Control Slc 5/04 PID

The Plc Kid

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Feb 2009
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I have a several pid instructions in a slc 5/04.

Each pid is on a rung by itself and we have logic to write a zero to the cv when heaters are turned off but there is an aux contact for the main mechanical heat contact before the on/off contact both xic.

Problem with it being this way even if the heat is off but the main contactor is open it does not write a 0 to the cv so the pid is still driving.

I want to monitor when the pid is telling the heater to cycle on/off and with it this way it would tell me the pid is on even though no heating is taking place because with the main contact open power is removed from the circuit.

So i guess my main question is would it be ok to put the main contactor aux xic contact in front of the pid instruction?

Will this keep the pid from executing when the main contact is off?

Is this a good method?
 
So i guess my main question is would it be ok to put the main contactor aux xic contact in front of the pid instruction?

No. Don't do this. In the SLC500 the PID must be on an unconditional rung. This is not the case for PLC/5 or CLX, but it is for the SLC - refer to the manual.

Will this keep the pid from executing when the main contact is off?
The instruction will execute and it will cause the 32 bit integral sum (word 17-18) to clear. A false rung does not mean that an instruction does not execute, it merely affects the way it executes. For example, OTE executes all the time, if the rung it true it executes and writes a 1. If the rung is false it executes and writes a 0.


Use logic to place the PID into manual mode (word 0, bit 1) and then write a 0 to the CV word.


edit: can you post the program or relevant portion of the program? I'm sure we can come up with a good solution.
 
Last edited:

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