Micrologix 1400 Watchdog error

Hello people, it's been a while...


Hoping someone here with patience can help me detect a problem with this code. Its a cyclic state machine working on a Micrologix 1400



The program had been installed and working for almost a month, and somehow the PLC got blocked blinking the FAULT led, and showing the WATCHDOG TIMER EXPIRED error code.


Can anyone detect a loop? What other problem can cause this error to happen? This has never happened to me before.


I am attaching the file. You will need RSLogix 500 v 8.10 or above to open it.



Thank you in advance!

The main Ladder Logic routine is only 50 or so rungs long, and there are no subroutines, and no jump/label instructions that could create a loop.

The maximum observed scan time (S:22) is 1.000 milliseconds, and the last scantime (S:35) was 0.400 milliseconds.

But the Watchdog (S:3) is set to 10 x 10 = 100 milliseconds.

So the problem is a firmware bug, or a hardware failure, or maybe a storm of network traffic that caused the controller to spend too much time on sorting out communications.

I wish I could give you a more accurate diagnosis, but this might just be something you have to wait to happen again, or replace the controller.

I got the same conclusion as Ken ...

question for you:

since the program name is "Watchdog problem.RSS" is there any chance that the program that you've posted is not the EXACT same program that was actually loaded in the processor when you had the fault? ...

specifically, are you 100% sure that the program file that you posted is EXACTLY the same as the program that faulted? ...

here's a LOOOOONNNNNGGGG shot idea from the "grasping at straws" department ...

it's possible for something like an incorrectly "aimed" message or command from an HMI or some other station on the network to "write" values into the Fault/Error locations in the processor's memory ... that type of mistake COULD cause the processor to fault - even though there is nothing actually wrong in the program code ...

(note: I've used this technique before in training classes - to test the students' ability to track down fault codes) ...

good luck with your project - and please come back and let us know how it finally turns out ... this is an interesting problem ...

Thanks Ken and Ron for your answers, you are always very helpful.


Just to clear some of your questions. I just took away the password protection of the code and saved it with that name to upload it here, otherwise its the same code that was running.


It has no HMI, just oldschool hardwired pushbuttons and digital outputs.


The technician in field is going to try to diagnose where its stucked today, and probably reinstall the code. I'll post any news.


Have a good day, and happy new year if we don't talk before!


Thank you for being there