Can I measure one sensor with two PLCs? I have a PLC that's part of a machine manufacturing stuff.

Another PLC is part of a robotic arm that needs to grab the piece.

It would be intresting to be able to read a number of sensors into both PLCs.

However I end up frying the sensors.

The sensor is a photovoltaic sensor that outputs a simple '1' (24V) or '0' (0V).

  • \$\begingroup\$ What communication protocol are you using? \$\endgroup\$ Nov 28, 2018 at 13:08
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    \$\begingroup\$ It might be easiest to just have one PLC read the sensor and then send that reading to the other. \$\endgroup\$
    – Hearth
    Nov 28, 2018 at 13:09
  • \$\begingroup\$ Just a simple sensor connected to the PLC. It only has to give '1' (24V) or '0' (0V) as an output \$\endgroup\$ Nov 28, 2018 at 13:10
  • \$\begingroup\$ @Felthry the problem is that it's very hard to access either PLC's \$\endgroup\$ Nov 28, 2018 at 13:10
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    \$\begingroup\$ You don't have to use a Communication Protocol (High usage) as you say, just wire a digital output from one PLC to an input on the other. \$\endgroup\$
    – Tyler
    Nov 28, 2018 at 16:14

1 Answer 1


a voltage signal can be shared between PLCs if they both share the same ground voltage.

that said the sensor needs a 24V supply that supply should be provided in such a way that with one of the PLCs turned off there will be no malfunction. this may mean carefully chosing which PLC is responsible for powering it, or powering it from both using a pair of diodes to prevent back-feeding power.

current signals (eg 4-20mA loop) cannot be shared as easily.

  • \$\begingroup\$ Instead of having a PLC provide power to the sensor, you can have a separate field power supply that's independent of either PLC power.current signals (eg 4-20mA loop) cannot be shared as easily?? I don't know where you got that from. A 4-20mA sensor/transmitter can drive many devices connected in series as long as you don't exceed the rated resistance and you are on a common power source. If on different power sources, you can use signal isolators. \$\endgroup\$ Nov 30, 2018 at 18:06
  • \$\begingroup\$ I was uinder the impression that most PLC 4-20 inputs were gounded one side and a voltage source the other, but yeah if there are passive inputs they could share it. \$\endgroup\$ Nov 30, 2018 at 19:00
  • \$\begingroup\$ No with 4-20mA, it's a current loop so you don't have grounds at the devices but at the voltage source itself. In most cases, there are no grounds at all. \$\endgroup\$ Dec 6, 2018 at 23:14

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