Simple question

Can you connect two binary inputs from two separate plc's in paralel in order to input bouth of them with one relay (or pushbutton) for example?
Thanks in advance.

You can but I don't believe its a good practice depending the input circuitry. With the lack of an input signal from a pushbutton or relay contact, there may exist bias currents between the two inputs that could create some issues. At the very least, it would be advised to research the inputs in more detail and possibly contact the mfg for a recommendation.

Thanks a lot, a was thinking the same but hoping that I was wronng
This is going to complicate my project a lot more...
Thanks again.

Use double contacts on the input
one contact for PLC 1 and other for PLC 2.

Dan Bentler

I was trying to avoid that (doubling the contacts), because I'm working on a reconstruction project and I was trying to use as much as I can of old equipement.

Use diodes for the inputs decoupling.

Take the inputs into one PLC and then message the input values to the second PLC

Are you talking about running two wires to two separate PLCs from one contact? I don't know of any reason why you can't. I have a situation where I attached a wire to a input terminal that already had a wire from a prox switch attached and then ran that to a timer. I may have violated a code that I don't know about but it works.

PhilipD said:

I don't know of any reason why you can't.

Click to expand...

google "input bias current"

Greetings beker ...

we MIGHT be misunderstanding exactly what you're asking for ... take a look at the sketch in Figure 1 (attached below) ... this shows ONE button connected to TWO separate input modules ...

notice that there's only ONE supply voltage – and only ONE neutral connection being used in the system that I've sketched – and notice that the supply voltage being used is 120VAC ... this is pretty much just a "plain vanilla" parallel connection – and I've never had any problem doing it this way ...

disclaimer: I've never done this in an actual industrial environment – but we do it quite often in the classroom ... it's a handy way to demonstrate conclusively just how much faster a ControlLogix system can scan its program when compared to something else – for example a PLC-5 ...

Figure 2 (attached below) shows a simple "scan counter" program for a PLC-5 at the top – and the same arrangement for a ControlLogix program at the bottom ... the idea is to press and then release the button as fast as possible – and see how many times each processor can scan its program while the button is making contact ... comparing the numbers that are generated gives the student a pretty good feel for just how fast these things are scanning ...

I'm not sure but I have a feeling that some of the issues raised by the other forum members might come into play with DC circuits - and in particular TTL connections ... so what voltage levels are you planning to use? ...

if we haven't fully answered your question, consider posting again and telling us WHY you're trying make these connections ... maybe we can offer more detailed help ...

and welcome to the forum ...

I was told that Inputs to a PLC should be wired in using that same power supply that the PLC itself uses. (when using a 24VDC sensor, etc.)
Bascially, if you want to send a signal between PLCs, it HAS to go through a relay. Doesn't this remain true if you want to sent the same signal to 2 PLCs? Shouldn't this input from the push button go to a double contact relay where each contact has power from the respective PLC?

Greetings seaston_65 ...

I was told that Inputs to a PLC should be wired in using that same power supply that the PLC itself uses. (when using a 24VDC sensor, etc.)

Click to expand...

maybe I'm just not understanding what you mean – but taken at face value, I'd say that the rule you've given is not exactly correct ... many PLC systems use several DIFFERENT voltages for their individual input and output signals ... yes, you have to match each signal's electrical characteristics to the proper I/O module – but most people don't use an interposing relay each time the input signal doesn't match the PLC processor's power supply ... note that I'm NOT saying that it's NEVER done (I suppose that sometimes isolation issues might come into play) – but as a general rule, you're not going to find a lot of extra relays tied in the way that (I think) you're describing ...

Bascially, if you want to send a signal between PLCs, it HAS to go through a relay.

Click to expand...

take a look at page 8 from the following manual:

http://samplecode.rockwellautomation.com/idc/groups/literature/documents/in/1771-in023_-en-p.pdf

this shows an "official" method of sending the output of one PLC to the input of another ... this isn't exactly what the original poster was asking about – but it does show that signals CAN (at least in some cases) be passed from one system to another without using an interposing relay ...

be sure to take a look at the note which says (in part):

"Use the same ac power source to power both modules to ensure proper phasing and prevent module damage." ... that's runs along the same lines as the notes I made in my earlier post – and the wiring sketch that I attached ...

Shouldn't this input from the push button go to a double contact relay where each contact has power from the respective PLC?

Click to expand...

I'm willing to say that in SOME cases that approach might indeed be the best (safest/most-reliable) method to use ... but maybe our original poster's system isn't very sophisticated at all – and maybe just a couple of wires would do the trick ... we can't tell for sure until (and unless) he adds some more detail to his original question ...

thank you for your input ...

It does not matter what powers the CPU's power supply. What matters is the reference voltage to the input module. As long as both input modules are referenced to a common neutral AND the 120vac hot to the input is in phase with the other 120vac hots to the same module it will work. It will work electrically.

If both PLC's share a common 120vac power source then hook 'em up without isolation.

If the PLC's each have their own 120vac power source then it is not an "acceptable" way to do it. You should isolate them from each other. Use two contacts on the pushbutton, one for each input module. Or use the pushbutton to energize a relay with two contacts, one for each input module.

The main considerations I have about not using isolated contacts:
1. You would not want someone to turn off the power to one plc and think it is safe to work on it - power could still be going to the input module from the external source.
2. It's never a good idea to connect two different 120vac power sources to each other because someone might re-phase the transformer primaries at some point and then you'd have 240vac going to the 120vac input module.

Repeat: If both PLC's share a common 120vac power source then hook 'em up without isolation.

jstolaruk said:

google "input bias current"

Click to expand...

Hey jstolaruk,

I did as you requested and googled "input bias current." Didn't quite understand all that I read -- amplifiers, FETs and, of course, this: the 80 µV and 400 nV/°C numbers of the bipolar, single-supply LT1013A. But I think, and correct me if I'm wrong, your main objection is the risk of "feedback" and that is something that we should all be aware of if doing something like the OP states. It is a risk, but it is a risk that exists in all circuits and each person has to weigh the benifits against the risks.

First, thanks for all your inputs I really appreciate it!

Now I will try to explain my situation a little bit better...

I am working on a reconstruction project therefore I'am looking to change a little as a can (budget an so on...).

I have eight plc's, all of them hooked up, programed and running.
These are small plc's designed for HVAC, so they do not have a communication capability.
Inputs to these plc's (24 V DC) are done through relays, but they do not have double contacts, which is the source of my problem...
Cause, a have a task to gather all these inputs that are going in above mentioned plc's and feed them to a new plc
which will drive the scada system but only to monitor the process on a remote station.
So this new plc will not control the process, that will remain the job for the old ones.
I was thinking to connect these inputs from the old ones parallel with the new one(24 VDC), so I don't have to replace all the relays.

And that's the whole story...