Connecting DO of PLC to DI of another PLC

I would like to connect the Digital outputs of a Siemens 1215C to the digital inputs of a Crouzet XD26.

Am I correct in assuming I connect the - terminal of the 24V supplies of each of the PLCs together and then connect the outputs of the Siemens to the inputs of the Crouzet?

Does the XD26 have isolated inputs? If it has, you connect the 0V from the Siemens to 0V on the digital inputs (they should have their own 0V/GND terminal). Now the outputs and inputs have the same voltage potential. Then the output from the Siemens go to the input of the XD26.

If the XD26 doesn't have isolated inputs you can hook both systems together by connecting 0V to 0V (absolutely not 24V). Now both systems have the same potential. This could be problematic though (ground currents) but if they are close together it would work.

Otherwise you have to use relays. The Siemens output to the coil of the relay. And then 24V from the XD26 over the relay contacts and to the input on the XD26. There are really small relays available for this purpose, for instance from phoenix. This is always a safe alternative.

.

I would never hook the output of PLC1 to PLC2, even if they are on the same supply. You are asking for trouble. Please use a simple interposing relay .

The distance between the PLC's will be around 10m or 32ft.

I'm not sure what you mean by this.

If the XD26 doesn't have isolated inputs you can hook both systems together by connecting 0V to 0V (absolutely not 24V)

Click to expand...

Can I connect it like this?

http://imgur.com/R5lphNs

Or would you recommend always using relays?

Your connection will work but I also agree with boneless and suggest the use of an interface relay.
I do what you are trying to do but only when both units are inside the same panel like for example the start/stop of a drive from the output of a PLC without using a relay.

Throw together an interposing relay as the people above have suggested. Even if it's on the same supply, it's just good practice and has the potential to save many, many headaches.

Thanks you ASF and boneless, I will do as you guys suggested.

Just adding if anyone is going to opt not to use an interposing relay, an important specification to check is the leakage current of inputs/outputs.

There is a "maximum off state leakage current" of a Digital output, also a "maximum off state current" of a digital in. It is possible that PLC2 will see the input as ON even when it is off! This problem is not present when using an interposing relay.

No problem to put them direct together (yes i know put relays in between is secure, however very slow)
I use this technique to talk with I2C to other PLC,
the only thing is same power supply and i use a few resistors of 2500 ohms to gnd to be sure the leakage is gone (and yes i use a LED to see what happens in same line.

Leakage is not a concern when you have isolated inputs.

The reason for that is that an isolated input is done by having in opto-coupler on the input. Basically the output is driving a LED. And the input has a phototransistor that sees the light from the LED. So the input is electrically isolated from whatever output is generating the input signal. And you need a fair bit of current and voltage drop over the LED to light it up.

Most PLCs have isolated inputs. But since these where small units it is possible they skimped on that part.

It is not good practice to put relays when there is no need for it. It's just an additional failure point. I'd call it lacy electrical engineer practice.

With that I don't mean to say there is or isn't a need for relays in this application but sometimes it's just frustrating to see relays between output from a PLC and inputs on a VFD when they are in the same cabinet and driven from the same 24VDC.

PS. This is not a dig at anyone. I'm just a little grumpy having worked with the electronic design of control systems.