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Old December 1st, 2021, 01:53 PM   #1
Sprayman
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Question Combing 2 separate PLC outputs into a single connection

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!
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Old December 1st, 2021, 01:57 PM   #2
mylespetro
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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.
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Old December 1st, 2021, 04:04 PM   #3
VAN
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Quote:
Originally Posted by Sprayman View Post
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.
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.
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Old December 1st, 2021, 04:51 PM   #4
Steve Bailey
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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?
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Old December 1st, 2021, 05:48 PM   #5
VAN
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Quote:
Originally Posted by Steve Bailey View Post
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?
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.
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Old December 1st, 2021, 06:15 PM   #6
Steve Bailey
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Quote:
I refuse to do relay logic unless its for voltage separation or I actually have no other options.
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.
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Old December 1st, 2021, 06:57 PM   #7
Steve Bailey
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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=131342
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Old December 1st, 2021, 07:51 PM   #8
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Quote:
Originally Posted by Steve Bailey View Post
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=131342

Hey, there are 2 PLC's and 2 solenoids, so seems logical to have 2 threads.

Last edited by padees; December 1st, 2021 at 07:53 PM.
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Old December 1st, 2021, 07:51 PM   #9
VAN
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Quote:
Originally Posted by Steve Bailey View Post
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.
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.
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Old December 1st, 2021, 08:03 PM   #10
padees
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Quote:
Originally Posted by VAN View Post
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.

Too many people mis-apply PLC outputs and fire loads they are not intended for.

Last edited by padees; December 1st, 2021 at 08:05 PM.
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Old December 1st, 2021, 11:19 PM   #11
plvlce
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Quote:
Originally Posted by VAN View Post
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.
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.
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Old December 2nd, 2021, 06:24 AM   #12
saultgeorge
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Quote:
Originally Posted by padees View Post
Too many people mis-apply PLC outputs and fire loads they are not intended for.
+1 on that!!
__________________
"Instant success teaches one nothing. Failure provides the tools and instructions one needs to succeed."
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Old December 3rd, 2021, 04:37 PM   #13
VAN
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Quote:
Originally Posted by padees View Post
Too many people mis-apply PLC outputs and fire loads they are not intended for.
PLC = logic controller, not using the output as what supplies power...

Quote:
Originally Posted by plvlce View Post
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.
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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