External 24 VDC power supplies for PLC inputs.

Hello everyone, I have a PLC question, in regards to using several external 24 VDC power supplies, from different machines ! I only have (2) commons on my PLC inputs, but I need (1) more common. Can I use any -24 VDC common or does it have to be from that specific power supply ? Can my PLC 24 VDC inputs come from any external source, without that sources -24 VDC common ?

The input is an LED and a series resistor. The LED is inside an optoisolator. Treat it like a low current lamp.
All 24DC supplies need to have their -24 sides tied to each other and the PLC common.
If not, then you'll need to add more input cards.
Are the (2) commons you have now isolated?

You can common the -24vdc s of all the power supplies together. Such is the case if they bond to ground. This would insure your inputs would work.

I have (2) isolated commons on my inputs that I would tie the (2) separate power supplies -24 VDC too, but I have a (3rd) power supply to deal with. All (3) power supplies are exactly the same, does it matter if their isolated, or can I tie all my -24 VDC commons together, and not worry about isolation ?

If you are using any power supply to drive PLC inputs the common from that Power Supply must be connected to the common on the PLC input card.
Doesnt matter how many power supplies or what brand/type.




The easiest thing and most common approach is to connect all the input card commons and Input Power supply commons together.
Set up a few jumpered terminal blocks closest to the Input card and pull all your commons to them.
You dont need to isolate the input groups unless you have a SPECIFIC reason to do so.