Combining PLC outputs to a single output

Sprayman said:

But ya I think backfeeding 24V into an output is a bad idea ...

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Maybe, if the output is a FET that can source some amount of current at 24V.

But what if the output is itself a relay (e.g. most MicroLogix 1100 outputs)? Then it does not matter.

FWIW, the OP created two threads on the same subject and there has been discussion on the other one.

http://www.plctalk.net/qanda/showthread.php?t=131341

#1 rule of electronic design is: DONT FORCE OUTPUTS. Trying to tie the two outputs together is forcing outputs. It will probably work for a little while, but eventually you will fry 1 or both outputs.

I would do 1 of 2 options:

(1) use an interposing relay on both outputs and wire the relays in parallel with the solenoids as the load so if either relay comes on the solenoid turns on

(2) wire the output of one PLC to an input of the other PLC and just use a mapping rung that anytime that input is on turn on the other output. This requires a spare input on each PLC, but is much preferred from tying the two outputs together.