micrologix 1200 output has voltage

Unlike relay output models, the ML with transistor outputs need to have a DC com connection made for each bank of transistors. Most people forget this or only hook up the DC com to one bank- and then get strange "stuck on" outputs like you're experiencing. Check that first.
 
This machine has been running fine since it's install 2 years ago.
The output is commanding an allen bradley powerflex 525, forward is output 7 and reverse is output 8.
I've discovered that someone has wired an AB 854K-B24SA3 stack alarm to this same output, probably about a year ago.
I'm thinking this is drawing to much, and has caused the contacts to fuse together. Should have been wired to the control relay...
 
New PLC came in, downloaded program and installed, working like a champ.
I will have to figure out some kind of workaround for the warning alarm.
 
You might also want to look into installing a 'reverse-bias diode or other back-EMF snubber across the output' to try to protect the output from having this happen again.
Poet.
 

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