Foundation Fieldbus End Users Council Australia Inc.
9 Corcoran St Duncraig, WA 6023
P.O. Box Z5546 Perth, WA 6831
Benefits derived from the FF specification FF-831
Why we need it, and why it improves interoperability and stability
Andreas Agostin
Pepperl+Fuchs Pte Ltd Singapore
In March 2004, the Fieldbus Foundation released the “Fieldbus
Power Supply Test Specification” FF-831. This specification
describes tests and measurement procedures for Foundation
Fieldbus power supplies and conditioners.
FF-831 is based on the existing physical layer specifications IEC
61158-2: 2003, FF-830 1.5 and FF-816 1.5, but considers tests
beyond the requirements of these standards that determine if the
power supply is fit for its intended purpose of powering a fieldbus
network. Therefore some implementations will meet the requirement
of the standard IEC 61158-2 but will not pass the conformance test
FF-831.
The paper will describe the performed tests and in what aspects
FF-831 exceeds the requirements of the IEC standard. It will explain
why extended requirements are necessary and will show what
techniques are required for the next generation of FF power
supplies. One of the already registered Fieldbus Power Supplies of
the next generation, the Fieldbus Power Hub, will serve as an
example to investigate the required technology.
The paper will conclude with the positive impact FF-831 has on
Fieldbus stability in general and user installations in particular.
Keywords: Fieldbus Power Supply, Fieldbus Power Conditioner,
Physical layer, FF-831 specification, IEC 61158-2 standard
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Benefits derived from the Foundation Fieldbus specification FF-831
Table of content
1.
Introduction .......................................................................................................3
2.
Differences in the supplies...............................................................................4
3.
Standardisation compliance.............................................................................7
4.
FF-831:2004 Tests .............................................................................................8
4.1. Applicability ......................................................................................................8
4.2. Test No. 1: Output voltage .............................................................................10
4.3. Test No. 2: Noise and Ripple .........................................................................10
4.4. Test No. 3: Timing..........................................................................................10
4.5. Test No. 4: Resonance ..................................................................................10
4.6. Test No. 5 and 6: Startup ...............................................................................11
4.7. Test No. 7: Short circuit..................................................................................11
4.8. Test No. 8: Add device...................................................................................11
4.9. Test No. 9: Galvanic isolation ........................................................................12
4.10. Test No. 10: Multi Segment test – short circuit.............................................12
4.11. Test No. 11: Multi Segment test – Signal isolation (crosstalk) .....................12
5.
Conclusion.......................................................................................................13
6.
List of abbreviations .......................................................................................14
7.
References .......................................................................................................15
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
Page 2 of 15
Benefits derived from the Foundation Fieldbus specification FF-831
1. Introduction
During the last three years of presentations at the “Jump Aboard” meeting in
Australia, Pepperl+Fuchs highlighted the technical advances of the network
interconnection topologies, leading to more and more supply power even for
hazardous (potentially explosive) areas. Additionally, Fieldbus concepts like FISCO,
FNICO, the Fieldbus barrier and others further diversify the topology solutions.
This led to a larger number of supplies and conditioners available on the market
today, and the customer now has the choice to select the most appropriate one for
his application.
But what are the differences? Is it just the amount of output voltage and current, or is
there more than that? And if the supplies all comply with the physical layer
specification (IEC 61158-2), why do the supplies perform differently?
The new Foundation Fieldbus specification FF-831 (Version 1.0) goes beyond the
IEC 61158-2 specification, and considers network behaviour rather than component
(isolated) behaviour.
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
2. Differences in the supplies
With Foundation Fieldbus, data communication (information) and power are on the
same cable; the communication is superimposed on the power (figure 1).
Communication is fully digital. The field instruments do consume current, but this
current is constant (typically somewhere around 20mA) and does not represent any
process information like with a 4-20mA instrument. The constant current is required
to operate the device; all signal values (measurement value like flow, temperature,
pressure, etc) are communicated digitally (similar to the HART communication).
UBus
“Normal” bus voltage
t
Superimposed
communication
Figure 1: Communication superimposed to the power
To allow communication to be on top of the supply power, the communication must
be decoupled from the voltage regulation units (Power Supply), as a voltage
regulation unit tries to maintain a fixed output voltage at its output terminals, hence
this would eliminate any signal that tries to be imposed on the network power. This
decoupling is done via a “low pass filter”: the DC voltage can pass, but the higher
frequency communication signal cannot pass the filter and is decoupled from the
voltage regulator (DC/DC converter) (see figure 2).
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
To host communication controller
FF Power Supply
Standard (bulk) Power
Supply, e.g. 24V
+
-
DC/DC
converter
+
-
Low Pass Filter
(f < 3kHz pass)
To field
Figure 2: Isolated Fieldbus Power Supply
For non-isolated Fieldbus Power Conditioners (figure 3), the additional power
conversion unit to provide symmetry is eliminated, but the filter itself must be
symmetrical to achieve the required impedance characteristics.
To host communication controller
FF Power Conditioner
Standard (bulk) Power
Supply, e.g. 24V
+
-
Low Pass Filter
(f < 3kHz pass)
To field
Figure 3: Fieldbus Power Conditioner
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
Page 5 of 15
Benefits derived from the Foundation Fieldbus specification FF-831
This becomes more obvious looking at the schematic drawing (principle) (figure 4).
FF Power Conditioner
FF Power Supply
Z/2
Z
Non-symmetrical filter
Symmetrical filter
Z/2
Figure 4: Schematic drawing of Power Supply and Power Conditioner
Additionally, there are different methods of how to build a low-pass filter. Today,
there are two methods used: an “active” filter and a “passive” filter.
An active filter uses an electronic circuit called “Gyrator” to build the required
impedance; passive filters use either inductors and/or resistors to build the required
impedance.
Active filters are usually smaller in size, but can only be built non-symmetrically due
to the electronic control circuitry1, which again requires additional galvanic isolation.
Passive filters are symmetrical (at least if the conditioner is according to the FF-831),
so that no galvanic isolation is required to provide any symmetry.
To summarize the differences:
•
A FF Power Supply always has galvanic isolation, and can be used with either
active or passive filter.
•
A FF Power Conditioner always has a passive (and symmetrical, to comply
with FF-831) filter, and does not have galvanic isolation.
1
If active circuitry would be used symmetrically, two parallel control units would compete against each
other, which would result in unstable operation.
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
3. Standardisation compliance
All Foundation Fieldbus equipment, whether it is a host, instrument, supply or
conditioner, has to comply with the IEC61158-2 standard. The current 3rd edition was
published in 2003 and previous releases are no longer valid.
The IEC61158-2:2003 defines the physical layer of different field buses. Its official
name is:” Digital data communications for measurement and control – Fieldbus for
use in industrial control systems – Part 2: physical layer specification and service
definition”.
This standard covers wire media and optical transmission, and different transmission
speeds.
The Fieldbus Foundation released a number of specifications that are to be applied
for FF products. Among these is the FF-816 Ver. 1.5, defining that FF-H1
(Foundation Fieldbus, 31.25kBit/s) uses the ‘medium attachment units for a wire
medium with 31.25kBit/s transmission speed and voltage signalling’.
It classifies a number of “types” that identify whether a supply is IS or non-IS, FISCO,
instrument, supply, etc.
The FF-830 Ver. 1.5 defines test procedures to ensure that the device conforms to
the IEC61158-2.
The FF-831:2004 was released in March 2004 and defines a number of tests to
ensure proper network behaviour of supplies and conditioners.
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
4. FF-831:2004 Tests
4.1. Applicability
The FF-831 specification is applicable for both isolated Fieldbus Power Supplies and
for non-isolated Fieldbus Power Conditioners. The following types are defined:
•
Galvanically isolated power supply with integrated conditioners: “Fieldbus
Power Supply”
•
Two types of non-isolated conditioners (I.e. no integrated additional power
supply).
Type 1: used with non-specific general purpose bulk power supplies
Type 2: used with specific (i.e. limited choice) general purpose bulk power
supplies.
The compliance test is to be performed by the manufacturer, and the test report FF832 must be submitted to the Foundation Fieldbus organization.
If the supply or conditioning systems use redundant parts/modules, the redundant
parts/modules have to be taken out of the system before the tests are performed
(see figure 5). This guarantees proper operation with single (non-redundant)
modules.
Figure 5: Remove redundant components while performing the tests
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
One example of the new generation of FF-831 compliant devices is the
Pepperl+Fuchs “Fieldbus Power Hub”. The unit has two entries on the Fieldbus.org
website. The following table shows how the entries are listed:
Manufacturer - Pepperl + Fuchs
Manufacturer - Pepperl + Fuchs
Product Name - Fieldbus Power Hub
Product Name - Fieldbus Power Hub
Model Number - HD2-FBCL-1.500 (Module) with
MB-FB-*.* (Backplane)
Model Number - HD2-FBPS-1.500 (Module) with
MB-FB-*.* (Backplane)
Revision - 1.0 (HD2-FBCL-1.500); 1.0 (MB-FB*.*)
Revision - 1.0 (HD2-FBPS-1.500); 1.0 (MB-FB*.*)
Type - Power Conditioner Type 1
Type - Power Supply
H1 Profile - 132
H1 Profile - 132
Campaign - PS000600
Campaign - PS000700
Registration Date -
April 30, 2004
Registration Date - April 30, 2004
The modules:
Backplane with modules
Power Supply and Conditioner look the same
Figure 6: Modules and backplane of the ‘Fieldbus Power Hub’
The registration information in the table above shows two types of modules:
A “Power Conditioner Type 1” (for the module HD2-FBCL-1.500) and a “Power
Supply” (for the module HD2-FBPS-1.500). The board (MB-FB-*.*) is used
universally and connects to redundant bulk supplies.
What is so special about the Fieldbus Power Hub? – With the same design and the
same backplane, the user can select between (redundant) Conditioner and
(redundant) Power Supply. The choice will be based on features and price.
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
4.2. Test No. 1: Output voltage
The supply/conditioner has to maintain the output voltage that is specified in the
datasheet, with a 20mA (simulates one device), 50% and 100% load.
4.3. Test No. 2: Noise and Ripple
The IEC61158-2 specifies noise levels only for transmitting devices. As a
supply/conditioner is not transmitting, the respective specification does not apply.
Hence, FF-831 creates a specification for these components.
4.4. Test No. 3: Timing
Like noise, timing also applies to transmitting devices only. The test in FF-831 makes
sure that a supply does not have an impact on such transmitting devices.
As the test environment does not contain cable, surge protectors or external noise,
the requirement for the timing parameter is slightly more stringent that the IEC
standard.
4.5. Test No. 4: Resonance
This test simulates different earth faults at the supply side, which are not allowed to
have an impact on the supply/conditioner. The test is to ensure that the bulk power
supply used has no impact on the proper behaviour of the supply/conditioner.
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
Page 10 of 15
Benefits derived from the Foundation Fieldbus specification FF-831
4.6. Test No. 5 and 6: Startup
Switching-on the bulk supply with all instruments connected could be one likely
scenario. This test simulates a load that draws up to 3 times the normal operating
current during startup.
A second scenario is to have the supply powered and then connect the Fieldbus
cable with all instruments wired. Again, the supply/conditioner must be able to
provide up to 3 times the nominal load current.
4.7. Test No. 7: Short circuit
The supply/conditioner must be able to withstand a continuous short circuit. Short
circuits can happen during commissioning, troubleshooting or maintenance.
After removing the short circuit, the supply/conditioner must recover within 20ms to
normal operation.
4.8. Test No. 8: Add device
During this test, a 20mA load (representing 1 instrument) is connected in addition to
the Fieldbus segment running at maximum current less than 20mA, so that after
connection, the bus is running at full load. 20ms after the connection, the voltage
must be at least 95% of the specified minimum output voltage of the
supply/conditioner.
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
4.9. Test No. 9: Galvanic isolation
This test applies to galvanically isolated Fieldbus power supplies only. The isolation
must withstand 500VACrms.
4.10. Test No. 10: Multi Segment test – short circuit
A short circuit on one segment must not have a significant impact on any other
segment, whether isolated supplies or non-isolated conditioners are used on the
same bulk power supply. The impact is insignificant if the voltage drops not more
than 20% within 100us (microseconds), and is within +/-5% after 100us.
4.11. Test No. 11: Multi Segment test – Signal isolation (crosstalk)
Multiple segments could be powered by the same bulk power supply or by a set of
multiple galvanically isolated Fieldbus Power Supplies. In such cases, there could be
crosstalk from one segment to the other via supply.
This test measures the crosstalk between segments, which must be a maximum of
14.5mVpp (approx. 1.5% of the communication signal).
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
5. Conclusion
Having described the tests, one can say that FF-831 performs multiple tests that are
not part of any other specification, e.g. physical layer spec. IEC 61158-2:2003.
One of the main statements of this specification is that both galvanically isolated
Fieldbus Power Supplies and non-isolated Fieldbus Power Conditioners can be used
to power Fieldbus networks, and as long as they comply with the tests, both of them
perform equally well.
A second statement is that multiple supplies or conditioners can be powered from
one and the same bulk supply. Such a powering method has no negative impact on
the behaviour of the Supply/Conditioner, as long as the Supply/Conditioner passes
the FF-831 tests.
The tests of FF-831 ensure proper Network behaviour of both isolated supplies and
non-isolated conditioners, and go beyond simple component definitions as in e.g. the
IEC61158-2:2003.
The tests consider proper operation of such units even in case of wiring faults,
overload, startup or maintenance.
To have a common platform to measure Power Supplies and Conditioners is
beneficial for the end user and ensures interoperability and improves stability,
because the test suite ensures that the Supply/Conditioner fulfils this purpose under
all normal and even many abnormal operating conditions.
Supplies/Conditioners that pass the FF-831 test suite are listed on the Fieldbus.org
website, currently on
http://www.fieldbus.org/productsandservices/registeredpower_conditioners.
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
6. List of abbreviations
CAN
Controller Area Network. Bus system used in cars.
DMM
Digital multimeter.
EMC
Electromagnetic compatibility.
Ex e
Protection method “increased safety”. Predominantly defining mechanical
requirements for cabling, connections, housings, etc.
Ex i
see IS
FF
1. Fieldbus Foundation (organisation).
2. Foundation Fieldbus (protocol defined by the Fieldbus Foundation).
FF-H1
Foundation Fieldbus with 31.25kBit/s transmission rate
FF-HSE
see HSE
FISCO
Abbreviation for “Fieldbus Intrinsically Safe COncept”. Method of
designing a Fieldbus network based on experiences and tests of the
German certification institute PTB, for Zone 1 and Zone 0 applications.
FNICO
Abbreviation for “Fieldbus Non-incendive COncept”. Method of designing
a Fieldbus network based on FISCO, but for Zone 2 applications.
FuRIOS
German abbreviation for “Fieldbus and Remote I/O system comparison”.
A study initiated by two huge German end users, ‘Aventis Pharma’ and
‘InfraServ Hoechst’, comparing a existing plant using remote-I/O
technology and conventional ‘point to point’ technology with a designed
plant using fieldbus.
HSE
High Speed Ethernet. Uses standard Ethernet with 10MBit/s, 100MBit/s
or 1GBit/s (future) for the data transmission.
IS
Intrinsic Safety. Protection method based on energy limitation for use in
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
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Benefits derived from the Foundation Fieldbus specification FF-831
potentially explosive (hazardous) areas.
ISO
International Standardisation Organisation.
MAU
Medium attachment unit. Electronic/mechanical/optical circuit that
connects to the transmission medium (cable/fiber optics/antenna)
P+F
Abbreviation for “Pepperl+Fuchs”.
TCP/IP
Internet transmission protocol.
uP
Micro processor.
USB
Universal Serial Bus. Bus used in personal computers (PC) for the
connection of periphery (e.g. printers, scanners).
OSI
Open System Interconnection. Model defining a networking framework for
implementing protocols in seven layers.
7. References
IEC61158-2:2003 “Digital data communications for measurement and control –
Fieldbus for use in industrial control systems – Part 2: physical layer specification
and service definition”
FF-816 1.5 “FOUNDATION™ Specification. Physical Layer Profile.“
FF-830 1.5 “FOUNDATION™ Specification. 31.25kBit/s Physical Layer
Conformance Test.“
FF-831:2003 “FOUNDATION™ Specification. Fieldbus Power Supply Test
Specification.”
Andreas Agostin, Pepperl+Fuchs Pte Ltd Singapore
Page 15 of 15