Short answer

PNP and NPN describe the output transistor's switching direction. A PNP output sources current toward the load; an NPN output sinks current toward 0 V.

PNP sensor to PLC input wiring diagramA three-wire PNP sensor sends positive voltage to a PLC input.PNP / SOURCING OUTPUTSENSOR3-wire DCPLC INPUTsinking / common −I0.0+24 V / brownsignal / black0 V / blue Original diagram · Signal Field Guide / field note wiring

What to look for

In a typical three-wire DC sensor, brown is positive supply, blue is 0 V, and black is the switched output. Wire colours are common, not universal law: the datasheet and connector pinout win whenever they disagree.

The practical check is the input common. A PLC input arranged to receive current from a PNP sensor needs a suitable 0 V reference on the other side. If that reference is missing or the input mode is wrong, a perfectly good sensor appears dead.

FIELD CASE / REASONING PATH

When the symptom appears in the plant

A machine builder shipped a packaging line to a new plant, but several proximity sensors failed to trigger the PLC inputs during commissioning. The machine was originally designed for the European market using PNP (sourcing) sensors, but the local maintenance team had replaced a damaged input card with an NPN (sinking) compatible module common in their older Asian equipment. The sensors were sending 24V correctly, but the PLC input card was expecting a path to 0V (ground) to register a 'true' state.

DIAGNOSTIC SEQUENCE

Five checks before replacement

  1. Identify the sensor output type by looking at the part number, datasheet, or physical wiring diagram printed on the sensor body.
  2. Check the PLC input card specifications to determine whether it is a sinking (requires PNP) or sourcing (requires NPN) module.
  3. Measure the voltage between the sensor output wire (black) and 0V (blue) while triggering the sensor; a 24V reading confirms a PNP output.
  4. Measure the voltage between the sensor output wire (black) and 24V (brown) while triggering; a 24V reading confirms an NPN output.
  5. Verify that the common terminal on the PLC input card is wired correctly (0V for sinking inputs, 24V for sourcing inputs).
PRACTICAL NOTES

PNP sensors are the standard in Europe and North America; NPN is more common in parts of Asia.

Some modern PLC input cards are configurable for either sinking or sourcing depending on how the common terminal is wired.

If you must mix types, use an interposing relay to convert the signal.

BEFORE YOU REPLACE THE PART

Bench checklist

  • Confirm the supply at the device terminals.
  • Confirm the meter is in the correct mode and position.
  • Confirm the input range matches the signal type.
  • Confirm the reference/common is intentional.
  • Record the reading before changing the wiring.

Reference families

  • IEC 61131-2 Standard
  • Sensor Manufacturer Technical Notes

Always verify the current edition and the exact device manual for the installation.

READ THE PRIMARY MATERIAL

Official references

These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.

This note is an educational reference, not a substitute for the equipment manual, site procedures, or qualified electrical work.