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Old December 16th, 2005, 09:11 AM   #1

wtakacs is offline
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Question Allen Bradley RIO

Thanks for your interest!

Does anyone know how to determine the scan frequency of the flexbus remote I/O mechanism. Does it occur asynchronously to the PLC scan time? At the moment I'm only interested in discrete I/O. Its my understanding that analog I/O must be read using BTs. Perhaps I just dont fully understand the mechanism in which case please explain it!

As an example, I have an SLC5/05 with a 1794-SN installed in Rack 0. A 1794-ASB is intalled and connected with blue hose to the SN. There is a 1794-IB16 on a 1794-TB3 with one wired input to a switch.

Any help is appreciated!
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Old December 16th, 2005, 09:51 AM   #2

wtakacs is offline
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Smile Correction

In the example it should be a 1747-SN not a 1794-SN. OOPS 8-)
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Old December 16th, 2005, 10:32 AM   #3
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The adapter scans its I/O on an independent cycle. The main rack's scanner polls the adapters. The processor gets it's copy of the I/O data table from the scanner module during it's I/O scan cycle. In short, the RIO is updated asynchronously to the program scan. The I/O image that the processor is working with does not change during logic solving (with SLCs and PLC5s anyway). With a PLC5, I have read some old A/B manuals that stated that the change of state of a discrete input, for example, may be reported to the processor up to 125ms after it actually occurs. Likewise for an output bit. It may take up to 125ms after the PLC turns on the output before that information reaches the remote output point. Repeatability of this delay is random. It might be 5ms one time and 100ms the next. The number of remote racks will affect this as well. A/B has worksheets that can help you accurately determine maximum RIO delays in their system design manuals.

This is something to always keep in mind at design time. Always try to put your time critical I/O in the local rack.

Hope this helps.
Paul C.
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