CLX watchdog fault

[sandy5020]

Hi,

I have an issue with AB Redundant PLC which was running perfectly for several years and all of a sudden both PLC was showing fault and stopped the plant. Any advice is highly appreciated!!
Error says- " watchdog fault-Task watch dog expired may have been caused by an infinite loop, complex program or a higher priority task

attached the screenshot


Regards
Sandy

 

[AlfredoQuintero]

As suggested in the screenshot you have posted, it seems there is an issue with the program itself. It may be some very rare state for the program which only happens under very special circumstances, reason why it worked for years without having experience a problem. I advice that you check the STRAINER program. If the program goes into an infinite loop that does not let the logic circuit complete its execution, the watchdog will kick-in with such an error.

 

[Maxkling]

Look for jumps and labels.

 

[drbitboy]

1. Unless the program is posted (PDF best, or .zip with program files) and the process is described here, this forum cannot provide much specific help.
2. If it ran for several years and only faulted now, then something changed. See 1 above.
3. Although there could be other reasons, the fault message provides three likely possibilities:
4. Infinite loop: are there any loops in the main program? See 1 above.
5. Is the MainRoutine doing a lot of processing and calculations on each scan? Was the per-scan time close to the watchdog timeout before the problem occurred, and has something in the process changed that would increase the amount of processing per scan? See 1 above.
6. Are there tasks other than the Main task in the PLC program? Are they higher priority than the Main task? Has anything in the process changed to make thos higher priority tasks run longer or more frequently? See 1 above.

Do you know what a watchdog is? If not, it is a PLC behavior where, if any one scan of the Main Task does not complete, from beginning to end, within a set time, then the watchdog task timeout expires and the PLC faults.

A watchdog is analogous to, but not implemented as, the following logic running in parallel with the Main Task:

  Main_Task_Scan_Complete         Watchdog_Task_Timer

----------]/[--------------------------[TON]------------



    Watchdog_Task_Timer.DN               Watchdog_Fault

----------] [--------------------------------( )----------



 

N.B. Again, the ladder logic above does not exist on the PLC as such, I use it here only to describe what a watchdog does.

If both Redundant PLCs faulted for a watchdog timeout, then that suggests this is not a problem with the PLC hardware, but something common to both PLCs, e.g. the program, or the process, or both.

See 1 above.