detecting a controllogix plc failure

I need an output on a controllogix card to turn off if for whatever reason the PLC fails. The off state of the output enables float control of various sump pumps in a plant. During normal operation these pumps are controlled by the PLC based on milltronics level readings.

What is the best way to determine if the PLC has failed and subsequently turn off the failure output? Using logic to do this doesn't make sense to me as if the PLC has truly failed, all logic should stop executing and therefore the output will remain high.

You can configure a fault routine that runs before the PLC shuts down, in this routine you can set the outputs to the desired state. However, I suspect that all outputs go false on a fault.

If the Clogix CPU owns the output and the CPU drops out of run mode, the output points owned by that processor should go to their fault states. I believe each point on a Logix output card has a configurable fault state.

The fault states are programmed in each card.

Try going into the properties of the output card in the I/O tree and look for a fault tab which has all of those options.

Paul

As Paul suggested, on the Configuration Tab of Module Properties, you can specify Fault Mode states.

I wonder though if this covers all circumstances.

If the application is pretty critical, you might consider an external watchdog timer circuit. The PLC would operate a scheduled task to cycle an output, at say 1Hz, to an external timer relay. The relay remains energized as long as the input pulse from the plc cycles as expected.

You can also use a GSV to monitor.
Knowledge base Answer ID 47624

Check out the Controller Fault Handler on the ControlLogix.

thanks for the great ideas guys.