other Protocols to
FF Flexible Function Block)
Transducer
Block #1
HART
Transducer
Block #z
CMD
Response
Profibus
Transducer
Block #w
Q
A
Channel Map
Ch 200
Ch 201
Ch 208
.
.
Flexible
Function
Block #1
RIO Function
Block #1
Transducer
Block #2
Channel Map
Ch 300
Ch 316
.
.
.
Transducer
Block #n
Channel Map
Ch 400
.
.
.
.
Channel List
200 - FT100
316 - AT101
.
201
400
208
.
28
32- or
64-bit
string
Control
As can be seen on the right in Figure 9-3, the specifications make use of the
Flexible Function Block to create a single Block capable of between 4 and
16 bits (22 to 26). The multiple-point combined-structure Boolean input
Function Block (MBI-64) and multiple-point combined-structure Boolean
output Function Block (MBO-64) provide a single-structure communications of a group of 64 Boolean points without status. Similar to the existing
MAO (Multiple Analog Output) Function Block, the multiple-point combined-structure analog output Function Block (MAO-16) receives a single174
NEW DEVELOPMENTS
structure communication of a group of 16 analog points, each with status.
These structures are intended to support the PLC model of programmed
access to an indexed set of values.
Because Remote Input/Output units are made of modular components,
each I/O card and module in the WIO device must be described by a set of
Device Description (DD) files. Each basic component in the RTU is an
extension of the head-end module and the specification defines an Association Block as a special form of the Transducer Block class that is used to
describe the expected configuration of each directly subordinate remote
input/output component, the actual remote input/output component, and
its current status. An Association Block is also used to describe the expected
configuration of each non-native field device, the actual non-native field
device, and its current status. An Association Block may also be used to
group Transducer Blocks associated with a component.
The Association Block is flexible enough to fully define the RTU to have
conventional I/O, specialized I/O, gateways to non-FF I/O, FF H1 links, or
non-I/O functions, such as controllers executing FF Function Blocks. The
Association Block is also used to describe the expected configuration of
each directly subordinate non-native field device, the actual non-native field
device component, and its current status as seen by the gateway. Non-native
field devices may address a single I/O point or multiple I/O points.
The Association Block is intended to associate the modular components
that function in conjunction with a gateway or head-end component. The
Association Block is also intended to associate the external non-native field
devices that function in conjunction with an HSE-RIO component. A third
use of the Association Block is to group Transducer Blocks associated with a
basic component. The Association Block indicates the expected configuration of each of the elements based initially on off-line configuration, indicates each of the actual elements encountered on-line by the unit, and
indicates the status of each element. The actual individual process points
are identified at the Transducer Block level by its (tag) name and are referenced by input or output class FF Function Blocks by that name (via
CHANNEL_TAG or via an element of a CHANNEL_TAG_* array).
Because these are highly modular devices, external viewers, such as operational hosts, configuration tools, and maintenance technician systems, need
NEW DEVELOPMENTS
175
to be able to easily see and understand the hierarchical organization of the
modular device and to allow the technician to quickly identify the relevant
device with its associated position in the slot card/terminal to which it is
physically connected. The WIO specifications include parameters to assist
in making these relationships.
9.3 Wireless
Despite the fact that we have been using wireless technology in SCADA
(Supervisory Control and Data Acquisition) systems for several decades,
industrial wireless connectivity is the “next frontier” of industrial digital
communications. The difference between the new developments and
SCADA systems is that the new products are designed to connect directly
to sensors and to build sensor networks, using ISM (Industrial Scientific
Medical) license-free radio frequency bands within the plant environment.
SCADA systems, on the other hand, are traditionally designed to connect
widely distributed controller systems over licensed radio frequencies or
other “long haul” technologies, such as microwave, satellite, or even conventional land-line telephony.
There have been a number of proprietary networks on the market for
approximately a decade; however, it was only in 2008 that an industrial
standard for plant sensor networks was released. WirelessHART is an industry consortium-led standards activity building on both the widely used
wired HART technology for the upper layers of the OSI (Open System
Interconnect) model and IEEE 802.15.4 wireless network infrastructure for
the lower layers of the OSI model.
The Fieldbus Foundation has also started a new activity to be coordinated
with the HSE RIO activities. The Wireless I/O work is being coordinated
with the Wireless Backhaul activities of the ISA100 committee, which will
implement a standards-based wireless Ethernet backhaul network to replace
the copper or fiber between the HSE node and the control system.
Because Fieldbus H1 and HSE have a requirement to support multicast
communications, most Fieldbus protocols using standard IEEE 802.11
wireless Ethernet are very difficult to implement and do not make sense versus installing a copper or fiber backbone. The WIO and ISA100 cooperative
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NEW DEVELOPMENTS
effort will address this problem as well as the challenges associated with
cybersecurity, quality of service / message prioritization, and coexistence
with other wireless (IEEE 802.15) networks.
9.4 Host System Interoperability
It is not necessary for a Host to have Function Blocks, and in fact the most
widely used generic Host, National Instrument Configurator (as shown in
Figure 9-4, which is a photograph of the USB H1 modem commonly used
to test devices in development or as a generic H1 interface) does not have
any Function Block capability.
Figure 9-4 — National Instruments USB H1 modem
A Host system with an H1 interface should have a Foundation-registered
communication stack and Foundation-conformant physical layer interface.
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177
Hosts that include an HSE interface should have a Foundation-registered
communication stack.
A Host profile defines a minimum set of Foundation-specific features that
must be implemented by a Host to achieve compliance with a specific Host
class as defined by FF-859. At present the Fieldbus Foundation defines the
following Host profile classes:
Class 61 – Integrated Host: Primary, on-process Host that manages the
communications and application configuration of all devices on a network. This is the Class of Host that will be part of a DCS.
Class 62 – Visitor Host: Temporary, on-process Host with limited
access to device parameterization. Typically represented by a handheld device used by maintenance technicians to communicate with
field devices.
Class 63 – Bench Host: Primary off-process Host for configuration
and setup of a non-commissioned device.
Class 64 – Bench Host: Primary off-process Host with limited access
to device parameterization of an off-line commissioned device.
Class 71 – SIF Integrated Host: Primary on-process Host for safety
instrumented functions.
Operators have access to the Integrated Host through operator workstations, while maintenance accesses the Host through plant asset management applications.
Host registration commenced in 2009 with Profile A, and starting in 2010,
all Host systems had to register against Profile B. The following Host features, which were optional in Profile A, are now Mandatory in Profile B.
Block Instantiation – allows for full utilization of Fieldbus devices
supporting instantiable Function Block and is primarily of benefit for
Control-in-Field applications.
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Multiple Capability Levels – for devices where certain blocks or features are optional (licensed), the standard or higher capability level
can be set in the tag placeholder during system configuration to prevent unsupported blocks from being used in a control strategy. This is
intended to prevent surprises during commissioning while also making device replacement easier.
Enhanced Function Blocks – supports the use of enhanced blocks as
defined by individual device manufacturers.
Profiled Custom Function Blocks – full utilization of non-standard
blocks as long as they are defined by the DD file.
Configuration of Scheduled Control Function Blocks – allows developers to build Control-in-Field control strategies.
DD V5.1 Device Level Access – support for enhanced Electronic
Device Description Language (EDDL) with cross-block, which makes
Fieldbus devices easier to use by enabling a dashboard with all diagnostics on the same page, all the setup on one page, the ability to
archive data, plus more, regardless of which block the information
resides on.
Profile B also incorporates the requirement for support of NAMUR NE 107
(http://www.namur.de) field diagnostics, which added a fourth device status
category, “Function Check,” and which is normally used to indicate that a
device is undergoing maintenance.
The requirement for an FF “Check Mark” on Host systems will help alleviate the concerns about Device/Host interoperability that has plagued FF
projects in the past.
NEW DEVELOPMENTS
179
Appendix A — Nomenclature
Description
Units
A
Attenuation
decibels (dB)
Cc
Cable capacitance
nanofarads (nF)
CD
Capacitance of device
nF
Ceq
Equivalent capacitance
nF
Ci
Residual capacitance
nF
CS
Capacitance of wire for network
nF
CT
Total capacitance of the network
nF
E
Error in process loop (setpoint–PV)
Process units
fr
Frequency range
hertz (Hz)
ID
DC current draw of field device with largest minimum voltage
milliamps (mA)
Ii
Interconnection current
mA
Ik
Maximum network current
mA
IN
Remaining or net current on net- work
mA
Io
Power supply current
mA
Kp
Process gain
Dimensionless
Lc
Cable inductance
millihenries (mH)
Li
Residual inductance
mH
LMaxx
Maximum length of cable x
M
LMaxy
Maximum length of cable y
M
LS
Spur cable length
meters (m)
LT
Trunk/home-run cable length
m
LTMAX
Maximum voltage of trunk cable
volts (V)
Lx
Length of cable x
M
Ly
Length of cable y
M
NC
Number of communications with the HMI
NP
Number of Publishers (devices on the
network)
Pi
Interconnection power
Watts (W)
Po
Power supply power
W
Rc
Cable resistance
ohms/km (Ù /km)
NOMENCLATURE
181
RS
Spur cable resistance
Ù/km
RT
Trunk/home-run cable resistance
Ù/km
tLOAD
Time to execute all Function Blocks in
loop
Milliseconds (ms)
Ui
Interconnection voltage
Volts
Uo
Power supply voltage
Volts
Us
Maximum intrinsically safe voltage
Volts
VD
DC voltage available at the field device V
VMin
Minimum voltage at device
V
VMIN
Largest minimum voltage of all field
devices
V
VPS
Power supply voltage
V
182
NOMENCLATURE
Appendix B — Fieldbus
Foundation Specification List
FF-061 Foundation Specification – System Architecture for WIO
FF-103 Foundation Specification - Common File Format
FF-131 Foundation Specification - Standard Tables
FF-569 Foundation Specification - Host Interoperability Support Test
Profile and Procedures
FF-581 Foundation Specification - System Architecture
FF-586 Foundation Specification - HSE Presence
FF-588 Foundation Specification - Field Device Access (FDA) Agent
FF-589 Foundation Specification - HSE System Management
FF-593 Foundation Specification - High Speed Ethernet Redundancy FF-801
Foundation Specification - Network Management
FF-633 Foundation Specification - Function Block Application Process
WIO – Part 6
FF-801 Foundation Specification - H1 Network Management
FF-803 Foundation Specification - HSE Network Management
FF-806 Foundation Specification - Data Link Protocol Specification Bridge
Operation Addendum
FF-816 Foundation Specification - 31.25 kbits/sec Physical Layer Profile
FF-821 Foundation Specification - Data Link Services Subset
FIELDBUS FOUNDATION SPECIFICATION LIST
183
FF-822 Foundation Specification - Data Link Protocol Specification FF-831
Fieldbus Power Supply Specification
FF-830 Foundation Specification - 31.25 kbits/sec Physical Layer
Conformance Test
FF-831 Foundation Specification - Fieldbus Power Supply Test Specification
FF-844 Foundation Specification - H1 Cable Test Specification
FF-846 Foundation Specification - Foundation Device Coupler Test
Specification Phase 1
FF-870 Foundation Specification - Fieldbus Message Specification
FF-875 Foundation Specification - Fieldbus Access Layer (Services and
Protocol)
FF-880 Foundation Specification - System Management
FF-883 Foundation Specification - System Management Addendum for
Software Download
FF-890 Foundation Specification - Function Block Application Process Part 1
FF-891 Foundation Specification - Function Block Application Process Part 2
FF-892 Foundation Specification - Function Block Application Process Part 3
FF-893 Foundation Specification - Function Block Application Process Part 4
FF-894 Foundation Specification - Function Block Application Process Part 5
FF-900 Foundation Specification - Device Description Language
FF-901 Foundation Specification - DDL Interoperability Specification
FF-902 Foundation Specification - Transducer Block Common Structure
184
FIELDBUS FOUNDATION SPECIFICATION LIST
FF-903 Foundation Specification - Pressure Transducer Block
FF-904 Foundation Specification - Temperature Transducer Block
FF-906 Foundation Specification - Advanced Valve Positioner Transducer
Block
FF-908 Foundation Specification - Flow and Flow Totalizer Transducer Block
FF-912 Foundation Specification - Standard Diagnostic Profile
FF-940 Foundation Specification - Communication Profile
FF-941 Foundation Specification - HSE Profile
FF-946 Foundation Specification - Device ITK Profile
FIELDBUS FOUNDATION SPECIFICATION LIST
185
Appendix C — Bibliography
C.1 Printed Materials
Berge, J., Fieldbuses for Process Control: Engineering, Operation, and Maintenance, ISA, Research Triangle Park, NC, 2001.
Fieldbus Application Guidelines for the Process Industry, Engineering Equipment
and Material Users Association, London, UK, 1997.
Fieldbus Book, A Tutorial, Technical Information bulletin, TI.38K02A01-01E,
Yokogawa Electric Corporation, Tokyo, Japan, May 2001.
Fieldbus Preliminary Application Note on Intrinsic Safety, Revision 1.1, Fieldbus
Foundation, Austin, TX, 1995.
Fieldbus Wiring Design and Installation Guide (Available at
www.relcominc.com), Relcom Inc., Forest Grove, OR.
Fieldbus Troubleshooting Guide (Available at http://www.relcominc.com/fieldbus/fbapnotes.htm), Relcom Inc., Forest Grove, OR.
Foundation Fieldbus Discovery Course, Southern Alberta Institute of Technology, Calgary, Alberta, 2001.
Goeldner, H.D., Johansmeyer, U., Schebstadt, F. and Storck, H. PTB W39
and 53a, Wirtschaftsverlag NW, Bremerhaven, Germany, 1989.
IEC 61158-2 (2007-12), Industrial communication networks - Fieldbus specifications - Part 2: Physical layer specification and service definition, International
Electrotechnical Commission, Chicago, IL, 2007.
IEC 65C/178/CDU–IEC 61158–3, Data Link Layer–DLL Service. Part 3,
International Electrotechnical Commission, Chicago, IL, 1999.
BIBLIOGRAPHY
187
IEC 65C/179/CDU–IEC 61158–4, Data Link Layer–DLL Protocol. Part 4,
International Electrotechnical Commission, Chicago, IL, 1999.
ISA-TR50.02, Part 9-2000, Fieldbus Standard for Use in Industrial Control Systems: User Technical Report, ISA, Research Triangle Park, NC, 2000.
AN9027 FNICO Non-Incendive Fieldbus System, MTL Instruments Application Note, June 2004.
Fieldbus Foundation Application Guides
AG-140 Wiring and Installation 31.25 kbit/s, Voltage Mode, Wire Medium
Application Guide, Fieldbus Foundation, Austin Texas, 1996.
AG-163 31.25kbit/s Intrinsically Safe Systems Application Guide, Fieldbus
Foundation, Austin Texas, 1996.
AG-181 Revision 3.1, FOUNDATION™ Fieldbus System Engineering Guidelines, Fieldbus Foundation, Austin Texas, 2010.
C.2 Web Sites
www.fieldbus.org
www.isa.org/fieldbus
www.iceweb.com.au/fieldbus
www.wib.nl
http://ourworld.cs.com/rahulsebos
www.namur.de
www.iaona-eu.com
188
BIBLIOGRAPHY
C.3 Fieldbus Certified Training Centers
Waseda University
Kitakyushu, Japan
Shipping & Transport College, Brielle
Brielle, The Netherlands
Lee College
Baytown, Texas
Southern Alberta Institute of Technology
Calgary, Alberta
King Mongkuts Institute of Technology
Ladkrabang, Thailand
SINOPEC Yanshan Simulation Center (YSC) Beijing, China
Trine University
Indianapolis, Indiana USA
University of Miskolc
Egyetemváros Hungary
C.4 Free Segment Design Tools
Application, Designmate software, Fieldbus Foundation –
http://www.fieldbus.org under the Technical Resources page.
Excel Spreadsheet, “FFSegCheck” – Yokogawa Corporation
http://www.yokogawa.com/fbs/fbs-download-en.htm
Excel Spreadsheet and Application, MTL Instruments –
http://www.mtl-inst.com/products/soft-tools/soft_tools.htm
Excel Spreadsheets, Hawke Systems –
http://www.ehawke.com/fieldbus/fieldbus_download.htm
Application, Pepperl+Fuchs,
http://www.segmentchecker.com
Application, Emerson Process Management “Segment Checker,”
www.emersonprocess.com/systems/support/segment
BIBLIOGRAPHY
189
Appendix D — Acronyms
D.1 Acronyms
Acronym
Description
non-FF Acronyms in Book
AWG
American Wire Gauge
DCS
Distributed Control System
DCS
Digital Control System
HMI
Human Machine Interface
I/O
Input/Output
IANA
Internet Assigned Numbers Authority
IEC
International Electrotechnical Commission
IEEE
Institute of Electrical and Electronics Engineers
IETF
Internet Engineering Task Force
IGMP
Internet Group Management Protocol
IP
Internet Protocol
IS
Intrinsically Safe
ISA
International Society of Automation
ISM
Industrial Scientific Medical
ISO
International Standards Organization
mA
milliamps
NIS
Non-Intrinsically Safe
OLE
Object Linking and Embedding
OPC
OLE for Process Control
PTB
Physikalisch Technische Bundesanstalt
SCADA
Supervisory Control And Data Acquisition
SP
Set Point
VDC
Volts Direct Current
XML
eXtensible Mark-up Language
ACRONYMS
191
Acronym
Description
FF Acronyms in Book
A
Arithmetic
AA
Analog Alarm
AHI
Analog Human Interface
AI
Analog Input
AO
Analog Output
Auto
Automatic
BG
Bias/Gain Station
C
Calculate
CAO
Complex Analog Output
Cas
Cascade
CDO
Complex Discrete Output
CS
Control Selector
DA
Digital Alarm
DC
Device Control
DD
Device Description
DHI
Digital Human Interface
DI
Discrete Input
DL
Data Link
DO
Discrete Output
DT
Deadtime
DTM
Device Type Manager
FDI
Field Device Interface
FDT
Field Device Tool
FFB
Flexible Function Block
FISCO
Fieldbus Intrinsically Safe Concept
H1
Fieldbus Low Speed Data Network
HSE
High Speed Ethernet
Iman
Initialization Manual
IP
Internet Protocol
IS
Input Selector
IT
Integrator (Totalizer)
LAS
Link Active Scheduler
LL
Lead Lag Controller
LM
Link Master
LO
Local Override
LS
Link Scheduling
192
ACRONYMS
Acronym
Description
FF Acronyms in Book
MAC
Management Access Control
MAI
Multiple Analog Input Block
Man
Manual
MAO
Multiple Analog Output Block
MDI
Multiple Discrete Input Block
MDO
Multiple Discrete Output Block
MIB
Management Information Base
ML
Manual Loader
OOS
Out Of Service
OS
Output Splitter Block
PD
Proportional Derivative
PD
Process Data
PI
Pulse Input
PID
Proportional Integral Derivative
RA
Ratio
RB
Resource Block
RCas
Remote Cascade
ROut
Remote Output
RX
Receive/Receiver
SC
Signal Characterizer
SOPID
Step Output PID
SRG
Set Point Ramp Generator
SS
Signal Splitter
STP
Shielded Twisted Pair(s)
T
Timer
TCP
Transmission Control Protocol
TD
Transducer Block
TX
Transmit/ Transmitter
UDP
User Datagram Protocol
UTP
Unshielded Twisted Pair(s)
VCR
Virtual Communication Resource
VFD
Virtual Field Device
XD
Transducer
ACRONYMS
193
D.2 FOUNDATION Fieldbus Acronyms
Acronym
Description
Ack
Acknowledge
AE
Application Entity
AI
Analog Input
AO
Analog Output
AP
Application Program
AP
Application Process
APDU
Application Layer Protocol Data Unit
AR
Application Relationship
AREP
Application Relationship End Point
ARP
Ethernet Address Resolution Protocol
ARPM
Application Relationship Protocol Machine
ASN.1
Abstract Syntax Notation 1
BOOTP
Bootstrap Protocol
CAP
Control Application Process
Cas
Cascade
CD
Compel Data
CFF
Capability File Format
CFF
Common File Format
CNF
Confirmation
CNLS
Connectionless
COM
Communication
Config
Configuration
CONN
Connection-Oriented
CSMA/CD
Carrier Sense Multiple Access / Collision Detect
DAP
Device Application Process
DCS
Distributed Control System / Digital Control System
DD
Device Description
DDL
Device Description Language
DevId
Device Identifier
DHCP
Dynamic Host Configuration Protocol
DI
Discrete Input
DL
Data Link
DLCEP
Data Link Connection End Point
DLL
Data Link Layer
Dlme
Data Link Layer Management Entity
DLPDU
Data Link Protocol Data Unit
194
ACRONYMS
Acronym
Description
DLSAP
Data Link Service Access Point
DMA
Direct Memory Access
DNS
Do Not Select
DO
Discrete Output
DPS
Draft Preliminary Specification
DS
Data Structure
DTC
Data Transfer Confirmed
DTU
Data Transfer Unconfirmed
DUT
Device Under Test
DV
Dynamic List of Variable List
EFD
(High Speed) Ethernet Field Device
EGP
Exterior Gateway Protocol
EP
Ethernet Presence
EPI
Ethernet Presence Interface
ERR
Error
EU
Engineering Unit
FAS
Fieldbus Access Sublayer
FB
Function Block
FBAP
Function Block Application Process
FDA
Field Device Access
FDA
Field Device Application (Agent)
FDA
Field Device Access Agent
FDL
Fieldbus Data Link Layer
FF
Fieldbus Foundation
FFB
Flexible Function Block
FMS
Fieldbus Message Specification
FMS
Fieldbus Messaging System
FS
Final Specification
FSA
Fail-Safe Active
FSA
Fault Statefail -Safe Active
FSPM
FDA Service Protocol Machine
Gen
Generic
H1
Hunk 1
HFD
HSE Field Device
HL
Link Designator of a four-octet LONG DL-address
HL.N.S
Four-octet LONG DL-address
HLD
Higher Level Device
ACRONYMS
195
Acronym
Description
HMA
HSE Management Agent
HSE
High Speed Ethernet
IA
Initialization Acknowledge
ICMP
Internet Control Message Protocol
ICS
Implementation Conformance Statement
ID
Identifier
Id
Identifier
IMan
Initialization Manual
IND
Indication
INFO
Informational
IR
Initialization Request
IUT
Implementation Under Test
LAS
Link Active Scheduler
LD
Linking Device
LDM
Last Delivered Message
LLC
Logical Link Control
LME
Layer Management Entity
LO
Local Override
LOP
Local Operator Panel
LRE
LAN Redundancy Entity
LS
Link Schedule
LSB
Least Significant Bit
LUV
Last Usable Value
MAC
Medium Access Layer
MAC
Medium/Media Access Control
MAI
Multiple Analog Input
MAO
Multiple Analog Output
MAU
Medium Attachment Unit
Max.
Maximum
Mb/s
Megabits per second
MDI
Multiple Digital Input
MDI
Multiple Discrete Input
MDO
Multiple Digital Output
MDO
Multiple Discrete Output
MDS
Medium Dependent Sublayer
Mfr.
Manufacturer
MIB
Management Information Base
196
ACRONYMS
Acronym
Description
MIB-II
Management Information Base II
MSB
Most Significant Bit
Msg
Message
MVC
Multi-Variable Container object
N.S.
Sub-link Selector of a four-octet LONG DL-address
NI
Not Invited
NM
Network Management
NMA
Network Management Agent
NMIB
Network Management Information Base
NST
Network Status Table
NTP
Network Time Protocol
OD
Object Description
OD
Object Dictionary
OSI
Open System Interconnection
PD
Physical Device
PD Tag
Physical Device Tag
PDU
Protocol Data Unit
PhPDU
Physical Layer Protocol Data Unit
PHY
Physical Layer
PICS
Protocol Implementation Conformance Statement
PID
Proportional Integral Derivative (control algorithm)
PLC
Programmable Logic Controller
PN
Probe Node PDU
PR
Probe Response PDU
PROP
Proposed
PS
Preliminary Specification
PT
Pass Token
RCas
Remote Cascade
RED
Redundant
Ref
Reference
REQ
Request
RFC
Request For Comments
RFU
Reserved for Future Use
ROut
Remote Output
RSE
Remote Single Layer Embedded
RSP
Response
RT
Return Token
ACRONYMS
197
Acronym
Description
SM
System Management
SMI
Structure and Identification of Management Information
SMIB
System Management Information Base
SMK
System Management Kernel
SMKP
System Management Kernel Protocol
SMKPM
System Management Kernel Protocol Machine
SMPM
Socket Mapping Protocol Machine
SNMP
System Network Management Protocol
SNTP
Simple Network Time Protocol
STD
Standard
SUT
System Under Test
TCP
Transmission Control Protocol
TCP/IP
Transmission Control Protocol / Internet Protocol
TD
Time Distribution
TOS
Type Of Service (IP definition)
TTCN
Tree & Tabular Combined Notation
TTL
Time To Live (IP definition)
UDP
User Datagram Protocol
VCR
Virtual Communication Relationship
VCRL
Virtual Communication Relationship List
VFD
Virtual Field Device
198
ACRONYMS
Appendix E — FF Segment Design
Example Exercise
As we proceed through this example exercise, which is based on a hypothetical process, remember that there is no single correct answer for any design
because the solution is a function of physical layout, control philosophy
(especially with regard to Control-in-Field and interacting loops), macrocycle length, and Area Classification.
E.1 Project Drawings and Specifications
The first requirement for any design is gathering the basic background
information, such as specifications and documentation/drawings. Since that
is the case, that is how we will start as well.
Figure E-1 shows a simplified P&ID of a typical distillation tower with its
steam boiler, which will be used for this exercise. Note that we have not
shown the majority of local indicators, pressure relief/safety valves, or
pump controls.
Table E-1 shows the associated Instrument Index for the distillation tower
in Figure E-1.
Also required is a Plot Plan, and ideally an Instrument Location drawing as
well as the Area Classification drawing. These three drawings are shown in
Figures E-2, E.-3 and E-4.
FF SEGMENT DESIGN EXAMPLE EXERCISE
199
Figure E-1 — Simplified P&ID of a distillation tower
Vessel
Service
C-1
Distillation Column
Simplified P&ID
V-1
Reflux Accumulator
E-1
Reboiler
E-2
Feed Heat Exchanger
E-3
Feed Preheat Exchanger
FC
102
FV
102
TJR
101
TJT
101
PDIT
102
E-4
TIT
105
FC
100
FV
100
SP
TIT
100
E-1
TI
108
Reflux Pump
P-3
Feed Pump
Top Product
LV
101
LC
101
Cooling Water
TIT
103
TIT
104
E-3
E-2
FIT
101
TE
101B
TC
100
Bottom Product Pump
P-2
C-1
TE
101C
PT
100
я FIT
100
FE
100
P-1
TI
109
TE
101E
TE
101D
Ovhd Condensor
TI
107
TIT
106
TE
101H
TE
101F
Service
E-4
я FIT
104
я FIT
102
TIT
102
TE
101G
Vessel
FV
101
FC
101
TI
110
DIT
101
TE
101A
LIT
101
V-1
LV
102
LC
102
LIT
102
PI
101
P-1
Feed
я FIT
103
P-2
P-3
Bottom
Product
Steam Out
Figure E-2 — Fieldbus system design – plot plan
Fieldbus System Design – Plot Plan
Interface
Room
N
300 m
Distillation Operating Unit : 01-1
NOTES:
E-1 / E-2 and E3 / E-4 are
stacked above
each other. E-2
and E-4 on
upper level
Height
between levels
of platform is
4.5 m
C-1 height is 8
m.
V-1
C-1
E-1
&
E-2
E-3
&
E-4
P-2
P-1
P-3
Pipe rack
V-1 height is 3
m
200
FF SEGMENT DESIGN EXAMPLE EXERCISE
Figure E-3 — Instrument location drawing
Elev
8m
TIT
102
TI
Elev 109
5m
TI
110
TIT
105
FIT
101
FCV
100
C-1
PDIT
102
TJT
101
TIT
100
LIT
102
Elev
5m
E-1
&
E-2
Elev
5m
FIT
102
FCV
101
TIT
106
FIT
104
P-1
FIT
100
P-2
P-3
LCV
101
PI
100
FIT
103
LIT
101
TIT
103
E-3
&
E-4
FCV
102
LCV
102
V-1
TI
Elev 107
5m
TI
108
TIT
104
Pipe rack
0
1
2
3
metres
4
Figure E-4 — Fieldbus system design – area classification
N
Fieldbus System Design – Area Classification
TI
Elevv 109
5m
m TI
110
TIT
105
E-1
&
E-2
LCV
Elevv 102
5m
FIT
103
FCV
100
FIT
100
FIT
101
Elev PDIT
8m 102
TIT
102
TJT
101
C-1
TIT
100
LIT
102
V-1
TI
TIT
104
FIT
Elev 102
5m FCV
102
Elevv 107
5m
m TI
TIT
103
108
FCV
101
E-3
&
E-4
LIT
101
TIT
106
FIT
104
P-1
P-2
P-3
LCV
101
PI
100
Pipe rack
Zone IIB (Class 1 Division 2 Group D)
Distillation Operating Unit: 01-1
FF SEGMENT DESIGN EXAMPLE EXERCISE
Zone IB (Class 1 Division 1 Group D)
0
1
2 3
metres
4
201
Processes, by their nature, are prone to interaction between loops. For
example, a change in level can easily affect the associated flow rate. There
are two schools of thought (two design approaches) as to how to group
devices on segments on interacting loops in Fieldbus implementations
depending on if they are dependent or independent.
Loops are called dependent when the two valves or process control strategies
have the ability to shut down the same piece of process equipment. Conversely, independent means that these two valves or process control strategies
affect different pieces of process equipment.
One design approach that can be used is to group interacting loops on the
same segment to enable control-in field as much as possible. The risk here is
that should control of the segment be interrupted, there is no way to interact with and control the process other than the next layer of protection,
such as ESD (Emergency Shut Down) or mechanical methods.
The second approach places interacting or dependent loops on separate segments. This strategy provides operators with a means whereby they might
be able to recover some form of control in the event of a segment failure.
We have chosen to use this second approach in our segment layout for the
overhead reflux line and final control elements FCV-102 (reflux return to
tower) and LCV-101 (overhead product).
Table E-1 is the Instrument Index for this project and includes the assigned
segment numbers in the Instrument Segment Diagram (ISD) column as
well as the associated Data Sheet (DS) number for each field device. The
calculations in the balance of this Appendix will confirm that the assignments are compliant with the project specifications.
Table E-1 — Instrument Index
Tag No.
Service
I/O Type
ISD
D.S
01-FIT-100
Steam Flow to Reboiler
H1
01-Seg-1
01-1-100
01-PT-100
Reboiler E-1 Steam Pressure
01-FV-100
Reboiler E-1 Steam Flow Control H1
01-Seg-1
01-1-101
01-TIT-100
Distillation Tower C-1 Bottoms
Temperature
01-Seg-1
01-1-102
202
H1
FF SEGMENT DESIGN EXAMPLE EXERCISE
01-TJT-101
Distillation Tower C-1 Tray Temperatures
H1
01-Seg -2
01-1-103
01-TE-101
Distillation Tower C-1 Tray Thermocouples
mV
N/A
01-1-104
01-TIT-102
Distillation Tower C-1 Overhead
Temperature
H1
01-Seg-1
01-1-105
01-FIT-102
Reflux Flow to Distillation Column C-1
H1
01-Seg-2
01-1-106
01-FCV-102
Reflux Flow Control to Distillation H1
Column C-1
01-Seg-2
01-1-107
01-PDIT-102
Distillation Column C-1 Differen- H1
tial Pressure
01-Seg-1
01-1-108
01-LIT-102
Distillation Column C-1 Bottoms
Level
H1
01-Seg-1
01-1-108
01-LCV-102
Distillation Column C-1 Bottoms
Level Controller
H1
01-Seg-1
01-1-109
01-FIT-103
Distillation Column C-1 Bottoms
Product Flow
H1
01-Seg-1
01-1-110
01-FIT-101
Distillation Column C-1 Mass
Feed Flow
H1
01-Seg-1
01-1-111
01-DIT-101
Distillation Column C-1 Feed
Density
01-FCV-101
Feed Flow Controller
H1
01-Seg-3
01-1-124
01-TIT-105
Distillation Column C-1 Feed
Temperature
H1
01-Seg-1
01-1-112
01-TIT-106
Reflux Temperature
H1
01-Seg-3
01-1-117
01-TIT-103
Feed PreHeat Exchanger E-3
Inlet Temperature
H1
01-Seg-2
01-1-118
01-TIT-104
Feed PreHeat Exchanger E-3
Outlet Temperature
H1
01-Seg-2
01-1-119
01-FIT-104
Overhead Product Flow
H1
01-Seg-3
01-1-120
01-LIT-101
Reflux Accumulator V-1 Level
H1
01-Seg-3
01-1-121
01-LCV-101
Reflux Accumulator V-1 Level
Control
H1
01-Seg-3
01-1-122
01-PI-100
Reboiler E-1 Steam Pressure
Gauge
Local
N/A
01-1-123
01-TI-107
Cooling Water Inlet Temperature Local
Overhead Condenser E-4
N/A
01-1-113
01-TI-108
Cooling Water Outlet Temperature Overhead Condenser E-4
Local
N/A
01-1-114
01-TI-109
Bottoms Temperature to Feed
Heat Exchanger E-2
Local
N/A
01-1-115
01-TI-110
Bottoms Temperature from Feed Local
Heat Exchanger E-2
N/A
01-1-116
FF SEGMENT DESIGN EXAMPLE EXERCISE
203
E.2 Design Basis
Many project leaders will select one of two methods as the basis for the
Physical Layer of their design:
• Actual voltage drop calculations for each segment
• Worst-case segment(s) and actual voltage drop calculations for any
exceptions to the resulting guidelines.
We will use both methods in this example to demonstrate how each might
be implemented.
Because we will be using the same instruments on the different segments,
the Macrocycle calculations for both of the above cases will be similar and
therefore will only be done for the actual voltage drop case.Once we have
determined which devices are assigned to which segment we will then have
Function Block execution time information on the device from the individual manufacturers on which to base this calculation.
Basic information on which the design will be based is as follows:
1.
Home Run trunk length: 300 meters
2.
Maximum of 12 devices per segment (this is less than the maximum
of 16 devices supported by most hosts, to allow room for potential
future expansion)
3.
Maximum of two control loops per segment
4.
Minimum voltage at Fieldbus device: 11 volts (this is above the FF
minimum of 9 volts, to allow room for potential future expansion)
5.
Control–in-Field
6.
Macrocycle: 1 second
7.
Compel Data: 25 milliseconds
8.
FF Power Supply: Stahl Series 9412 with integrated advance diagnostics, 24 VDC output, 500 mA
204
FF SEGMENT DESIGN EXAMPLE EXERCISE
9.
Field Device Coupler: Pepperl+Fuchs R2-SP-N12; short-circuit current = 58 mA
10.
Current load of Device Coupler: 4.5 mA
11.
Current load of Host: 20 mA
12.
Current Load of Handheld: 10 mA
13.
Resistance of cable: 44 Ohm/km (return) (Type “A”)
The basis for the “Worst Case” design, which will apply to the complete
plant and not just this unit operation, is based on the following additional
assumptions:
14.
Spurs: 120 m (worst case as per FF Specification)
15.
Current load of Field Device: 20 mA (median for all FF devices is 17
mA)
16.
current load of Final Control Element: 24 mA
The manufacturer, model number, current load and Function Block macrocycle time for each of the devices are summarized in Table E-2, with the
font coding (italics, bold, or underlined) shown for each of the control
loops on each of the three segments.
To minimize cable installation costs, a four pair cable is run from the Interface Room to a conventional junction box located in the approximate center of the Distillation Operating Unit in the pipe rack near the “south” end
of the E-3 and E-4 heat exchangers as shown in Figure E-5.
FF SEGMENT DESIGN EXAMPLE EXERCISE
205
Table E-2 — Device characteristics summary
Current FB1 (msec)
(mA)
Tag
Manufacturer Model
01-FIT-100
Yokogawa
EJX
15
AI 30
01-PT-100
Yokogawa
EJX
15
AI 30
01-FV-100
Fisher
DVC 6000
19
AO 25
01-TIT-100
ABB
TTX 300
12
AI 10
01-TJT-101
Pepperl+Fuchs
F2DO-TIEX8
26
MAI 40
01-TIT-102
ABB
TTX 300
12
AI 10
01-FIT-102
Yokogawa
EJX
15
AI 30
01-FCV-102
Fisher
DVC 6000
19
AO 25
PID 30
01-PDIT-102
Yokogawa
EJX
12
AR 30
AI 30
01-LIT-102
Yokogawa
EJX
12
AI 30
01-LCV-102
Fisher
DVC 6000
19
AO 25
01-FIT-103
Yokogawa
EJX
12
AI 30
01-FIT-101
Endress+Hau
ser
ProMass 83
11
AI 18
AI 18
01-FCV-101
Fisher
DVC 6000
19
AO 25
PID 30
01-TIT-105
ABB
TTX 300
12
AI 10
01-TIT-106
ABB
TTX 300
12
AI 10
01-TIT-103
ABB
TTX 300
12
AI 10
01-TIT-104
ABB
TTX 300
12
AI 10
01-FIT-104
Yokogawa
EJX
15
AI 30
01-LIT-101
Magnetrol
705 3x
15
AI 15
01-LCV-101
Fisher
DVC 6000
19
AO 25
206
FB2 (msec)
PID 30
PID 30
PID 30
FF SEGMENT DESIGN EXAMPLE EXERCISE
Figure E-5 — Junction box location drawing
FF JB
Seg 1
Elev
8m
TIT
102
TI
Elev 109
5m
TI
110
TIT
105
FCV
100
E-1
&
E-2
Elev
5m
FIT
101
C-1
FIT
100
TJT
101
TIT
104
TIT
100
LIT
102
Elev
5m
FIT
102
FCV
102
FF JB
Seg 1
LCV
102
PDIT
102
FF JB
Seg 2
TI
Elev 107
5m
TI
108
V-1
FCV
101
E-3
&
E-4
TIT
106
FIT
104
P-1
P-2
P-3
LCV
101
PI
100
FIT
103
LIT
101
TIT
103
FF JB
Seg 3
Field JB
Pipe rack
0
1
2
3
metres
4
Figures E-6, E-7, and E-8 show the Instrument Segment Diagrams for each
of the segments.
FF SEGMENT DESIGN EXAMPLE EXERCISE
207
Figure E-6 — Instrument Segment Drawing 01-Seg-1
Field
Field Junction Box
01-LIT-102
RCP001
01-LCV-102 /
01-FJB-01-3
01-FCV-100 /
01-FJB-01-7
01-FIT-103
+
–
01-FIT-103 /
01-FJB-01-6
01-FJB-01-3 /
01-LCV-102
01-TIT-100
01-TIT-100 /
01-FJB-01-5
+
–
01-PDIT-102 /
01-FJB-01-1
01-PDIT-102
–
–
+
–
–
7
4
8
–
01-FJB-01-6 /
01-FIT-103
01-FJB-01-7 /
01-FCV-100
S
RPC001-Fieldbus / 01-JB-01
–
G
Trunk In
1
3
Primary Power Supply
+
–
G
UPS002 /
Ckt 13
L
N
G
WPS002
+
24 VDC –
+
G
2
–
G
T
+
G
–
Pair 1
+
+
–
+
–
01-FJB-01-5 /
01-TIT-100
01-FJB-01-2
–
+
–
RPC001-B / WPS001
G
Trunk Out
01-FJB-01-2 /
01-PDIT-102
01-FJB-01-2 /
01-TIT-102
+
–
3
+
G
01-TIT-102 /
01-FJB-01-2
–
G
G
01-FJB-01-2 TIn /
01-FJB-01TOut
–
+
6
+
–
+
–
2
+
–
G
T
G
+
01-TIT-102
5
01-FJB-01 TOut /
01-FJB-01-2 TIn
RPC001-A / WPS002
G
+
1
+
G
01-FJB-01-4 /
01-FIT-100
+
–
+
G
01-FJB-01-2 /
01-LIT-102
–
+
–
+
–
WPS001 /
RPC001-B
+
–
S
Secondary
H1 Card
Controller: 02
Card: 05
Slot: 01
Secondary Power Supply
+
4
G
G
UPS001 /
Ckt 12
L
N
G
WPS001
24 VDC
WPS002 /
RPC001-A
01-FCV-100
G
Trunk In
–
01-FJB-01-1 /
01-TIT-105
Trunk Out
01-FIT-100 /
01-FJB-01-4
–
T
01-FJB-01 Tin / 01-FJB-01
01-JB-01
+
+
+
01-FIT-100
+
–
+
01-LCV-102
+
–
Port 4
RPC001-Host / 02-04-01 P1
12
+
–
Primary
H1 Card
Controller: 02
Card: 04
Slot: 01
01-JB-01 /
RCP001 / Pair 1
1
2
3
4
Port 3
01-LIT-102 /
01-FJB-01-2
01-JB-01 /
01-FJB-01 TIn
Port 2
+
–
Host I/O
Port 1
+
–
01-TIT-105 /
01-FJB-01-1
Marshalling
01-JB-01
01-TIT-105
+
–
Segment 1 has a secondary Fieldbus junction box mounted near the top of
the distillation column for PDIT-102 and TIT 102.
208
FF SEGMENT DESIGN EXAMPLE EXERCISE
Figure E-7 — Instrument Segment Drawing 01-Seg-2
Field
Field Junction Box
01-FCV-102
RCP001
RCP001-Host / 02-04-01 P2
01-FCV-101 /
01-FJB-01-3
01-TIT-103
+
–
01-TIT-103 /
01-FJB-01-6
+
–
01-FJB-02-3 /
01-FCV-101
01-TJT-100 /
01-FJB-02-5
–
–
5
–
–
+
6
–
G
G
+
+
–
3
7
4
8
G
+
–
01-FJB-02-5 /
01-TJT-100
–
01-FJB-02-6 /
01-TIT-103
01-FJB-02-7 /
01-TIT-104
–
S
Pair 2
RPC001-Fieldbus / 01-JB-01
G
+
–
+
–
RPC001-B / WPS001
G
T
2
+
–
G
+
1
+
G
01-FJB-02-4 /
01-FIT-101
01-TJT-101
+
G
01-FJB-02-2 /
01-FCV-102
RPC001-A / WPS002
+
–
G
Primary Power Supply
+
–
UPS002 /
Ckt 13
L
N
G
WPS002
+
24 VDC –
+
–
Secondary Power Supply
+
–
UPS001 /
Ckt 12
L
N
G
WPS001
WPS001 /
RPC001-B
01-TIT-104 /
01-FJB-01-7
+
–
+
–
+
–
Secondary
H1 Card
Controller: 02
Card: 05
Slot: 01
WPS002 /
RPC001-A
+
–
G
Trunk In
–
01-FJB-02-1 /
01-FIT-102
01-TIT-104
S
T
Trunk Out
01-FIT-101 /
01-FJB-01-4
–
01-FJB-02 Tin / 01-FJB-02
01-JB-01
+
+
+
01-FIT-101
+
–
+
12
01-FCV-101
+
–
Primary
H1 Card
Controller: 02
Card: 04
Slot: 01
01-JB-01 /
RCP001 / Pair 2
Port 4
3
4
5
6
7
Port 3
01-FCV-102 /
01-FJB-01-2
01-JB-01 /
01-FJB-02 TIn
Port 2
+
–
Host I/O
Port 1
+
–
01-FIT-102 /
01-FJB-01-1
Marshalling
01-JB-01
01-FIT-102
+
24 VDC –
Segment 2 with most of the instruments associated with the heat exchangers
in the “center” of the unit has a single Fieldbus junction box near the center
of the E-3 and E-4 heat exchangers.
FF SEGMENT DESIGN EXAMPLE EXERCISE
209
Figure E-8 — Instrument Segment Drawing 01-Seg-3
Field
Field Junction Box
01-LIT-101
+
–
G
Trunk In
–
01-FJB-03-1 /
01-TIT-106
+
–
S
T
+
–
+
–
+
–
01-FJB-03-3 /
01-LCV-101
5
2
–
+
3
7
4
8
S
–
Pair 3
RPC001-Fieldbus / 01-JB-01
G
+
G
–
–
G
+
–
+
–
–
G
–
+
–
RPC001-B / WPS001
G
+
6
G
01-FJB-03-4 /
01-FIT-104
+
–
G
T
+
–
RPC001-A / WPS002
Secondary
H1 Card
Controller: 02
Card: 05
Slot: 01
+
1
G
01-FJB-03-2 /
01-LIT-101
Trunk Out
01-FIT-104 /
01-FJB-01-4
–
01-FJB-03 Tin / 01-FJB-03
01-JB-01
+
+
+
01-FIT-104
+
–
+
01-LCV-101 /
01-FJB-01-3
+
–
Port 4
RPC001-Host / 02-04-01 P3
01-LCV-101
+
–
Primary
H1 Card
Controller: 02
Card: 04
Slot: 01
01-JB-01 /
RCP001 / Pair 3
Port 3
01-LIT-101 /
01-FJB-03-2
6
7
8
9
10
11
12
Port 2
+
–
01-JB-01 /
01-FJB-03 TIn
Host I/O
RCP001
Port 1
+
–
01-TIT-106 /
01-FJB-03-1
Marshalling
01-JB-01
01-TIT-106
+
–
G
UPS002 /
Ckt 13
L
WPS002
+
24 VDC –
N
G
+
–
Secondary Power Supply
+
–
UPS001 /
Ckt 12
L
WPS001
N
G
WPS002 /
RPC001-A
Primary Power Supply
+
–
WPS001 /
RPC001-B
+
–
+
24 VDC –
Segment 3 has a single Fieldbus junction box near Pump 2 and is mounted
on the reverse side of the pipe rack support from the pull through junction
box.
E.3 Worst-Case Calculation Physical Layer
For this example we will assume 12 devices with 2 control loops (valves) and
10 transmitters and will base our calculation on the assumption that we
have the load installed at the end of the longest spur to simulate the highest
possible voltage drop.
Vd = Vp - [ΣId + I HH + (ISC-IDmin) + IC + IH] × (R × (Lt + LspurL))
where:
Vd
Vp
Id
210
= Voltage level at field device
= Voltage available from power supply
= Current consumption of each field device
FF SEGMENT DESIGN EXAMPLE EXERCISE
IHH
ISC
IDmin
IC
IH
R
L
Ltmax
LspurL
= Current consumption budget for handheld meter
= Current load for short circuit protection
= Current consumption of field device with minimum level of
current required
= Current load of device coupler
= Current load of Host
= Resistance of cable
= Length of cable, meters
= Maximum possible length of trunk, meters
= Length of longest spur
Total cable budget is 1900 meters. Calculate the maximum permissible
trunk length with 12 spurs of 120 meters each.
Ltmax = 1900 - (12 × 120) = 460 meters
Now calculate the device voltage as if the entire load were at the end of the
longest spur, as this will exceed the worst case of a single valve (24 mA) at
the end of the spur.
Vd= 24 - ({[(10×20)+(2×24)] + 10 + (58-20) + 4.5 + 20}/1000) ×
{44 × [(460+120)/1000]}
Vd = 24 - [(248+10+38+4.5 +20)/1000] × (44 x 0.58) = 24 - (0.3205
× 25.52) = 24 - 8.18 = 15.82 Volts
Therefore, the worst-case situation assumption provides more than sufficient voltage at the device furthest from the FF power supply.
E.4 Individual Spur Calculations
Because we have the design constraint of a maximum of two control loops
on a single segment, and we have five control loops in the unit, the project
will have to use three segments. Formulas to be used are as follows:
FF SEGMENT DESIGN EXAMPLE EXERCISE
211
Voltage at the device coupler:
VC = Vp - [ΣId + I HH + (ISC-IDmin) + Ic + IH] x (LT × 44Ω/km)
And voltage at each individual device, which reflects the voltage drop along
the spur from the device coupler to the individual field device:
Vd = VC - [Vc + (Id × (LSpur × 44Ω/km))]
where:
VC
Vp
Id
IHH
=
=
=
=
ISC
IDmin
Ic
R
=
=
=
=
LT
Vd
LSpur
=
=
=
Voltage level at field device coupler apparatus
Voltage available from power supply
Current consumption of each field device
Current consumption budget for handheld meter (typically 10
mA)
Current load for short circuit protection
Current consumption of field device with minimum level
Current consumption of coupler
(L × Rcable) Resistance of cable Rcable (typically 44 Ohm/km for
Type “A” cable)
Length of trunk cable in km
Voltage level at field device
Length of spur cable in km
WARNING: Watch the units used in your calculations since current is
normally in milliamps, cable length in meters and the corresponding
calculations are based on Resistance being ohms/kilometre of cable.
Rather than calculate all the spurs individually, which can easily be done
with a free sizing program such as Designmate (available from the FOUNDATION Fieldbus website) the calculation will be done for the two worst cases:
longest total cable length and spur with highest current consumption. The
longest cable and trunk length combination of 371 meters is on Segment 1
to tag PDIT-102, while the largest current consumption device is on Segment 2 (device TJT-101) at 26 mA with a trunk and spur length of 357
meters.
212
FF SEGMENT DESIGN EXAMPLE EXERCISE
Longest Cable Length Calculation:
Calculate voltage at the device coupler:
VC = Vp - [?Id + IHH + (ISC-IDmin) + Ic] x (LT × 44Ω/km)
Vc = 24 - ({[12+19+19+15+15+15+12+15+12+15] + 10 + (58-12)
+ 4.5 + 20}/1000) × {44 ×[(350+18)/1000]}
Vc = 24 - {149 + 10 + 46 + 4.5 + 20}/1000) × {44 × [368/1000]}
Vc = 24 - (229.5/1000) × (44 × 0.368) = 24 - (0.2295 × 44 × 0.368)
= 24 - 3.716 = 20.28 Volts
Calculate the voltage drop along the spur from the device coupler to PDIT102:
Vd = VC - [(Idc + Id) × (LSpur × 44Ω/km))]
Vd = 20.28 - [(4.5 + 15)/1000] × [(3/1000) × 44]
Vd = 20.28 - (0.0195 × 0.003 × 44) = 20.28 - 0.00257 = 20.28 Volts
Based on this calculation, we have more than sufficient voltage.
Largest Load Segment Calculation:
Calculate the voltage at the device coupler:
VC = Vp - [ΣId + I HH + (ISC-IDmin) + Ic] × (LT × 44Ω/km)
Vc = 24 - ({[15+19+19+11+12+12+26] + 10 + (58-11) + 4.5 + 20}/
1000) × {44 × (340/1000)}
Vc = 24 - {114 + 10 + 47 + 4.5 + 20}/1000) × {44 × 0.340]}
Vc = 24 - (195.5/1000) × (44 × 0.340) = 24 - (0.1955 × 44 × 0.340)
= 24 - 2.92 = 21.08 Volts
Calculate the voltage drop along the spur from the device coupler to PDIT102:
FF SEGMENT DESIGN EXAMPLE EXERCISE
213
Vd = VC - [(Idc + Id) × (LSpur × 44Ω/km))]
Vd = 21.08 - [(4.5 + 26)/1000] × [(17/1000) × 44]
Vd = 21.08 - (0.0305 × 0.017 × 44) = 21.08 - 0.0228 = 21.05 Volts
Therefore, we have more than sufficient voltage based on this calculation as
well so the segments will all function with capacity to spare.
Mixed cable calculation:
As a final Physical Layer exercise, let us assume that we are replacing an
existing analog device with a Fieldbus device and then adding two additional Fieldbus devices as a new control loop. The trunk is 300 meters (statistically, this is the average length of a trunk) and is a type “C” cable. The
one existing spur (30 m) will also be a Type “C” cable, while the new spurs
(35 m and 50 m respectively) for the field device (17 mA) and control valve
(20 mA) will be a Type “A” cable.
• FF Power Supply: Phoenix Contact FB-PS-PLUG-24DC/28DC/
0.5/EX (Module) with FB-PS-BASE/EX (Base), 28 VDC output
• Field Device Coupler: Relcom FCS-MB8-SG, short-circuit current
= 59 mA
Calculate the cable budget using the ratio of maximum lengths of each
cable that can be used:
Type “C” – maximum cable budget is 400 meters
Type “A” – maximum cable budget is 1900 meters
Type “C” ratio = (300 + 30)/400 = 0.825
Type “A” ratio = (35 + 50)/1900 = 0.045
Total cable budget used = 0.825 + 0.045 = 0.87
The sum of the ratios is less than 1.0 so we are within the cable limitations
of the FF specifications.
214
FF SEGMENT DESIGN EXAMPLE EXERCISE
Determine maximum total spur length remaining:
1900 (1.0 - 0.825) = 1900 × 0.175 = 332.5 meters
E.5 Macrocycle Calculation
The manufacturer, model number, and Function Block Execution time for
each of the devices are shown in Figure E-9.
Segment 1 is the most heavily loaded segment, and since the devices in this
example have similar execution times, Segment 1 is the one for which the
sample macrocycle calculation will be completed. The other factor to be
considered in Segment 1 is that PDIT-102 is a calculated measurement,
comparing the AI from LIT-102 as the lower pressure measurement to determine the differential tower pressure (PDIT-102) and thereby avoiding the
expense and associated maintenance as well as operational issues (plugging,
lag, etc.) associated with having to run longer impulse lines over the height
of the vessel.
Figure E-9 — Segment 1 macrocycle calculation
AI
PID
LIT-102
AI
AO
LCV-102
PDIT-102
CD -1
AI
LIT-102
LCV-102
PID
AI
FIT-100
AO
FCV-100
CD -2
PID
AO
PDIT-102
AR
AI
FIT-100
AI
FCV-100
PID
AO
0
25
50
75
100
125
FF SEGMENT DESIGN EXAMPLE EXERCISE
150
175
200
225
250
275
215
Note that the total macrocycle time is 30 + 25 + 25 + 30 + 25 = 135 milliseconds, well under the 40% or 400 milliseconds of the 1 second macrocycle budget. In the above equation the LIT-102 block is the initial 30
milliseconds; we then have the two sequential Compel Data commands at
25 milliseconds each (in italics) and then FCV-100 control valve at 30 milliseconds (PID) and 25 milliseconds (AO) in bold.
The other reason to make the LIT-102 AI block first is so that the PDIT-102
Arithmetic block (AR) can use this input to calculate the differential pressure for the associated AI block. Because PDIT-102 is not being used for
control it does not require an additional Compel Data at the end of its execution because monitor-only signals can be captured using Client/Server
Views.
216
FF SEGMENT DESIGN EXAMPLE EXERCISE
Index
.cff 103
.ffo 103
acyclic 101, 117
AI (analog input) 7, 12–13, 55, 74, 93–94,
96–97, 109, 113, 119–121, 123–124, 158,
171–172, 192, 194, 206, 215–216
alarm priority 150
alarms 106
analog input (AI) 93
analog output (AO) 93
application layer 1, 4–5, 54, 194
area classification 77, 79–80, 85, 87–88, 199,
201
asynchronous 24–25
attenuation 44
BAD 27, 107–110, 159, 161
bandwidth 117
BASIC 8, 24, 30, 56, 92–93, 105, 120, 175,
199, 204
black channel 169–171
block mode 101
bridges 17, 22, 24, 183
buffered 105
bulk power supply 79, 118
bus with spurs 20
bypass 9, 159
cable 19, 21–22, 41–48, 52, 55, 57, 61–62,
64–68, 70–72, 74–75, 77–79, 82–84, 87–
88, 90, 100–101, 129–134, 139, 141–143,
145–146, 156, 181–182, 184, 205, 211–
214
calculation
macrocycle 18, 25–27, 97, 113–114, 116–
125, 199, 204–205, 215–216
INDEX
voltage drop 70, 79–80, 85, 88, 90, 101,
117, 204, 210, 212–213
capabilities file (.cff) 103
capacitance 44, 75, 92
capacity 53, 80, 89, 214
cascade loops 108
certification 35, 70, 73, 134
CFF 93, 95, 147, 194
channel 7, 13, 19, 95, 103, 106, 113, 149,
169–171, 174–175
characteristic impedance 41–42, 44
chickenfoot 22, 46
CIF (control in field) 118
client-server 104–105
commissioning 35, 92, 103, 129, 133–134,
139, 146, 179
communication stack 1
compel data 113, 115, 120, 124–126, 194,
204, 216
condition monitoring system 29
configuration 110
connector blocks 50
contained parameters 12, 27
control
loop 98, 121
network 58–59, 116
strategy 106, 150, 179
CRC 170
cyclic 101, 105, 113, 122, 126, 170
daisy chain 21
damping 110
DART 85–87
data link
layer (DLL) 1, 4–5, 25, 54, 103, 187–188,
194–195
service access point 24, 195
data sheet 202
DCS 27, 38, 78, 121, 127, 162, 173, 178,
191, 194
217
DD 7–8, 10–11, 14–15, 29, 33, 36, 92–93,
95, 101, 103, 106, 154, 175, 179, 192, 194
DD services 10
decision analysis 98
default node address 129
derivative time 158
device
code 23
coupler 22, 48–49, 52, 87, 184, 205, 211–
214
description 7, 11, 15, 29, 35, 92, 103, 147,
154, 175, 179, 184, 192, 194
ID 147
diagnostic 56, 106
DTM 30, 32–33, 35, 192
earth 61
EDDL 29–30, 34, 36–37, 60, 179
end-to-end 22
enhanced block 8
enterprise 28, 32, 59
equipment classification 73
ethernet 8, 22, 54, 58
ethernet, high speed 172, 183, 192, 196
execution 6, 25, 74, 92, 113, 115, 122, 124,
204, 215–216
extended block 8, 10
failure strategies 109
FAS (also fieldbus access sublayer) 5, 195
fault state 108
FDI 35–38, 60, 192
FDT 30, 32–35, 39, 192
FDT/DTM 30, 32
FF-569 183
FF-831 70, 74, 184
FF-844 43–44, 71, 184
FF-846 184
FFB (also function blocks) 8–10, 12, 56, 192,
195
fieldbus
access sublayer (FAS) 1, 5, 195
barrier 83–85, 90
218
intrinsically safe concept (FISCO)
69, 74, 192
message specification (FMS) 1, 5, 184,
195
non-incendive concept (FNICO) 69, 81
final operational address 129
FISCO 69, 74–85, 88, 100, 192
floating-point 13
FMS (also fieldbus message specification) 5, 55,
195
FNICO 69, 74, 78, 81–82, 188
fully flexible function block (FFB) 8–9
function block VFD 25
function blocks 1, 6, 8–10, 12–13, 24–25,
27, 33, 37, 56, 92–93, 96, 101, 113, 115,
117, 120–122, 125, 149–151, 169–170,
172, 175, 177, 179, 182
GAIN 158
gateways 17, 35, 172–173, 175
GOOD 27, 53, 57, 61, 64, 71, 74, 78, 108,
110, 112, 116, 141, 153, 157
ground 52, 60–64, 71, 73, 131, 134, 139
H1 56, 170
hardware address 23, 55
HART 29–30, 33, 35–36, 154, 163, 173, 176
high energy trunk 83, 88, 90
high speed ethernet (HSE) 172, 183, 192,
196
HMI 1, 27, 33–34, 112–113, 181, 191
host interoperability 179, 183
host interoperability support test
(HIST) 183
HSE 8, 22, 41, 54–56, 58–60, 154, 172–176,
178, 183, 185, 192, 195–196
HSE class 56
idle current 69
IEC 9, 12, 29, 32, 34, 36, 38, 65, 74–75, 77,
81–82, 129–131, 136, 141, 169, 187–188,
191
impedance 70
INDEX
industrial scientific medical (ISM) 176, 191
input parameter 12, 27, 106
instantiation 178
instrument segment diagram 202
integral reset 158
interlock 9, 107
intrinsically safe 69–70, 90
IP address 56
IS 69–70, 72, 74–77, 82, 88, 90, 93, 100–101,
191–192
ISD (instrument segment diagram) 202
ISM (instrument scientific medical) 176,
191
jitter 166
LAS (also link active scheduler) 147, 165
limit values 150
link
active scheduler (LAS) 4, 6, 23, 25, 107,
147, 192, 196
master (LM) 24
object 25
scheduling 25
live list 24, 133–134, 146
LM 24, 192
location drawing 199, 201, 207
loop diagram 91
MAC 55, 193, 196
macrocycle 117, 120, 122
MAI (also function blocks) 8–9, 12, 193, 196,
206
management information base (MIB) 6, 105
Manchester encoding 3–4
manufacturer code 23
MAO (also function blocks) 8, 12, 174, 193,
196
maximum overall length 44
MDI (also function blocks) 8, 12, 193, 196
MDO (also function blocks) 8, 12, 193, 196
medium access control (MAC) 55
MIB 105, 193, 196–197
INDEX
mixed 22, 214
MODE_BLK 148–149, 151
multivariable 96–98
network 1, 3, 6, 8, 17–19, 22–25, 28–29, 33–
34, 38, 41, 45–49, 51–55, 57–59, 64, 69–
72, 76, 79, 81, 84, 88–92, 98–99, 101, 103–
110, 112–114, 116–123, 125–127, 129–
130, 133–134, 146, 153–154, 157–158,
162, 164–166, 171, 173, 176, 178, 181,
183, 192, 197–198
diagram 91
management information base
(NMIB) 6, 197
NIS 69, 100–101, 191
NMIB 6, 197
node address 24
noise 3, 41, 52, 57, 60, 62–63, 67, 72, 134,
156–157, 165
non-intrinsically safe (NIS) 17, 69–70, 191
object dictionary 5–6, 197
OOS 111
OPC 29
open block 8
operating mode 111
OSI seven-layer model 1, 15
output parameter 12–13, 24, 27
P&ID 95–96, 199–200
packet 2, 55, 134–135
parameter
contained 8, 12, 27, 29, 39
input 7–8, 12–13, 25, 27, 30, 38, 59, 71,
84, 88, 91, 93, 95–96, 105–106, 112,
116, 118, 120, 125, 133, 141, 147–
150, 159, 171, 174–175, 191–194,
196, 216
output 7–8, 12–14, 18, 24–25, 27, 59, 65,
70, 73, 77–78, 80, 87–88, 90–91, 93,
95, 100, 105, 107–112, 116, 118, 120–
121, 123–125, 133, 154, 158–159,
172, 174–175, 191–197, 204, 214
219
physical layer 1
PID 7, 74, 93, 121
point-to-point 21–22
polarity 19, 70–71, 93, 95
power conditioner 49, 70, 79–81, 83, 88, 90,
118, 154, 164
power supply 17–18, 45–46, 53, 69–73, 78–
80, 86–89, 97, 100–101, 109, 118, 133,
181–182, 184, 204, 210–212, 214
Profibus PA 29, 33, 35–36
proportional gain 158
publisher 56, 106
publisher-subscriber communication 25
RATE 15, 65, 69, 97–98, 112, 118, 131, 145,
157–158, 202
redundant 19, 56, 78, 80–81, 119, 173, 197
registration 14–15, 178
remote I/O 28, 59, 172
repeaters 18–19, 57, 59, 79–80, 112
report distribution 106
RESET 13, 158
resource block 7, 27, 101, 112, 147–148,
151, 171, 193
risk 64, 78, 81–82, 88, 97–99, 106, 109, 116,
159, 202
safety bus 169, 171
sampling frequency 116
SCADA 27–29, 176, 191
schedule 24, 101, 125, 196
segment 18–20, 22, 46–50, 52–53, 63, 70,
72, 74–75, 78, 80–81, 85, 88–89, 91, 93,
95, 100–101, 105, 107, 109, 113–123, 134,
136, 142, 154, 156–158, 165, 189, 199,
202, 204, 207–213, 215
segment execution time 115
shielded twisted pairs 57
shielding 42
short circuit 47–49, 61, 89, 99–101, 142,
211–212
SIF 169–170, 178
SIL 169
sink-source 104
220
SMIB 6, 198
specifications 8–9, 14, 16, 30, 36, 42, 46, 54–
55, 59, 71, 130, 133, 137, 173–174, 176,
187, 199, 202, 214
splice 45
spur 99–100
stale rate 158
standard block 8
status 7, 13, 27–28, 107–108, 110, 112, 148,
158, 161, 166, 174–175, 179, 197
STP 57, 193
subscribers 56, 106
surge 64–65
switch 9, 57, 147, 171
system management information base
(SMIB) 198
TB 148, 173–174
TCP 22, 54, 193, 198
terminal block 76
terminator 49
testing 8, 14–15, 38, 88, 130–131, 133, 172
token 24–25, 123, 197
topologies
bus with spurs 20
chickenfoot 22, 46
daisy chain 21
end-to-end 22
mixed 22, 214
point-to-point 21–22
tree 198
transducer block 173
transmission control protocol (TCP) 54,
193, 198
tree 198
trunk 18, 21–22, 43, 45–46, 50–52, 64, 71,
77–79, 82–83, 85, 87–88, 90, 100–101,
133, 154, 181–182, 204, 211–212, 214
tuning 105–106, 158
twisted-pair 41, 57
type A cable 90
type B cable 42
type C cable 42
type D cable 42
INDEX
UDP 54, 193, 198
UNCERTAIN 27, 107–108, 110, 161
unshielded twisted pairs 57
user datagram protocol (UDP) 54, 193, 198
user layer 1, 6, 15, 29, 170
UTP 57, 193
VCR 5–6, 25, 93, 103–106, 193, 198
vendor-specific block 8
VFD 6–7, 25, 193, 198
virtual communication relationships
(VCR) 6
virtual field device (VFD) 6–7
waveform 18, 30, 134–136
WIO 59–60, 172–176, 183
wireless 59, 172–173, 176–177
write protection 148
XD_SCALE 149–150
XML 29
INDEX
221