PLC Output State in Stop Mode

Does anyone know of a PLC, be it old or a more modern version, who's associated discrete outputs would remain high or "on" when the processor has been put in the stop mode? Does this make sense to anyone? To me this would be an unsafe condition.

I don't know of any PLCs that behave that way, but sometimes you can configure remote I/O to hold their current state on loss of communications with the host. It's more often done with analog signals than with discretes.

Steve Bailey said:

I don't know of any PLCs that behave that way, but sometimes you can configure remote I/O to hold their current state on loss of communications with the host. It's more often done with analog signals than with discretes.

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OK, I can see it in that case, but we have a Modicon 584 with distributed I/O. The output cards we are using are not configurable this way and I don't see any other way to have the outputs retain state when the processor is stopped. Thanks for the reply.

some of the ControlLogix modules will let you do that ...

To me this would be an unsafe condition.

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I'm certainly not going to argue - but for better or for worse it's available ...

OR ...

how about a relay-type output module that has NC outputs? ...

OR ...

most people just use a regular output and wire it to a relay with NC contacts ... etc. ... tell us more about what you want it to do - maybe we can come up with something ...

Ron Beaufort said:

some of the ControlLogix modules will let you do that ...

I'm certainly not going to argue - but for better or for worse it's available ...

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This is also available for Omron PLCs, so there may be a way to do this with Modicon.
And like Ron, I won't go into the whys and wherefors of using this, just to reiterate, that it is there (or might be)if you want it!

Thanks everyone for your reply. I originally posted this question specifically on the Modicon 584 which is a dinasour we use in our nuclear power plant. The engineer for this system as well as the trainer, stated that the outputs will remain in thier last state when the processor is taken to stop mode. As I have found out through this forum, this would be true for a latched output. None of the outputs we were using were of this variety. Therefore our outputs all went to off. Fortunately, we had prepared for both scenarios by recording the position of all affected valves etc.. I did learn something about the old Modicon from all of this and that is a good thing. Thanks again.


Tech7

Perhaps if the PLC code had just required some cooling fans/pumps to be turned on you'd want to leave them on even though you'd gone out of Run mode?

Considering where you work please don't quote me!

Note that the Logix modules will not "Remain in last state", unless they are configured (channel by channel) to hold last state.