Logix5000 Controllers General Instructions - SLAC Confluence

514 Special Instructions (FBC, DDT, DTR, PID)The timestamp ranges from 0-32767 milliseconds. Monitor thetimestamp. When it changes, a new process variable sample has beenreceived. Every time a timestamp changes, execute the PID instructiononce. Because the process variable sample is driven by the analoginput module, the input sample time is very accurate, and the loopupdate time used by the PID instruction should be set equal to theRTS 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, ifthe RTS time is 250 msecs, you could put the PID logic in a periodictask that runs every 100 msecs to make sure that you never miss asample. You could even place the PID logic in a continuous task, aslong as you make sure that the logic would be updated morefrequently than once every 250 milliseconds.An example of the RTS method of execution is shown below. Theexecution of the PID instruction depends on receiving new analoginput data. If the analog input module fails or is removed, thecontroller stops receiving rolling timestamps and the PID loop stopsexecuting. You should monitor the status bit of the PV analog inputand if it shows bad status, force the loop into software manual modeand execute the loop every scan. This lets operator still manuallychange the output of the PID loop.Relay LadderPublication 1756-RM003I-EN-P - January 2007