AB PLC5 Outputs turning themselves off!

TCIG

Member
Join Date
Aug 2002
Posts
57
I've got an AB PLC-5 rack set up as a remote rack. The first eight bigs of a sixteen bit isolated output card keep turning off by themselves. There is continuity throughout the rung all the way to the output, and suddenly after a while the output will just turn off. Here's the troubleshooting I've done so far.

1) Verified that I wasn't momentarily losing some other bit in the software rung. I am, indeed, just losing the output even though there's continuity.

2) Swapped out the output card with a brand new isolated output card.

3) Called Rockwell technical support, sat on hold for 10 minutes, and talked to a guy who said either my chassis is bad (not likely) or I've got a block transfer instruction that is for some reason writing to that card. The fact that the problem is limited only to the first 8 bits would seem to give credibility to the second possibility. There USED to be an analog output module in that rack, but it was taken out. As far as I can tell, all the code, including the BTR and BTW for it, have been removed from the software. Besides, the analog out was in slot 7, whereas my problematic card is in slot 0.

4) After finding no block transfer instructions in the code going to that rack at all, and after verifying that there are no conflicts between the integer files used in the block transfers (before or after the analog cards were removed) and any I/O, I replaced the chassis.

The problem still persists.

I've moved all the outputs over to a different isolated output card and they aren't having any problems. I'd like, however, to find the root cause of the problem in case we have future needs to use those outputs.

Any suggestions???
 
Well there's a lot of Allen Bradley experts on this site that will probably pin-point the problem for you. It sound to me like you've proven the problem is not coming from the program. Even though you put in a brand new output card I would try using the card you are now using in that slot or put the brand new card in the slot you are using now, just to ensure the 'brand new card' is not defective. At that point you eliminate the card, the chassis and the software as the problem and then what is left? I would also verify the setup for that slot.
 
Now oddly enough I was reading that and thinking he had pretty well isolated the fact that it wasn't a hardware problem.

You have isolated that there is no block transfer addressing this location, but is anything else affecting it?

Try searching for that word address and see if any File or Word level instructions are writing to this address. So if the affected addresses are O:050/0 through O:050/7 then search for O:050.

It sure sounds like a program problem. It could also be someone writing a value to this location over the DH+. If this problem is happening all the time try removing the PLC from the network and see if the problem disappears.

Short of that, I would suggest posting your file for us to look at. Make sure you tell us what addresses are acting up.

If you don't want to post it, then send it to me via email at [email protected]

OG
 
In addition to seeaching for Output usage, do a Cross-reference, and scroll to the end of the O: listings to see if there are any Indirect Addressed outputs.

Indirect addressing won't show up in a search nor cross-reference for the address, but does show up in the cross-reference.

Not that I think this is the problem - just being thorough.
 
I don't know squat about PLC-5's, but I do suggest that you verify the capacity of the power supply powering the loops. I have had other brands that worked fine until the total current for the analog outputs got above teh capacity, and then the loop power supply would drop out. If you have a newer model switching power supply, they can drop out, and then reset themselves after the load drops. You show up ten minutes later and it all looks good!
 
Thanks for all your suggestions, guys.

The power supply IS capable of handling everything, as it's all running fine with the outputs on a different card.

Didn't find anything in the cross-reference either.

I'm still trying (so far unsuccessfully) to find a DH+ write-over like Operaghost described.

I can't try any other card swaps for a while until the machines using this PLC all go down for more than a few minutes. Doesn't happen very often, but I'll keep the suggestion in mind.

By the way, Operaghost, I'd love to post a copy of the file, but the company for whom I work would never allow it. I'm not even allowed to just email it to you. Thanks, though.

If anybody thinks of anything else, feel free to post it! :cool:
 
Maybe change out the network adapter. You didn't say if it's RIO or ControlNet, but the adapter is the only thing that hasn't been changed in this setup.

I'm not entirely grasping at straws; I don't think this is a misaddressed block transfer because it affects all 8 bits. Block transfers gone amuck result in several bits changing state as the BT control and data cycle through their 8-bit data space. There was even a wild bug years ago in which BTR or BTW instructions that were deleted from the processor kept executing !

Think about the way the 1771-ASB adapter works; because of it's heritage as an 8-bit platform, I think the high 8 bits are handled by different circuits than the low 8 bits. But the network adapter isn't faulting, so I'd look at the interface between the adapter and the chassis; make sure that you clean the card edge thoroughly with alcohol swabs.
 
I am yet to see a PLC program shared that would cause a company to lose a competitive edge by its release. Are you telling me that you actually asked someone if you were allowed to release the program?
 
I had this exact situation once and it did turn out to be the network adapter. We were using remote I/O. The outputs would turn on and off at random, without any changes in the programming logic. It only affected one card in the remote rack and just a few of the outputs. Changing the network adapter took care of the problem.
 

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