Industrial Automation
Automation Industrielle
Industrielle Automation
3
Industrial Communication Systems
3.2
Field bus operation
Buses de terreno: modo de trabajo
Bus de terrain: mode de travail
Feldbus: Arbeitsweise
Fieldbus - Operation
3.1 Field bus types
3.2 Field bus operation
Data distribution
Cyclic Operation
Event Driven Operation
Real-time communication model
Time distribution
Networking
3.3 Standard field busses
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Field Bus Operation 3.2 - 2
Objective of the field bus
Distribute process variables to all interested parties:
• source identification: requires a naming scheme
• accurate process value and units
• quality indication: {good, bad, substituted}
• time indication: how long ago was the value produced
• (description)
source
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value
quality
time
description
Field Bus Operation 3.2 - 3
Data format
source
value
quality
time
description
minimum
In principle, the bus could transmit the process variable in clear text (even using XML..)
However, this is quite expensive and only considered when the communication network
offers some 100 Mbit/s and a powerful processor is available to parse the message
More compact ways such as ASN.1 have been used in the past with 10 Mbit/s Ethernet
ASN.1: (TLV)
type
length
value
Field busses are slower (50kbit/s ..12 Mbits/s) and thus more compact encodings are used.
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Field Bus Operation 3.2 - 4
Datasets
Field busses devices have a low data rate and transmit always the same variables.
It is economical to group variables of a device in the same frame as a dataset.
A dataset is treated as a whole for communication and access.
A variable is identified within a dataset by its offset and its size
Variables may be of different types, types can be mixed.
dataset
binary variables
analog variables
dataset
identifier
wheel
speed
0
air
pressure
16
bit offset
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line
voltage
32
size
time
stamp
48
64 66
70
all door closed
lights on
heat on
air condition on
Field Bus Operation 3.2 - 5
Dataset extension and quality
To allow later extension, room is left in the datasets for additional variables.
Since the type of these future data is unknown, unused fields are filled with '1".
To signal that a variable is invalid, the producer overwrites the variable with "0".
Since both an "all 1" and an "all 0" word can be a meaningful combination, each
variable can be supervised by a check variable, of type ANTIVALENT2:
dataset
correct variable
error
undefined
variable value
check
0 1 0 1 1 1 0 0
0 1
0 0 0 0 0 0 0 0
0 0
1 1 1 1 1 1 1 1
1 1
chk_offset
var_offset
00 = network error
01 = ok
10 = substituted
11 = data undefined
A variable and its check variable are treated indivisibly when reading or writing
The check variable may be located anywhere in the same data set.
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Field Bus Operation 3.2 - 6
hierarchical or peer-to-peer communication
PLC
“master”
central master / slave: hierarchical
AP
alternate
master
PLC
AP
all traffic passes by the master (PLC);
adding an alternate master is difficult
(it must be both master and slave)
“slaves”
input
peer-to-peer: distributed
PLCs may exchange data,
share inputs and outputs
allows redundancy
and “distributed intelligence”
devices talk directly to each other
PLC
PLC
AP
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“masters”
AP
input
separate bus master from application master !
output
PLC
AP
“slaves”
output
AP
Application
Field Bus Operation 3.2 - 7
Broadcasts
A variable is read in 1..3 different places on avergage
Broadcasting messages identified by their source (or contents) increases efficiency.
application
processor
plant
image
application
processor
plant
image
…
application
processor
instances
…
plant
image
=
=
distributed variable
data base
plant
image
bus
Each device is subscribed as source or as sink for a number of process variables
Only one device is source of a certain process variable (otherwise collision)
The bus refreshes plant image in the background
The replicated traffic memories can be considered as "caches" of the plant state
(similar to caches in a multiprocessor system), representing part of the plant image.
Each station snoops the bus and reads the variables it is interested in.
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Field Bus Operation 3.2 - 8
Decoupling Application and Bus traffic
decoupled (asynchronous):
coupled (event-driven):
application
processor
application
processor
events
(interrupts)
traffic
memory
(buffer)
queues
bus
controller
sending: application writes data into memory
receiving: application reads data from memory
the bus controller decides when to transmit
bus and application are not synchronized
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bus
controller
sending: application inserts data into queue
and triggers transmission,
bus controller fetches data from queue
receiving: bus controller inserts data into queue
and interrupts application to fetch them,
application retrieves data
Field Bus Operation 3.2 - 9
Traffic Memory: implementation
Bus and Application are decoupled by shared memory, the Traffic Memory, (content
addressed memory, CAM, also known as communication memory); process
variables are directly accessible by application.
Application
Processor
Traffic Memory
Ports (holding a dataset)
Associative
memory
an associative memory decodes
the addresses of the subscribed
datasets
Bus
Controller
two pages ensure that read and
write can occur at the same time
(no semaphores !)
bus
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Field Bus Operation 3.2 - 10
Freshness supervision
Applications tolerate an occasional loss of data, but no stale data, which are at best
useless and at worst dangerous.
Data must be checked if they are up-to-date, independently of a time-stamp (simple
devices do not have time-stamping)
To protect the application from using obsolete data, each port in the traffic
memory contains a freshness counter.
This counter is reset by the bus or the application writing to that port. It is incremented
regularly, either by the application processor or by the bus controller.
The application should always read the value of the counter before using
the port data and compare it with its tolerance level.
The freshness supervision is evaluated by each reader independently, some readers may
be more tolerant than others. Bus error interrupts in case of severe disturbances are not
directed to the application, but to the device management.
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Field Bus Operation 3.2 - 11
Example of Process Variable application interface (program)
Access of the application to variables in a traffic memory is simple:
ap_put (variable_name, variable value)
ap_get (variable_name, variable value, variable_status, variable_freshness)
Rather than fetch and store individual variables, access is done by clusters
(predefined groups of variables):
ap_put_cluster (cluster_name)
ap_get (cluster_name)
The cluster is a table containing the names and values of several variables.
The clusters can correspond to "segments" in the function block programming.
Note: Usually, only one variable is allowed to raise an interrupt when received:
the one carrying the current time (sent by the common clock)
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Field Bus Operation 3.2 - 12
Transmission principle
The previous operation modes made no assumption, how data are
transmitted.
The actual network can transmit data
•
cyclically (time-driven) or
•
on demand (event-driven),
•
or a combination of both.
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Field Bus Operation 3.2 - 13
Cyclic versus Event-Driven transmission
cyclic: send value strictly every xx milliseconds
individual
period
misses the peak
(Shannon-Nyquist!)
always the same,
why transmit ?
time
hysteresis
event-driven: send when value change by more than x% of range
how much hysteresis ?
- coarse (bad accuracy)
- fine (high frequency)
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limit update
frequency !,
limit hysteresis
nevertheless transmit:
- every xx as “I’m alive” sign
- when data is internally updated
- upon quality change (failure)
Field Bus Operation 3.2 - 14
Fieldbus: Cyclic Operation mode
3.1 Field bus types
Classes
Physical layer
3.2 Field bus operation
Data distribution
Cyclic Operation
Event Driven Operation
Real-time communication model
Time distribution
Networking
3.3 Standard field busses
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Field Bus Operation 3.2 - 15
Cyclic Data Transmission
address
Bus
Master
1
2
3
4
5
devices
(slaves)
6
Poll
List
plant
The master polls the addresses in a fixed sequence, according to its poll list.
Individual period
1
2
3
4
5
Individual period
6
1
2 3
4
5
N polls
6
1
2 3
4
5
6
time [ms]
RTD
address
10 µs/km
(i)
read transfer
data
(i)
address
(i+1)
time [µs]
The duration of each poll is the sum of
the transmission time of address and
data (bit-rate dependent)
and of the reply delay of the signals
(independent of bit-rate).
44 µs .. 296 µs
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Field Bus Operation 3.2 - 16
Round-trip delay of master-slave exchange
master
closest data sink
repeater
T_m
most remote data source
repeater
t_repeat
t_repeat
The
round-trip
delay limits
the extension
of the bus
propagation delay
(t_pd = 6 µs/km)
T_m
t_source
t_mm
t_ms
access delay
(t_repeat < 3 µs)
T_s
t_repeat
t_sm
T_m
distance
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Field Bus Operation 3.2 - 17
Cyclic operation principle
Data are transmitted at fixed intervals, whether they changed or not.
The delivery delay (refresh rate) is deterministic and constant.
The bus is under control of a central master (or distributed time-triggered algorithm).
No explicit error recovery needed since a fresh value will be transmitted in the next cycle.
Only states may be transmitted, not state changes.
Cycle time is limited by the product of the number of data transmitted and the
duration of each poll (e.g. 100 µs / point x 100 points => 10 ms)
To keep the poll time low, only small data items may be transmitted (< 256 bits)
The bus capacity must be configured beforehand.
Determinism gets lost if the cycles are modified at run-time.
Cyclic operation is used to transmit the state variables of the process.
These are called Process Data (or Periodic Data)
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Field Bus Operation 3.2 - 18
Source-Addressed Broadcast
Process Data are transmitted by source-addressed broadcast.
Phase1: The bus master broadcasts the identifier of a variable to be transmitted:
bus
master
device
device
sink
source
subscribed devices
sink
sink
devices
(slaves)
bus
variable identifier
Phase 2: The device that sources that variable responds with a slave frame
containing the value, all devices subscribed as sink receive that frame.
bus.
master
subscribed
device
subscribed
device
sink
source
subscribed devices
sink
sink
devices
(slaves)
bus
variable value
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Optimizing Cyclic Operation
Problem: Cyclic operation uses a fixed portion of the bus's time
The poll period increases with the number of polled items
The response time slows down accordingly
Solution: introduce sub-cycles for less urgent periodic variables
that are a power of 2 multiple of the base period:
2 ms period
1
2
4a
8
16
1 4b
4 ms period
1
2
3
64
1
4a
time
1 ms period
(basic period)
1 ms
1 ms
group with
period 1 ms
The poll cycles should not be modified at run-time (non-determinism)
A device exports many process data (state variables) with different priorities.
If there is only one poll type per device, a device must be polled at the
frequency required by its highest-priority data.
To reduce bus load, the master polls the process data, not the devices
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Field Bus Operation 3.2 - 20
Cyclic Transmission and Application
cyclic
poll
cyclic
algorithms
cyclic
algorithms
cyclic
algorithms
cyclic
algorithms
application
1
application
2
application
3
application
4
bus
master
Periodic
List
source
port
Traffic
Memory
Ports
Ports
Ports
sink
port
bus
controller
bus
controller
Ports
sink
port
bus
controller
bus
controller
bus
controller
bus
port address
port data
The bus traffic and the application cycles are asynchronous to each other.
The bus master scans the identifiers at its own pace.
Bus and applications are decoupled by a shared memory, the traffic memory,
which acts as distributed database updated by the network.
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Field Bus Operation 3.2 - 21
Application Of Cyclic Bus
The principle of cyclic operation, combined with source-addressed broadcast, has
been adopted by most modern field busses
It is currently used for power plant control, rail vehicles, aircrafts, etc...
Deterministic behavior, at expense of reduced bandwidth and geographical extension.
The poll scan list located in the central master (which may be duplicated for
availability purposes) determines the behavior of the bus. It is configured for a
specific project by a single tool, which takes into account the applications’
requirements. This guarantees that no application can occupy more than its share of
the bus bandwidth and gives control to the project leader.
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Field Bus Operation 3.2 - 22
Example: delay requirement
publisher
application instance
device
subscribers application instances
device
device
applications
bus
bus instance
Worst-case delay for transmitting all time critical variables is the sum of:
Source application cycle time
8 ms
Individual period of the variable
16 ms
Sink application cycle time
8 ms
= 32 ms
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Field Bus Operation 3.2 - 23
Example: traffic pattern in a locomotive
% periodic time
occupancy is proportional to surface
total = 92%
number of devices: 37 ( including 2 bus administrators)
30 frames of 128 bits
65 frames of 64 bits
37 of 16 bits
18 of 32
49 frames of 256 bits
period
16 ms
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32 ms
64 ms
128 256
1024
Field Bus Operation 3.2 - 24
Fieldbus: Event-driven operation
3.1 Field bus types
3.2 Field bus operation
Data distribution
Cyclic Operation
Event Driven Operation
Real-time communication model
Time distribution
Networking
3.3 Standard field busses
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Field Bus Operation 3.2 - 25
Event-driven Operation
• Events cause a transmission only when an state change takes place.
• Bus load is very low on average, but peaks under exceptional situations
since transmissions are correlated by the process (christmas-tree effect).
Multi-master bus: uses write-only transfers
intelligent
stations
eventreporting
station
eventreporting
station
eventreporting
station
sensors/
actors
plant
Detection of an event is an intelligent process:
• Not every change of a variable is an event, even for binary variables.
• Often, a combination of changes builds an event.
• Only the application can decide what is an event, since only the application
programmer knows the meaning of the variables.
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Field Bus Operation 3.2 - 26
Bus interface for event-driven operation
application
filter
driver
Application
Processor
• Each transmission on bus causes an interrupt.
• Bus controller checks address and stores data in
message queues.
• Driver is responsible for removing messages of queue
memory and prevent overflow.
• Filter decides if message can be processed.
message (circular) queues
interrupt
Bus
Controller
bus
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Field Bus Operation 3.2 - 27
Response of Event-driven operation
Caller
Application
Transport
software
Bus
Transport
software
Called
Application
request
interrupt
indication
confirm
time
Since events can occur anytime on any device, stations communicate by
spontaneous transmission, leading to possible collisions
Interruption of server device at any instant can disrupt a time-critical task.
Buffering of events can cause unbounded delays
Gateways introduce additional uncertainties
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Field Bus Operation 3.2 - 28
Determinism and Medium Access In Busses
Although the moment an event occurs is not predictable, the bus
should transmit the event in a finite time to guarantee the reaction delay.
Events are necessarily announced spontaneously
The time required to transmit the event depends on the medium access
(arbitration) procedure of the bus.
Medium access control methods are either deterministic or not.
Non-deterministic
Collision
(CSMA/CA)
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Deterministic
Central master,
Token-passing (round-robin),
Binary bisection (collision with winner)
Field Bus Operation 3.2 - 29
Events and Determinism
Deterministic medium access is necessary to guarantee delivery time bound
but it is not sufficient since events messages are queued in the devices.
events
producers
& consumers
input and
output queues
bus
acknowledgements
data packets
The average delivery time depends on the length of the queues, on the bus
traffic and on the processing time at the destination.
Often, the applications influence the event delay much more than the bus does.
Real-time Control = Measurement + Transmission + Processing + Acting
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Field Bus Operation 3.2 - 30
Events Pros and Cons
In an event-driven control system, there is only a transmission or an operation
when an event occurs.
Advantages:
Can treat a large number of events – but not all at the same time
Supports a large number of stations
System idle under steady - state conditions
Better use of resources
Uses write-only transfers, suitable for LANs with long propagation delays
Suitable for standard (interrupt-driven) operating systems (Unix, Windows)
Drawbacks:
Requires intelligent stations (event building)
Needs shared access to resources (arbitration)
No upper limit to access time if some component is not deterministic
Response time difficult to estimate, requires analysis
Limited by congestion effects: process correlated events
A background cyclic operation is needed to check liveliness
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Fieldbus: real-time communication model
3.1 Field bus types
3.2 Field bus operation
Centralized - Decentralized
Cyclic Operation
Event Driven Operation
Real-time communication model
Time distribution
Networking
3.3 Standard field busses
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Field Bus Operation 3.2 - 32
Mixed Data Traffic
Process Data
represent the state of the plant
Message Data
represent state changes of the plant
short and urgent data items
infrequent, sometimes long
messages reporting events, for:
• Users: set points, diagnostics, status
• System: initialisation, down-loading, ...
... motor current, axle speed, operator's
commands, emergency stops,...
-> Periodic Transmission
of Process Variables
-> Sporadic Transmission of
Process Variables and Messages
Since variables are refreshed periodically,
no retransmission protocol is needed to
recover from transmission error.
Since messages represent state
changes, a protocol must recover lost data in
case of transmission errors
basic period
basic period
event
time
sporadic
phase
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periodic
phase
sporadic
phase
periodic
phase
Field Bus Operation 3.2 - 33
Mixing Traffic is a configuration issue
Cyclic broadcast of source-addressed variables is the standard solution in field busses
for process control.
Cyclic transmission takes a large share of the bus bandwidth and should be reserved
for really critical variables.
The decision to declare a variable as cyclic or event-driven can be taken late in a
project, but cannot be changed on-the-fly in an operating device.
A message transmission scheme must exist alongside the cyclic transmission to carry
not-critical variables and long messages such as diagnostics or network management
An industrial communication system should provide both transmission kinds.
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Field Bus Operation 3.2 - 34
Real-Time communication stack
The real-time communication model uses two stacks, one for time-critical variables
and one for messages
time-critical
process variables
time-benign
messages
7
Application
6
Presentation
Remote Procedure Call
5
Session
connection-oriented
4
Transport (connection-oriented)
3
Network (connectionless)
2"
Logical Link Control
medium access
2'
Link (Medium Access)
media
1
Physical
implicit
implicit
Logical Link
Control
connectionless
connectionless
common
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Management
Interface
Field Bus Operation 3.2 - 35
Application View Of Communication
Periodic Tasks
R1
R2
R3
Event-driven Tasks
R4
E1
Variables Services
(Broadcast)
node
E3
Message Services
Traffic
Memory
Process Data
E2
Queues
Supervisory
Data
Message Data
(unicast)
bus controller
bus
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Field Bus Operation 3.2 - 36
Cyclic or Event-driven Operation For Real-time ?
The operation mode of the communication exposes the main approach to
conciliate real-time constrains and efficiency in a control systems.
cyclic operation
event-driven operation
Data are transmitted at fixed intervals,
whether they changed or not.
Data are only transmitted when they
change or upon explicit demand.
Deterministic: delivery time is bound
Non-deterministic: delivery time vary widely
Worst Case is normal case
Typical Case works most of the time
All resources are pre-allocated
(periodic, round-robin)
Best use of resources
(aperiodic, demand-driven, sporadic)
object-oriented bus
message-oriented bus
Fieldbus Foundation, MVB, FIP, ..
Profibus, CAN, LON, ARCnet
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Fieldbus: Time distribution
3.1 Field bus types
3.2 Field bus operation
Data distribution
Cyclic Operation
Event Driven Operation
Real-time communication model
Time distribution
Networking
3.3 Standard field busses
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Field Bus Operation 3.2 - 38
Time: TAI and UTC
TAI (Temps Atomique International) is the scientific time scale. It is continuously
incrementing and will never be reset or discontinued. It is the base of all other scales.
UTC (Universal time coordinated) is the legal time. It is the base for the clocks of all
countries. It indicates approximately 12:00:00 at solar noon in Greenwich at the Spring
equinox (it was formerly called Greenwich Mean Time). The Bureau International des
Poids et Mesures (BIPM), Paris, is responsible for the definition of UTC.
Rate: UTC and TAI proceed at exactly the same rate; both were identical back in 1961.
Leap Seconds: Since 1961, the earth rotation slightly slowed down, days became longer. When
the difference between UTC and solar noon exceed 0,9 s, which happens after some years, the
BIMP adjusts UTC by letting all clocks insert a leap second, so the last minute of a day lasts 61
seconds (the reverse case is also possible, but very unlikely).
Leap seconds cannot be anticipated, since irregularities of the earth’s rotation are unpredictable.
In 2011, UTC lagged behind TAI by 34 seconds: when TAI was 2011-02-04 12:00:00, UTC was
2011-02-04 11:59:26.
One cannot deduce TAI time from UTC time without a table of all elapsed leap seconds, and
UTC cannot be predicted for a given TAI, since the introduction of a leap second is a decision of
the BIPM. The system of leap seconds is still subject to discussion and could be revised in the
future – it could be abolished in 2015.
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Time-stamping and synchronisation
In many applications, such as disturbance logging and sequence-of-events,
the exact sampling time of a variable must be transmitted together with its value.
For this purpose, the devices are equipped with a clock that records the creation date of
a value (not the transmission time).
To reconstruct events coming from several devices, clocks must be synchronized.
considering transmission delays over the field bus (and in repeaters,....)
A field bus provides means to synchronize clocks despite propagation delays and
failures of individual nodes.
t1
t2
processing
input
input
input
t3
t4
bus
t1 val1
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Syntonization vs Synchronization
distinguish:
1) frequency synchronization (syntonization)
2) relative synchronization
3) absolute synchronization
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Field Bus Operation 3.2 - 41
Example: Phasor information
Phasor transmission over the European grid: a phase error of 0,01 radian is allowed,
corresponding to +/- 26 µs in a 60 Hz grid or 31 µs in a 50 Hz grid.
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Field Bus Operation 3.2 - 42
Time distribution
In master-slave busses, the master distribute the time as a bus frame.
The slave can compensate for the path delays. Time is relative to the master
In demanding systems, time is distributed over separate lines as relative time (e.g.
PPS = one pulse per second) or absolute time (IRIG-B), with accuracy of 1 µs.
In data networks, a reference clock (e.g. GPS or atomic clock) distributes the time.
A protocol evaluates the path delays to compensate them.
• NTP (Network Time Protocol): about 1 ms is usually achieved.
• IEEE 1588, all network devices collaborate to estimate the delays, an accuracy
below 1 µs can be achieved without need for separate cables (but hardware
support for time stamping required).
(Telecom networks typically do not distribute time, they only distribute frequency)
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Field Bus Operation 3.2 - 43
NTP (Network Time Protocol) principle
client
t1
network
server
time request
t2
network
delay 
time response
t3
t4

time request
(t 4  t1 )  (t3  t 2 )
2
t’1
t’2
time response
network
delay 
t’4
t’3
distance
time
Measures delay end-to-end over the network (one calculation)
Problem: asymmetry in the network delays, long network delays
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Field Bus Operation 3.2 - 44
IEEE 1588 principle (PTP, Precision Time Protocol)
Grand Master Clock
residence time
calculation
peer delay
calculation
MC
Pdelay-response
TC
Pdelay-request
TC
TC
MC = master clock
TC
TC
TC = transparent clock
OC = ordinary clock
OC
OC
OC
OC
Two calculations: residence time and peer delay
All nodes measure delay to peer
TC correct for residence time (HW support)
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Field Bus Operation 3.2 - 45
IEEE 1588 – 1 step clocks
time
ordinary
(slave) clock
t1
peer delay
calculation
Pdelay_Req
bridge
bridge
1-step
transparent
clock
1-step
transparent
clock
t2
t1
link delay
 t4
t3
Pdelay_Resp
(contains t3 – t2)
grand
master clock
Pdelay_Req
t1
Pdelay_Req
t2
Pdelay_Resp
t3
t4
t2
Pdelay_Resp
t3
t4
Sync
residence

time
residence time
calculation

t5
residence 
time
Sync
t5
t6
Sync
(contains all  + )
distance
Grandmaster sends the time spontaneously.
Each device computes the path delay to its neighbour and its residence time
and corrects the time message before forwarding it
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Field Bus Operation 3.2 - 46
References
To probe further
• http://www.ines.zhaw.ch/fileadmin/user_upload/engineering/_Institute_und_Zentr
en/INES/IEEE1588/Dokumente/IEEE_1588_Tutorial_engl_250705.pdf
• http://blog.meinbergglobal.com/2013/11/22/ntp-vs-ptp-network-timingsmackdown/
• http://blog.meinbergglobal.com/2013/09/14/ieee-1588-accurate/
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Fieldbus: Networking
3.1 Field bus types
3.2 Field bus operation
Data distribution
Cyclic Operation
Event Driven Operation
Real-time communication model
Time distribution
Networking
3.3 Standard field busses
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Networking field busses
Networking field busses is not done through bridges or routers,
because normally, transition from one bus to another is associated with:
- data reduction (processing, sum building, alarm building, multiplexing)
- data marshalling (different position in the frames)
- data transformation (different formats on different busses)
Only system management messages could be threaded through from end to end,
but due to lack of standardization, data conversion is not avoidable today.
Industrial Automation
2013
Field Bus Operation 3.2 - 49
Networking: Printing Example
MPS = Master Printing System
LS = Leitstand
(section supervision)
PM = Print Master
SS =Section Steuerung
(section control)
MPS
Production
Plant-bus (Ethernet)
Operator bus (Ethernet)
Console,
Section Supervision
LS LS LS PM
LS LS LS PM
LS LS LS PM
LS LS LS PM
Printing Towers
Section Busses (AF100)
B
C
Section Control
E
D
SSB
SSC
SSD
SSE
Line bus (AF100)
Reelstand-Gateways
RPB
RPC
RPD
RPE
Reelstand bus (Arcnet)
Reelstands
multiplicity of field busses with different tasks, often associated with units.
main task of controllers: gateway, routing, filtering, processing data.
most of the processing power of the controllers is used to route data
Industrial Automation
2013
Field Bus Operation 3.2 - 50
Assessment
What is the difference between a centralized and a decentralized industrial bus ?
What is the principle of source-addressed broadcast ?
What is the difference between a time-stamp and a freshness counter ?
Why is an associative memory used for source-addressed broadcast ?
What are the advantages / disadvantages of event-driven communication ?
What are the advantages / disadvantages of cyclic communication ?
How is time transmitted ?
How are field busses networked ?
Industrial Automation
2013
Field Bus Operation 3.2 - 51