Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 74)

Publication 1757-UM006A-EN-P - May 2002
2-30 Integrating Fieldbus into Rockwell Automation Logix System
Foundation Fieldbus
Performance Foundation Fieldbus, FF, is a very 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.
• Translates into slow computations in the transmitters.
• 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: