Connecting Two PLC outputs

[smithn1 0]

Can you connect two PLC outputs(sourcing) together? and if no why? For example, you want to turn on an indication light when ever 1 or both outputs are on and have it off when both are off. Forget the fact that this could be achieved within software.

[RussB 22]

Sure I can but I wouldn't. Edited 28 Feb 2014 by RussB

[smithn1 0]

Do you have a sound argument for not doing this? I do agree that you should not connect two outputs together, I am just looking for a good explanation on why not to. Edited 28 Feb 2014 by smithn1

[panic mode 246]

you could just add a simple diode (1N4001 for example) between two outputs, then load at one output would be turned only by that output, but load at second output would be on when either of two outputs are on. this however is defeating the whole point of using PLC. it is also not something anyone would expect to see so they would likely be chasing their tail for a while till figuring out what was going on. I agree with Russ, this is not standard practice in industry and I (like everyone else) would not expect or want to see this on a piece of equipment. what would be a sound reason to do this? the only thing I can think of is someone not having software or not knowing password (already guilty for getting themselves into such situation).

[Joe E. 259]

Some electronic outputs don't like having external voltage applied to them. If you absolutely must do this, use relays, with each relay coil controlled by an output and the contacts parallelled. I must echo the others, though: why would you need to do this? Are the outputs on different PLCs?

[BobB 58]

Absolutely stupid proposition in my view! Defenately not good practice even with diodes.

[panic mode 246]

Joe is right, some devices have universal outputs (ones that can both sink and source). Creating low resistance path (like a diode) between two such outputs will cause a short circuit.