Relay Ladder - Allen-Bradley Logix5000 Reference Manual

Chapter 13 Special Instructions (FBC, DDT, DTR, PID)
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The timestamp ranges from 0...32,767 ms. Monitor the timestamp. When it
changes, a new process variable sample has been received. Every time a
timestamp changes, execute the PID instruction once. Because the process
variable sample is driven by the analog input module, the input sample time is
very accurate, and the loop update time used by the PID instruction should be
set equal to the RTS time of the analog input module.
To make sure that you do not miss samples of the process variable, execute your
logic at a rate faster than the RTS time. For example, if the RTS time is 250 ms,
you could put the PID logic in a periodic task that runs every
100 ms to make sure that you never miss a sample. You could even place the
PID logic in a continuous task, as long as you make sure that the logic would be
updated more frequently than once every 250 ms.
An example of the RTS method of execution is shown below. The execution of
the PID instruction depends on receiving new analog input data. If the analog
input module fails or is removed, the controller stops receiving rolling
timestamps and the PID loop stops executing. You should monitor the status
bit of the PV analog input and, if it shows bad status, force the loop into
software manual mode, and execute the loop every scan. This lets the operator
still manually change the output of the PID loop.

Relay Ladder

Rockwell Automation Publication 1756-RM003N-EN-P - October 2011