Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 75)

Publication 1757-UM006A-EN-P - May 2002
Integrating Fieldbus into Rockwell Automation Logix System 2-31
• 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 through
independent function blocks, so each require their own 40 to 50
milliseconds.
The CN2FF operates on the Fieldbus side at the max. speed of the
Fieldbus, and at the ControlNet rate on the ControlNet side.
Therefore, the CN2FF is not a limiting factor in a Fieldbus systems
performance. When a CN2FF operates, the Fieldbus side and the CN
side run asynchronously. When the CN2FF receives data, it is stored in
the CN2FF and is Produced on CN at the NUT rate. Therefore, in a
typical CN2FF Fieldbus system, the controller will be receiving a lot of
redundant data.
In a PLX system, with a FIM fieldbus interface, the Fieldbus side
operates at the Fieldbus data rate, and the controller side operates at
the backplane rate, so again, it is not a restriction on the performance
of a fieldbus system.