watchdog timer help

bh307

Member
Join Date
Mar 2008
Location
HOUSTON
Posts
57
Can someone look at this and see if you see anything wrong with it . My watchdog relay keeps triping my main fuel line .I have replace the external relay .I think my program watchdog is missing some pulse and is triping it. It does not happpen all the time . It may run 24 hrs or this last time a week before it happens . I know it is the watchdog that is doing it. like I said I have replace the timer and check all wiring.
thanks for any help.And do you think maybe changing the scan timer may help.
 
It looks ok - giving a 1 second ON then 1 second OFF. Could it be that the external watchdog timer is set to closely too precisely this time thus any variance trips it?
 
So you are cycling a physical output every second? What type of output card because I wouldn't think a relay type would last too long under these conditions.
 
If your watchdog is set at 3 seconds and you're pulsing the PLC output every second you should be OK. If you suspect the watchdog is somehow ocasionally missing a pulse you could extend the watchdog timer out a bit (maybe 5 seconds) or speed up the PLC output pulse. I think I would start by extending out the wathdog timer and go from there.
 
Unless you have something weird going on with the task/routine, that output should be on for one second, and then off for one second. Are you always callling the routine? Is that timer, or output addressed anywhere else in the program? Is the output in a local rack, or a remote, and if remote, how is the communication set up?

What about the external circuit...is the output card a solid state output card? As BobO said, a relay card might not survive very many months like that, and could begin to stick.

Can you post some info for the external watchdog timer and wiring?
 
yes the output is in a local rack ,yes its call for allthe time it is the second subroutine in the main ladder there are 12 subroutines.yes the card is solid state.
 
well I made a mistake it is a relay output card just check. the system has been in opertion for about 6 months and the problem just started about three weeks ago. so maybe it is the relay card I may need to get a solid state card and replace the relay card.
thanks for the help.
 
At 1 second on, one second off, operating 24/7 for six months, you're looking at over 7.5 million on/off cycles. That's a lot of cycles for a relay output.

A quick check would be to reprogram to a different output point to see if the problem goes away. Assuming you have a spare output available.
 
Controllogix relay output modules:
ab knowledge base said:
Maximum switching frequency is 1 operation per 3 seconds (0.3Hz at rated load). This rate is specified so that the mechanical relays can keep up with the load and also to help the relays achieve their expected contact life of 300k cycles with resistive load / 100k cycles with inductive load.

Switching any faster and the relay may not be able to react to the command. This might cause the output to stay On when commanded to go Off or vice versa.
In addition the life expectancy of the contacts will be significantly reduced.
See the ControlLogix Digital I/O Modules manual page 7-59 (191 of the .pdf) for the complete 1756-OW16I Specifications.

I think their specs are conservative based on the full load rating of the relay.

I would suggest that you find out the voltage of the circuit feeding your external watchdog relay and switch to a matching solid state output if possible...

If that's not possible, then extend the timers as much as feasible for the application.
 
Last edited:

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