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Hi guys

I've been working on a project so long that I now can't see the solution to what I imagine is a simple problem. I have a v350 and I am taking a digital input to control an MB.

I have wired many digital inputs that use the same 24vdc power supply as the PLC. But this instance I need to take a switched 24vdc input from another piece of equipment, and I cannot figure out how to wire up correctly!

Please can someone help?!


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If you are using two distinct power supplies then I would recommend using a solid-state-relay between the other device's 24V output and our PLC's Digital input.


If the relay sees 24V from your other device, it will allow power to pass to our input.

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Adding a relay would do the job.


Disconnect your input device from the PLC and connect it to a 24VDC relay coil.

If you are using PNP PLC input ,connect +24V of your PLC power supply to the common pin (Movable contact) of the relay.

If you are using NPN PLC input, connect 0V of your PLC power supply to the common pin (Movable contact) of the relay


Last step is to connect the normally open contact pin of the relay to your PLC input.


If the input will be triggered very frequently, it's better to use a DC load switching solid state relay to avoid electro-mechanical relay failures.

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  • 2 weeks later...
  • MVP 2023

The easy way is to do what Gabriel suggested - tie the commons together.


@Zac - be careful with solid state relays driving a high resistance load such as a PLC input.  Yet another real-life lesson I've learned the hard way.


Depending on the type of device in the solid state relay, it may leak enough current to turn the input on.  Russ didn't say which V350 he's using, and a quick check of the input resistance on various models reveals a variance of 3K to 6.5K.  The Creators can clarify this.  On a PNP input, we list the guaranteed logical 1 voltage at 17 volts.  If you happen to be connected to an input that has the 6.5K resistance, you can get a logical 1 from 2.6 mA.  I've run into problems when you go to turn the thing off - unless you have sufficient load (typically 100 mA) they tend to float on.


This is the kind of problem you want to experiment with on the bench before you go building a whole system based on a spec sheet.  They're not always right.


If you want to use a relay, get one of the little ones that actually has dry contacts in it.


Joe T.

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