SLC 5/04 system stops in auto cycle run

Issue is with a SLC 5/04 PLC on a box glue machine built in 1999. The power supply was replaced and a new HMI installed 3 years ago. For the past few days the machine runs for 2-20 minutes and stops auto cycle run with no error. After replacing some sensors we now look for another reason that the machine stops. After a stop it has to be cycled in manual a few times before it will go back into auto cycle run again. I have not connected to the processor as the only port I can use is connected to the HMI. I doubt it will show anything but will check the processor status later.
What should we look for step by step?
See I/O configuration in attachement.

First thing to check when is stops, is the green CPU RUN LED still lit?

If so, then there is not a card fault or CPU fault, but some other cause making the process hang up.

CPU keeps running - no faults on PLC or HMI.

I'd say you need to get RSLogix 500 connected and study the code even if it means temporarily unplugging the HMI. There is something happening which the designer failed to trap and/or display that you probably will find faster looking at live ladder logic.

I'm thinking about PLC hardware going bad or any other thing that acts up from time to time. The fact that it does not go into autcycle on the first try could be an area to look for. We are thinking of replacing hardware starting with the most obvious but this can be a costly way to go. However the customer is nervous and willing to try anything. Right now we are cooling the cabinet (90 deg environment) to see if that changes anything.

ThomasGruetter said:

I'm thinking about PLC hardware going bad or any other thing that acts up from time to time. The fact that it does not go into autcycle on the first try could be an area to look for. We are thinking of replacing hardware starting with the most obvious but this can be a costly way to go. However the customer is nervous and willing to try anything. Right now we are cooling the cabinet (90 deg environment) to see if that changes anything.

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With a SLC, if anything was wrong with the CPU or the backplane comms to any of the cards, it would definitely fault the processor. Logic doesn't stray, or get flaky, because the CPU constantly checks for this and will wipe itself out and shut down. Same thing with the cards in the rack, any little hiccup on the backplane will cause a fault.

It is rare, but possible for a card to have a problem on its field side, but it is even more rare for that problem to be intermittent. The one type of PLC card that can be intermittent is the relay output card (look for OW or OX in the part number).

If the machine craps out every twenty minutes, it should not take very long to prove beyond a doubt the root cause by monitoring logic.

I should insert a caveat here, that the absolute worst written program I have ever seen was a small bag making machine that was so horribly written, it took four hours to figure out the root cause of a simple problem. I ended up writing a new program for that one.

So, you might discover a program written by a person trying to win a code obfuscation contest, but then you can share that with us and we can help you fix it.

I am not a fan of shotgunning parts at a machine. It might seem like the fastest way to solve a problem, but in my opinion it is always faster to find proof for the solution you think you need.

I have seen things like this where the programmer used OTL & OTU's for every mode and function.

There were a few conditions that were not considered that should have unlatched a bit and continued. Since you say it has to be cycled a few times to get it running I am guessing something is not unlatching and progressing. It could be one simple proxy or limit switch signal not coming, and that might not trip a fault display.

Generally if a machine stops in auto it is waiting for a condition to be met before it can step on, this could be a sensor or a latched condition thats waiting for an unlatch. Is the machine stopping in the same position each time, or is it stopping in random positions?


Steve

You need to go online with the plc when it stops.

1. where did the program stop.
2. was it still in auto cycle?
3. was it looking for a sensor input to goto the next step?
4. if it is not in auto, what can cause the program to go to manual.

5. if you cannot see the obvious, start the machine in automatic again and remove the hmi cable. I had an hmi terminal start to go bad and it was doing something similar. the touchscreen where the stop button was located was wearing out. we had to put in some timers looking at the bits to see the issue, after that, all hmi pb's had fault detection.

james