Basic signal wiring


Thread Starter


Ok, sorry for the basic question but I hope that someone is willing to help. I've got a device with multiple voltage outputs sharing a common. My PLC input module also has a common reference (terminals are connected to the same bus). For example:

Device side
D1 = + of Signal 1
D2 = Common
D3 = + of Signal 2

PLC side
PLC1 and PLC3 are voltage input points.
PLC2 and PLC4 are connected internally.

Would it make sense to use twisted pair wiring make the following connections?

Pair 1
D1 - PLC1
D2 - PLC2

Pair 2
D3 - PLC3
D2 - PLC4

Or does the fact that the commons are connected by multiple paths cause a problem? Leaving out the D2 - PLC4 connection should still work, but I lose the noise suppression of twisted pair. I realize the question would be moot if my PLC module had channel isolation, but it doesn't.
meh... I realized I over-simplified the case I am concerned about, but cannot edit the post to fix it. Lets try a slightly more complicated case.

Device 1 - signal reference is tied to the DC return of a 12 V supply.

Device 2 - signal reference is tied to the DC return of a 24 V supply.

PLC - signal + and reference from each device are connected to the same I/O module, which itself has a common reference bus.

DC return of 12 V PSU is tied to earth ground
DC return of 24 V PSU is tied to earth ground

12 V DC return terminal block is tied to 24 V DC return terminal block.

What do you think?

I think we're not clear about the question or the problem.

One should always use twisted, shielded pairs for each analog signal, even if the COM/RET of all individual power supplies are all referenced to the same earth/ground.

If the two devices are not located in the same place, and the cables interconnecting the two devices are not run in the same conduit or cable tray or raceway then even if the circuit power supply COMs/RETs are all referenced to the same ground, trying to use individual conductors for one, two or three of the two devices in your case will probably not work very well.

One things I've seen that frustrates many people is that they use a less-than ideal wiring circuit/method for one circuit and it works--surprisingly. Then, they try the same thing on another circuit and it doesn't work and they spend an incredible amount of time troubleshooting and blaming the control system and the device and everything but the wiring method. And, they openly admit that when they tried the method previously and it worked, they were surprised when it worked. And, so they just reasoned (wrongly) that it should work on every circuit--even though they admitted other circuits were wired in a standard method. Even when it's proven the new device works by only changing the wiring to industry-standard practices they still insist there is something wrong with the control system or the device--because of their one experience with non-standard wiring.

From the information provided, it doesn't seem there will be any trouble "interconnecting" power supplies by tying the COM/RET wires to the same earth/ground. According to what you've told us, both the 12V and the 24V power supply COM/RET wires are earthed/grounded AND there is a wire interconnecting the two earth/ground TBs.

Unless there's something about the non-isolated input capability (or lack of capability) of the PLC card that we don't understand. Perhaps if you told us the manufacturer and model/part number of the card you're using there might be someone here with experience with that card and multiple power sources.

Again, at least for me, I'm not exactly clear on the problem here. And, if this is a new installation of a PLC, or if you are adding a new input card to an existing PLC.

The more information you can provide, the more help we can be!
I think you've answered my question. There isn't a problem yet, just a little bit of last minute doubt before releasing the wiring instructions.

The I/O module is Opto22, so the logic side / field side isolation is fine, just no channel to channel isolation. This is a new installation, and I think that I was getting thrown off by the fact that the field devices may have several analog or digital input or output signals, but maybe only one or two terminals for COM. An example of this would be the ER-340i motor drive, which has 7 single ended analog input or output terminals (in addition to a few DIO signals) and just 2 COM terminals.

Using twin ferrules I can get a few more connections out of those COM terminals, but it seems odd to need a terminal block just so that each signal can be paired with its reference.

I had come to the conclusion that running the analog and high frequency digital signals with shielded twisted pair was correct, while the DIO could be left on discrete wires as long as the source and Opto22 references were pulled to the same DC Common. Your response has helped restore my confidence in that plan.

In a general case (assuming pos. side signaling from isolated sources), I can make as many connections as I like between the field signal references, the I/O module references, and the DC COM/RET block. Ground loops should only be a problem if there are multiple paths to earth.