CompactLogix RPI - Power of Two<?>

Somewhere in the past, I recall reading or hearing that the RPI setting for local modules on a Compact Logix 5000 have an effective value that is a power of 2. That is, 1, 2, 4, 8, 16, 32, etc.
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I searched all my documents, and, I can't find this reference.
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Does anyone have a reference that explains this?

Specifically, if an RPI is entered as '15', is the effective value 8, or is it 16.
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Or am I confused about this issue.

My local rep talked to be about this a few years ago, and you are correct about the RPI relationship and binary multiples, unfortunately I never understood the background process that this was really affecting.

For instance some typical RPI values for me:

Flex IO rack (All Digital IO): 20ms (base rate of 20ms x1)
Flex IO rack (w/Analog IO): 40ms (base rate of 20ms x2)
ASI Master module (Digital IO): 80ms (base rate of 20ms x4)
Hardy Weight Controllers : 160ms (base rate of 20ms x8)
Power Flex drives: 320ms (base rate of 20ms x 16)

I would have to dig into RPI detail more to better understand why this is a 'best practice'. Granted this was on a rather large system where the default RPI setting for everything would have been just overhead on the processor. By offsetting the RPI by device signal type the comms become more efficient. Small systems this probably won't matter but on larger ones it certainly does.

Look here, Page 72.

CompactLogix processors (at the time of that printing) only update I/O on a power of 2 ms basis. Specifying an RPI of 100ms results in an actual RPI of 64 ms.

ControlLogix processors do not have this restriction.

There are other rules for ControlNet; the module RPI MUST be an even multiple of the NUT. The manual cited above has an error allowing a 10ms NUT and a 10ms RPI... that does work, but from bench tests is somewhat less than reliable.

rdrast said:

Look here, Page 72.

CompactLogix processors (at the time of that printing) only update I/O on a power of 2 ms basis. Specifying an RPI of 100ms results in an actual RPI of 64 ms.

ControlLogix processors do not have this restriction.

There are other rules for ControlNet; the module RPI MUST be an even multiple of the NUT. The manual cited above has an error allowing a 10ms NUT and a 10ms RPI... that does work, but from bench tests is somewhat less than reliable.

Click to expand...

Thanks for that manual page. I just learned something I didn't even know I needed to learn!