A.Bradley PLC-5 WatchDog Error


Thread Starter

Moshe Markovich

Upon finishing Program downloading; A "Watch Dog Error" rises. After resetting the system 5-10 times, The error vanishes, and everything's fine.
This error rises each time we're downloading the program.

Michel A. Levesque, eng.

The Watch Dog timeout value is too low. Typically you might want to set the watch dog a t 2 to 3 times the normal scan time. the watch dog is meant to detect a program freeze, lockup or infinite loop condition.
the reason you only see this after a download is that you probably have initialisation routines that are activated on a restart, which adds to the normal scan time, which trips the watch dog.

Bob Peterson

Take a look at S:28. This is the watchdog timer setpoint. Its possible it has been set to a value which is too low. The default is 500 msec. Before putting the PLC back in run mode, set S:9 (max scan time) to zero. After you
put it back in run mode, check what your max scan time has been. If it is close to the watchdog timer setpoint, you need to increase the watchdog timer setpoint.

Bob Peterson

Daniel C. Rozok

I have seen a simular problem, but only when going from Program to Run Mode. It happens prior to the first scan of the application. The problem was due to indirect addressing that was out of range. The first scan of the application reset all the indirect addressing variables to a valid positions, but the PLC would fault prior to the first scan. This was due, after some lengthy research with AB and others, to a hardware 'pre-scan' of the logic that is preformed prior to the first program scan of the PLC.

During this 'pre-scan', amongst other things, addressing is validated. As you stated, a WatchDog fault would occur, then vanish. Again, once cleared, I would not see again until a new downloaded program was placed in the PLC. I lived with the problem and found no solution. One solution that comes to mind now, would be to check the indirect addressing in the OffLine mode and check to see that all the addressing and their respective pointers are valid. Something is telling me that would be harder to preform than it
sounds. Good Luck!!!

Daniel C. Rozok