AB1746-OB16 Hold Last State

Hello all,

I am currently trouble shooting a SLC 505 control system. The output modules that are used are the 1746-OB16 and I am trying to find out what happens to the outputs in the event of a control fault, and if it is possible to program a "hold last state" function.

Thanks,
Dena

Dena,

In the case of a PLC fault, the outputs generally go to the off state. However, the ControlLogix PLC output fault state can be programmed to be off or on.

If the application is important, you might build an external watchdog timer circuit. The PLC would operate a scheduled task to cycle an output to an external timer relay. The timer relay remains energized as long as the input pulse from the plc occurs on schedule. If the pulse does not occur, a fault may have happened, and the timer can activate or hold on the needed outputs.

On the SLC-500 platform, outputs always turn logically off in the event of a processor fault. It does not matter if the fault is caused by hardware, firmware, or the user program.

Faults caused by the user program (like indexing off the end of an array or placing a negative preset in a timer) can be reset by a Fault Routine in the controller, so that a fault is immediately reset (use an OTU S:1/13 instruction) and the controller attempts to run the program again. This method can be used to annunciate a user fault but is not the same as a "hold last state" function.

Important: Electronic circuits can fail in an energized state. You should never rely on a general purpose solid state or relay output from a general purpose PLC for a safety function.

Thank you both for your answers. We have incorporated a watchdog, and you have confirmed my hunch that the PLC would go to off state during failure.