wdfiller

PLC 5 ladder output unexpected behavior

5 posts in this topic

I hate to start a new topic on such an old processor but I forced to keep this thing working until I can upgrade it.  We are having funny issues with outputs.  At first they were on a PLC5/60.  We suspected a bad processor so we changed to a PLC5/40 we had in our tool crib and the problem didn't go away.  Some of our outputs in the ladder are being evaluated to true but not turning on.  We can force the output on and it will come on and when we remove the force it will turn back off.  I've never seen this problem before on any processor.  All the logic leading up to the output is true but the output is not turning on and is there not green.

I've already checked all the obvious causes.  The instruction is an OTE.  The output is not duplicated somewhere else in the logic that is turning it off.  The ladder file is being scanned.  I replaced the output with a B3 bit and on the next line had the B3 bit turn on the ouput.  The B3 output bit turned on but the output did not.  This output is on a remote I/O rack and other outputs on that rack are working.

Any ideas would be greatly appreciated.

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Can you post your program so we can have a look?  Going to be tough guessing without a little insight.

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OTE is not the only instruction that can write to an output, also is the rung scanned... (are you sure?)

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A slow processing PLC-5 program combined with poor PLC code can allow for such erratic behavior.  Could you post a screenshot of some of the code so we can see an example of how the code developer programmed such coil outputs?

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Any HMIs writing to the processor?  

Wonderware scripts turning it off would not be detectable in ladder. 

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