Wiring using relay contacts by looping from different sources of 24V DC

I'm not a PLC person but if "24 DC" means power supply then it looks like you are always shorting out one of the power supplies. Drawbacks would be many and would include melted relay parts flying through the air if the power supplies are large enough.

I'm only guessing, but, I suspect that this is what you really want

That way assuming that PLC A is always powered then PLC B will be powered when the relay is activated.

OR what are you trying to achieve?

The PLC A & PLC B have their own power supply that is 24V + and -

Is the purpose to operate PLC A in condition A (relay de-energized), and under condition B (relay energized) PLC A is unpowered and PLC B operates?

If this is correct, DPDT relay (already mentioned) will be required to make this scheme work. Some consideration regarding break-before-make function of relay should likely be given.

<...what are you trying to achieve?...>

Start with that. A few sentences will do.

PLC A & B require a DI (Digital Input) from the same relay. The relay do not have additional aux contacts on them, hence trying this!

I think I understand what you wish to do but you still have not made it perfectly clear. My guess is that you have been tasked with connecting two Programmable Logic Controllers (PLC) inputs to a relay that has only one remaining auxiliary contact. If my guess is correct you have omitted at least three critical pieces of information:

1. Where does the 24V (DC?) come from and how is it wired to power each of these PLCs?
2. What type of input circuitry (sink, source, other) do each of these PLC expect for valid detection?
3. Why are two independent PLC systems used?

Depending on the answers to these three questions, other questions may follow. What I hope for is my questions focuses you to solve your problem independently. If that happens, please come back and at least tell us it works now and why does it work.

OK. So the drawing in the original post to this thread doesn't describe the real-life installation. Fine. The words do.

The first problem is that of how the <...Digital Input...>s are to be connected. It is usual for the + rail of the power supply to be common to all inputs on the same input card. The contact connected to the input places a +ve on the input terminal when the contact is closed. This raises a bit in the input card, which the software uses in an "examine on" or "examine off" instruction in the logic. The -ve rail on the power supply is usually common to the input card, and the circuit is completed by connecting the - rail of the input card to the power supply. The drawing in the original post doesn't show this.

The next problem is that of isolation of supplies. Assuming that <...PLC A & B...> are fed from different DC power supplies it is inappropriate to put these onto a common return unless it is absolutely certain that there is no other connection between them. The forum has not been advised of this. Neither has it been advised as to where the coil of the <...relay...>, the contact for which is in the original drawing, is being fed from; is it the supply for <...PLC...A...>, or is it <...PLC...B...>, for example? Or are both <...PLCs...> fed from the same supply? The drawing in the original post doesn't show this.

Three solutions:

1. It would be better for the <...relay...> in question to switch a new slave relay and a clean contact from the new one to provide the input for the second <...PLC...>, thereby keeping the two supplies separate from each other. They are, after all, relatively inexpensive.
2. It would be better to replace the <...relay...> with one that has two changeover contacts, and wire one to each <...PLC...>'s input. They are, after all, relatively inexpensive.
3. An alternative would be to network the two <...PLC...>s, and use a 1 in one of them to become a 0 in the other, and vice versa.
4. There may be others.

Evaluation of the options is outside the domain of the forum and must remain with the original poster.

That deals with the what. The why is different. It remains a mystery to the forum why the <...relay...> in question has to toggle between PLCs' inputs, and might evermore remain so.

Your question and suggestions assume that the DI power supplies for PLC-A and PLC-B are operating at the same voltage to ground and that making their DC- terminals common is OK. I strongly doubt that. You will do best to have the available auxiliary contact on your device's relay energize a separate DPST or DPDT relay so the two PLC's DI's are kept electrically separate.

--JMM

Auxiliary contacts can be developed by using a double pole double thro relay instead of just using a single pole double throw in my opinion..

Your wiring diagram is incomplete...

Maybe you're looking for something like this....maybe

Whatever you intend to accomplish, it will be better to use a double pole - double throw relay, which will avoid any unexpected short circuits.