PLC5 Watchdog Timer Issue

It might be as you said i.e. someone has changed time out for WT. Also it depends on how were stored variables at PLC memory. For instance if it stored at NVRAM it might be caused the battery discharge.

For hazardous industrial conditions sometimes it used to be when data is losing at some of memory registers, especially if program for your particular PLC was completely downloaded to SRAM.

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Typically the PLC5 system is pretty reliable. A few months ago one of my plc5/11 systems gave up its CPU in a similar manner that you are asking about.

It started by giving me some intermittent strangeness, dumping some register values here and there over a couple of days, even changed a timer value once, and then faulted off the watchdog... same thing...register had blown it's value and was 0. I replaced the the CPU and have no other issues.

I am CoronaCameraMan -

My history goes back to the early 1980's with the A-B PLC's, haven't worked with them recently but worked with several in power generation where voltage surges could get thru the standard A-B power supply and shut the program down.

I also found a history where the electric utility at 34.5 kV was switching capacitors in a substation 7 miles away and 7 of 10 plant trips correlated to capacitor switching times. After installing a line filter ahead of the PLC power supply that problem went away.

I did have processor cards fail with line power filters installed.

Is it possible that an unexpected data value with an indexed register or other program "glitch" caused a value of 0 to be written to the watchdog timer's register?

--JMM