Wiring PLC output to another PLC input

[JJ1234]

I need one PLC to receive a signal from another PLC.

Can I wire the PLC output of one PLC to the input of another PLC? (same voltage of course).

Do i need to put some type of load in the line like a light?

 

[celichi]

I need one PLC to receive a signal from another PLC.

Can I wire the PLC output of one PLC to the input of another PLC? (same voltage of course).

Do i need to put some type of load in the line like a light?

Recommend you put in an isolation via relay.

In our panels this is done with Yellow Wire.

PLC A Output Drives Relay Coil. PLC B routes input power through contacts of Relay and back into input in PLC B chassis.

 

[JJ1234]

Recommend you put in an isolation via relay.

In our panels this is done with Yellow Wire.

PLC A Output Drives Relay Coil. PLC B routes input power through contacts of Relay and back into input in PLC B chassis.

Thank you,

Do i need a load on the PLC B line?

 

[celichi]

I need one PLC to receive a signal from another PLC.

Can I wire the PLC output of one PLC to the input of another PLC? (same voltage of course).

Do i need to put some type of load in the line like a light?

Thank you,

Do i need a load on the PLC B line?

On our AB Digital Input Cards, no load is required.

If the contacts are outside the panel, then it should be fused.

If the coil is outside the panel it should be fused.

 

[mylespetro]

Is it possible to do this over communications instead of wiring? Or are the PLCs not networked together?

 

[geniusintraining]

I have done it several times and never had an issue and no protection, we had a machine down and it would only run in manual, it was wrote in STL S5 and was password protected so no way of seeing the code to troubleshoot, the other was an HMI failed that was obsolete and the only HMI I had would not replace it, so install the HMI with a PLC to give the other PLC inputs and simulated we were in manual but the other PLC had no idea it just thought someone was pushing the buttons.

It worked great and since they were both sharing the same 24vdc and the same ground, I just ran the output from the one to the inputs to the other.

 

[JJ1234]

Is it possible to do this over communications instead of wiring? Or are the PLCs not networked together?

Not networked together and to get them networked together would be alot of work.

 

[mylespetro]

I have done it several times and never had an issue and no protection, we had a machine down and it would only run in manual, it was wrote in STL S5 and was password protected so no way of seeing the code to troubleshoot, the other was an HMI failed that was obsolete and the only HMI I had would not replace it, so install the HMI with a PLC to give the other PLC inputs and simulated we were in manual but the other PLC had no idea it just thought someone was pushing the buttons.

It worked great and since they were both sharing the same 24vdc and the same ground, I just ran the output from the one to the inputs to the other.

I've also done it, both digitals and re-transmitting analogs from one PLC to the other. I would also recommend an interposing relay between the two if they're using different 24V (or whatever voltage) power supplies.

 

[geniusintraining]

Not networked together and to get them networked together would be alot of work.

What brand of PLC's and what ports do they have available? there are some good protocol converters on the market that may work nice for you if you are needing to do a lot of IO. If its just one or two its not worth the time and just run the wires like you are doing.

 

[celichi]

I would not recommend mixing power supplies from two different panels, avoid at all costs. It may work, or you may end up having ground loops or other issues.

Also if you are bringing power in from a second panel, then it needs to be noted on the Panel that this disconnect does not remove all power sources, hence the Yellow Wire.

For the cost of a 24VDC relay, I see no point in taking that risk.

Depending on what the signal means, if it is critial, remember to make sure it is Fail Safe.

If you do not know what Fail Safe mean, you can google it, but basically, 1, or energized would be the Good or OK or Permissive State.

This way if a fuse is popped, or panel is powered down, or wire is cut, then the process waits or stops.

If you decide to send over Ethernet, then depending if the signal is critical, you would need a Comm Loss Alarm or something similar to detect communication loss.

 

[James Mcquade]

As already suggested use an interface relay.
just because both machines run on 120 vac for example, doesn't mean they have line phase power. even though they may be phased with L1 and L3, doesn't mean they are connected to the same power transformer or distribution panel and are grounded together with the same potential. you also need to use yellow wire for the interface wire to the panel without the relays to indicate an alternate source of power.
james

 

[Ken Roach]

I will join the chorus to emphasize that you need to do this interface with care. I have personally f'ed this sort of thing up in several ways:

• 120V AC derived from different phases of a transformer connected to the same Input module of a PLC-5.
• Triac (AC) outputs with ordinary output leakage causing AC inputs to be true even when the Output isn't true.
• DC outputs with ordinary output leakage doing the same thing.
• Conductors from another system remaining energized when the disconnect for my panel is switched off.

My general recommendations:

• Use 24V DC interfaces.
• Choose one of the systems to provide the DC power and common.
• Use yellow wire to indicate an external source of voltage in the "remote" cabinet.
• Dedicate an entire module to the interface feature on both sides.
• If you can't dedicate an entire module, dedicate an electrically isolated group of I/O points.
• If you can't dedicate an isolated module or group, use a set of interposing relays.
• If you strobe or multiplex the data, take any asynchronous features of the I/O system into account.

"I only need a couple of bits of data" almost always grows. Red Lion's protocol converting gateways are the gold standard for doing this sort of thing between PLCs from different vendors.

 

[arpus4KM]

I can't say I haven't done this. But I can definitely say that I've worked on a few that used this method almost exclusively to pass information.

system had 6 slc 5/04 spread through different sections of the system. These were all connected through dh+, but for some reason they didn't use any messaging. All information that was passed between controllers for part location or interlocking was done through outputs to inputs.

I can't say how it was wired electrically concerning the transformers and how that plays together, but this is what they did. I realized that happened when I went to add some messaging to cut out a latch at a specific time of day from one controllers status to anothers. I put the messaging in and then found that there was nothing but outputs to inputs for plc to plc status.


Large automated dipping system that spans a whole end of a plant, 20+ years old.

 

[Rson]

Just a note - I believe the color wire has changed from "Yellow" to "Orange" depending on where you are in the USA and which NFPA standard you are following locally.

 

[parky]

I agree with a lot that has been said here, the way we used to implement signals from one cabinet to the other is definitely isolate i.e. relays or analogue isolators, these would be contained in a see though lid box with a warning that equipment in this enclosure will be live and not isolated by this panel with a location of where these signals come/go to.
Again Orange cables to distinguish the other potential (regardless of voltage) .