Wiring Inputs from two machines

I have two seperate machines that are sending signal to one input module the signal is +24VDC. The input module only has one terminal for common. Can I connect the -24VDC from both machines to the same terminal so that inputs from both machines will work?

If you have multiple power supplies that are 24vdc and input signal to plc, always tie the 24vdc common together or common to same potential. Then it will work.

Thank you. I was just worried about causing some sort of issue by having the negatives of two different machines connected together.

Don't think of it as two different machines, but rather, two different power supplies. A machine can have multiple power supplies, so we are all assuming that you have 1 power supply per machine. Also, we are assuming that both power supplies are the same input and output voltages.

Each machine sending one signal to one input module. So each of those signals is ultimately coming from one power supply, so those commons can be tied together as long as output voltages from each power supply are the same. I'm unaware if the max current from each power supply has any affect, maybe someone else can clarify. I just know as long as output voltages are the same, then combining those commons together should be ok.

Hi

I would suggest to put 2 interposing relay. One for each input

joaco1993 said:

Hi

I would suggest to put 2 interposing relay. One for each input

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I second this suggestion

Don't mix you two machines power supplies, put interface relays

tespoors said:

Thank you. I was just worried about causing some sort of issue by having the negatives of two different machines connected together.

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That is a valid concern. I would agree with the others who suggest interposing relays, unless these are rapidly changing signals in which case I'd carefully study of the risks of connecting the commons or use solid state interposing relays.

the wiring will need to conform to NFPA 79 color coding standards.
2007 edition Section 13.
13.2.4.1 the color orange shall be used to identify ungrounded conductors that remain energized when the main supply circuit disconnecting means is in th off position.
'this means that you need orange conductors in the interposing relay panel to identify the conductors going to the other panel. you also need to have a sticker denoting that the panel also has more than one source of power.
james

OkiePC said:

That is a valid concern. I would agree with the others who suggest interposing relays, unless these are rapidly changing signals in which case I'd carefully study of the risks of connecting the commons or use solid state interposing relays.

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Another thing that might pop up here is if one of the power supplies negative is grounded and the other is not, or both are and the grounds between the 2 machines have [Ecm I think it was called - a voltage difference between the 2 grounds that on one application was 5 volts, enough to give ghost encoder signals to a servo drive]

It may be advisable to bond the 2 machine grounds together.