Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 43)

Publication 1757-UM006A-EN-P - May 2002
The Fieldbus Communication Model 1-29
Foundation Fieldbus
Performance Foundation Fieldbus is a powerful network providing both
communication and distributed control capability. However, fast
response is not one of its great capabilities. The screen capture below
reflects the time allocated for 18 function blocks to publish their
outputs on Fieldbus. The average time is about 40 ms. per published
value.
Therefore, in the application reflected in this schedule, you should
plan on a loop closure time of on the order of 1 second if you want
new data from all devices each time you run the loop calculation.
Part of the reason that fieldbus is slow is that Fieldbus devices operate
on very small amounts of current. 10 to 20 ma. per device is typical.
This translates into slow computations in the transmitters. It typically
takes 100 ms for a fieldbus transmitter to make a new measurement of
an input with all the associated calculations completed. Therefore,
when attempting to determine the performance of a fieldbus system,
please recognize these facts.
Performance Calculation Considerations
Some pressure transmitters will read their transducers and create a
new floating point digital readings of the PV every 100 ms. That value
can only be read every 40 to 50 milliseconds because of the Fieldbus
data rate, and of the Fieldbus protocol.
The data rate is 31.25 Kbps, or 31 bits per millisecond. Very very slow
by comparison with ControlNet or Ethernet.
• A minimum Fieldbus message uses 99 bits.
• A minimum response uses 150 bits.
Just to put those messages on the wire takes 8 ms. The protocol says
that you must allow time for each device to send nonscheduled
messages, in addition to the Publishing of the Precess Variables, that
are scheduled. The protocol also says that you must allow significant
time for a Fieldbus device to respond to a request for data or
information. The result of the slow data rate and the protocol dictate
that Fieldbus configuration tools allow 40 to 50 milliseconds for the
transmission of data from each Function Block.
Also, many pressure transmitters measures both the pressure and the
temperature. If the application dictates that both values must be used,
then 80 to 100 milliseconds will be allocated to communicating with
those two function blocks, in that one pressure transmitter. Both the
Pressure and the Temperature interface with other Fieldbus devices