Basic wiring for PLC


Thread Starter



I discussed with my friend today about wiring a switch to a digital input. The discussion was about the zero in the case in the picture (see link). Is this a proper wiring or do I have to wire the zero to the input-card? We both know there is more simple solutions but we couldn't agree about this example.

Yes, wire the -24 Volt to the card also. even if it's somehow connected through the ground system, it should be connected directly to negative to be correct.


Bruce Axtell

Yes, you would need to bring back what you call zero.

However, just because you CAN do it, doesn't mean you SHOULD do it.

Generally, it is not a good practice to mix voltages from different sources.

It would be preferable to use an isolation relay on the field side and wire the dry contact of the relay to switch the voltage of the PLC panel.