watcdog timer micrologix 100

lordloverocket · Jan 15, 2004

Has anyone ever encountered a watchdog timer error on the micrologix 1000. Our plant recently had a planned power shut-down which meant that we ran on generators for several hours. The unit in question was showing the fault LED when power was restored and when cheching the cause of the error I discovered that the watchdog timer had timed out. I have checked the scan time which shows a max recorded scan of 1 mS when running and the max allowed was set to 50mS. I have increased this to the maximum that is allowed, 255mS I think,but i can't understand the cause of the fault. The program isn't complex and there are no jumps or loops, could the PLC be faulty? Here's a rough copy of the program. Is there any code that will allow me to reset these types of error automatically without the need to connect a pc? Again I would like to thank you guy's for any help you can give me









        T1/DN    I1/0   I1/1	T1

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--------¦/¦------¦ ¦----¦ ¦-------¦            ¦

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                                  ¦  5 sec     ¦

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        T1/DN				SQO

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--------¦/¦-----------------------¦            ¦

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        I1/0        I1/1		MOV

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--------¦/¦---------¦/¦-----------¦            ¦

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                                  ¦____________¦

JesperMP · Jan 15, 2004

My guess is that the CPU entered a corrupt state when you changed over to or back from generator supply (noise ? spikes ? brownout ?).

It does not seem to be caused by the user program, and the fault is non-recoverable , so I guess that you cannot prevent it from happening again.

What you can do is to set S:1/8 Fault Override At Powerup. If the CPU is completely shutdown, it will clear the error and continue at powerup
You could combine this with an over/undervoltage relay, so that the CPU is shutdown if there are problems with the supply voltage.

zmanvortex · Jan 15, 2004

I need my memory refreshed here. Could you clear the watchdog timer fault with the fault routine which is scanned one time when a fault occurs or was this feature just on the SLC's.

JesperMP · Jan 15, 2004

The Micro has the fault routine feature as well, but the watchdog timeout error is non-recoverable (meaning you cant reset it with the fault routine and contine with main program).

zmanvortex · Jan 15, 2004

Thanks for refreshing my memory Jesper. I am amazed how few programmers use the fault routine in the AB processors as an exception handler.

watcdog timer micrologix 100

lordloverocket

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JesperMP

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zmanvortex

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JesperMP

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zmanvortex

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