A proximity sensor that "works on the bench" and then sits dark on the machine is usually a polarity mismatch, not a bad sensor. PLC digital inputs and sensor outputs have to agree on which side sources the current.

Sinking and sourcing

Current in a DC input circuit has a direction.

  • A sinking PLC input draws current into the input terminal. It pairs with an NPN (negative-switching) sensor. The sensor switches the load toward 0 V.
  • A sourcing PLC input pushes current out of the input terminal. It pairs with a PNP (positive-switching) sensor. The sensor switches the load toward +V.

The words are easy to swap because people describe either the sensor or the card. Read the card manual for sink versus source, then buy the sensor to match that card, not the other way around after the panel is built. A PNP sensor on a sinking-only input (or an NPN sensor on a sourcing-only input) does not give a reliable "1". It can look like an open wire.

Outputs follow the same split. A relay output tolerates mixed AC/DC loads and gives galvanic isolation, but it is slow and the contacts wear. A transistor output switches faster and lasts longer, but it is polarity- and voltage-specific: sink or source, and only the rated DC voltage. Do not treat "transistor output" as interchangeable with "relay output" on a contactor coil unless the datasheet says the transistor can carry that load.

What to write on the RFQ

  1. Name the PLC input card and whether each channel is sinking, sourcing, or selectable.
  2. Specify the sensor output as PNP or NPN to match that card, plus the supply voltage (commonly 24 V DC).
  3. If the plant already standardized on one polarity, keep it. Mixed PNP and NPN spares are the usual commissioning fault.
  4. For analog, keep this separate: 4-20 mA is a live-zero current loop (0 mA means a broken wire, not a process zero). It is not a PNP/NPN choice.

This is selection research, not a product offer. SpecForge is an industrial product specification encyclopedia for procurement engineers and automation integrators. It is not a manufacturer or a distributor, and it does not sell PLCs or sensors.

The I/O electricals chapter, including the sink/source pairing and the analog live-zero note, is in the SpecForge PLC selection guide:

https://www.sourcebyspec.com/encyclopedia/plc.html

How the same PNP/NPN output shows up on a proximity switch datasheet is in the proximity sensor guide:

https://www.sourcebyspec.com/encyclopedia/proximity-sensor.html

Verify the card and the sensor against the current manufacturer PDFs before you release the panel wiring.