PLC Fault Codes


Thread Starter

Bob Day

I have a SLC 5/03 in the field that recently faulted with a fault code of 22. WatchDog Timer expired. This Processor has been running for quite some time and this is the first time it has faulted out. Any suggestions?
reduce the number of JMP and LBL instructions if you have any. Try not to JMP backwards in the program excessive amounts of time.
The watchdog timer is a setting in the status file that basically tells the PLC what the max scan time is that it should allow. The fact that it decided to time out on you all of a sudden could be indicative of the program becoming more complex (or larger), or maybe the processor is just slowing down (Sometimes heat in the control room can do this). I assume you have not added any code to it lately, since you didn't say you did. So thats not the problem. I would probably look at any math i had in the processor and see if there was a way to simplify it. That would be my first suspicion for something that could bog down a processor. It might be as simple as a particular setpoint causing all the numbers to work out to irrationals or something. The second thing i would suspect is communications. If you have a lot of MSG blocks, make sure you are not firing them all off at once. that could be causing the processor to spend an inordinate amount of time on the com ports and not enough time on the solving of logic. Another thing, if you have a lot of sections of code that only get solved when a particular event happens, maybe it was simply that too many of those events happened at the same time and the PLC suddenly had too many ladder files to solve in one scan.

Of course, if i had this problem, (since i would *know* beyond a shadow of a doubt that I had done all of the above stuff correctly) the first thing i would do is increase the watchdog timer setting in the scan times tab in the status file. Then i would watch the PLC run for a while and monitor the rest of the info in there that tells you what the last/max/min/avg scan times were. Maybe the PLC has just been running really close to the watchdog timeout setting since day 1 and it finally went over, might be a 1 in a million shot whose number finally came up (and would theoretically keep coming up every few months). In that case, I'd say its not a big deal. If you do just increase that setting, You should keep an eye on the rest of those words to be sure the scan times aren't creeping up.

If fault appears again, before clearing error.
Go to S2 Status file "Scan times" this should show you the last scan time before faulting.
You may have to increase the Watchdog time, try 1ms.
Has program been changed recently?
Is 5/03 communicating with others?
As I have seen a "WatchDog" bit programmed so if "Loss of Comms" occurs then processor faults.



David Alvarez Quiroga

Watchdog is a system used for avoiding code hanging.

You should retrigger watchdog timer from time to time, if you do not do so timer expires and program restarts (it is like a reset).

As you can see this avoid hanging of the programm (or PLC). If you are in an infinite loop watchdog timer will expire and your system will restart.

I cannot give you any ideas since I do not know at all your equipment but hope this help you to find fault by your own.


Ranjan Acharya

Do you have jumps or something like that. They could cause your code to go
off into an infinite loop and trip the WD. Just a quick thought.

This program is rather small. Only 109 rungs no subroutines and very little math. I am using the rs-232 port to communicate with a remote Micrologix 1200 via modem and radio. My communications routine is fairly straighforward and will not fire the next message without the previous one either getting done or erroring out after the max number of retries.
I found out earlier today that this site has been experiencing some power problems. Hopefully this is the problem. This site has only had this one problem and is running fine now. I have many of these out in the field and this is the first time that I have encountered this type of error.
Check value of address S:3 if scan time exceeds S:3 X 10 you get fault. try increasing to see what scan time is. check JMP statments for endless loops.
Sorry about the necroposting, i hope you are still reading the thread.

On September 19, 2003, Bob Day wrote:
> I found out earlier today that this site has been experiencing some power problems. <

Ya know, i once had a site that kept getting odd errors. Tech support finally told me to check out the power cause bad power connections can do kooky things. They faxed me a huge list (like 3 pages i think) of things to check. Naturally i never got around to trying it out, because it was easier to keep fixing the PLC than to go off screwing around with the electricity. (Electricity and snakes are the only 2 things that truly scare me) But anyway, if you are still having trouble, call em up and see if they can send you this list.