PLC 5/15 weird problem

I had a strange problem with an old 5/15 that has been running untouched for years today that I have never seen.

PLC had a major fault in the middle of the night. Unfortunately my counterpart got to it before I did this morning and cleared the fault without looking at it.

He couldn't get the machine to run so he called me. With the program in remote run I looked at the input data table and every input in the entire table was 1 even though the inputs weren't on. If you changed one of them from a 1 to a 0 it would reset it back to 1.

I went through all the error bits and couldn't find anything. Cycled the power and the same thing happenned.

Downloaded the program into it, same thing. Cycled power again and and logix told me that the program in the processor had changed do you want to upload. Now everything is fine and it's running normally.

This is a pretty simple little system, no RIO and all discrete I/O in the main rack. It talks DH+ to one other 5/40.

Has anyone seen this before or have a clue what might of happened?

I'd first disconnect the DH+ connection and try again. If it works, someone on the DH+ is messing things up.

But, what you describe sure sounds like a bad rack ('chassis') to me. The ONLY problem I ever had with 1771- stuff was a bad rack. So, (even though its a pain), change out the chassis, and see what happens.

Also, it may be that there's a connection problem brewing in the back of the IO modules. Turn ALL the power off, and pull the I/O cards one at a time, and use a high quality contact cleaner (no, not an eraser), and clean the edge connectors. If nothing else, simply removing and re-seating the IO cards may clear the problem.

Good luck, and let us know how you fix everything.

jdbrandt said:

...it may be that there's a connection problem brewing in the back of the IO modules. Turn ALL the power off, and pull the I/O cards one at a time, and use a high quality contact cleaner...and clean the edge connectors.

Click to expand...

That was my same thought when I read the OP - hopefully its on the right track

(no, not an eraser)

Click to expand...

I admit a good contact cleaner is better, but a good hard eraser has been a valuable tool over the years that has solved many mysterious problems.

I think in most cases the eraser is better. I know enough chemistry to realilze that any solvent that can un-oxidize a corroded terminal could also damage some components, including my lungs, and leave residue in undesirable spots. With an eraser, you can control where the dry, unharmful residue falls.

allscott said:

. . .Downloaded the program into it, same thing. Cycled power again and and logix told me that the program in the processor had changed do you want to upload. . .

Click to expand...

You will always be prompted to upload from a classic PLC5/15. it does not provide the checksum for RSLogix to guarantee an exact program match, so RSLogix will always ask you to upload when going online. This is normal.

Thanks guys. The system is running fine now, the next time it goes down for maintenance I will pull the cards out of the rack and clean everything.

I'll post back if I find anything or if it happens again.