Combing 2 separate PLC outputs into a single connection

Sprayman

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Jan 2020
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Chicago
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Hi there,

Quick Question. I have 2 PLCs and 2 solenoids. I would like to wire these up so that If either one turns on, it will turn on both Solenoids.

My initial thought is to bring both outputs to a single wire and then wire both solenoids to this.

My question is. Would doing this destroy a PLC? For example.

If PLC-2 Outputs 24V and turns on both solenoids. Would PLC-1 see this potential and ignore it? or would I generate some magic smoke?

These PLCs are internally made so I am not sure about the schematic exactly. Just wondering what the rule of thumb is for this kind of setup.

Thanks!
 
Need a bit more information in my opinion, such as PLC and I/O card models, are the digital outputs fed by separate 24VDC supplies, etc.

My initial thought if they're fed from the same 24VDC supply would be to wire both outputs to a relay and have the solenoids wired to the NO contact(s) of the relay.
 
Sprayman said:
Quick Question. I have 2 PLCs and 2 solenoids. I would like to wire these up so that If either one turns on, it will turn on both Solenoids.

My initial thought is to bring both outputs to a single wire and then wire both solenoids to this.

My question is. Would doing this destroy a PLC? For example.
Click to expand...

1st, Why?

2nd, don't do this (EVER).

3rd, 1st PLC controls both solenoids, 2nd PLC communicates with 1st PLC and requests solenoids go active. With both feedback of permissive such as eSTOP or any other fault logic that wont activate the solenoids.

Is this a weird homework problem?

Because if it is and "communication isn't allowed", take output from 2nd PLC and go to relay that is voltage isolated and goes to input of 1st PLC, then 1st PLC controls solenoids.

I'm sure you could come up with some "genius plan" to have n+1 PLCs controlling an output... but why would you ever do that.

Thanks for coming to my TED talk.
 
This is a place for interposing relays. Have each PLC turn on a relay instead of the solenoid. Wire contacts from the relays to the solenoids in your desired configuration. You can use a single power supply for both solenoids rather than power supply for each. If you decide to go with a separate power supply for each solenoid, connect the negative side of the two power supplies together.

Edit: And why on earth did the OP post the same question twice?
 
Steve Bailey said:
This is a place for interposing relays. Have each PLC turn on a relay instead of the solenoid. Wire contacts from the relays to the solenoids in your desired configuration. You can use a single power supply for both solenoids rather than power supply for each. If you decide to go with a separate power supply for each solenoid, connect the negative side of the two power supplies together.

Edit: And why on earth did the OP post the same question twice?
Click to expand...

I refuse to do relay logic unless its for voltage separation or I actually have no other options. (SSD relay is probably fine but relays last for like a one hundred thousand cycles min... its such an annoying problem to fix when the relay stops working)

Also, what if I want 10 PLCs to turn these solenoids on... 10+ relays? If they all had an LED so I can tell which relays are turning them on but that sounds like a headache.
 
Steve Bailey said:
I'm willing to use relays when they are a good tool for an application. I try not to paint myself into a corner by being overly dogmatic.
Click to expand...

I think its more limiting using relays when you have a PLC... Literally the thing that was made to replace relays.

But, each their own.
 
VAN said:
I think its more limiting using relays when you have a PLC... Literally the thing that was made to replace relays.

But, each their own.
Click to expand...


Too many people mis-apply PLC outputs and fire loads they are not intended for.
 
Last edited:
VAN said:
I think its more limiting using relays when you have a PLC... Literally the thing that was made to replace relays.

But, each their own.
Click to expand...
A PLC replaces relay logic, not necessarily relays themselves. There are plenty of situations where you might want an interposing relay.

And in this case OP has stated the PLC logic cannot be changed; relay logic is left as the sensible option.
 
padees said:
Too many people mis-apply PLC outputs and fire loads they are not intended for.
Click to expand...

PLC = logic controller, not using the output as what supplies power...

plvlce said:
A PLC replaces relay logic, not necessarily relays themselves. There are plenty of situations where you might want an interposing relay.

And in this case OP has stated the PLC logic cannot be changed; relay logic is left as the sensible option.
Click to expand...

I think you guys are doing this on purpose, the other thread has you guys literally jumping hurtles to do logic with relays. So 2 PLCs can turn on a solenoid(s).

smh
 

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