PNP and NPN type outputs are available as 3-wire sensors with a DC voltage power supply (the voltage limits are dependent on the specific sensor):
PNP type: switching on the positive voltage load
NPN type: switching on the negative voltage load
These outputs are typical of photoelectric detectors, inductive proximity or other sensors.
The load (relay coil, contactor relay input, PLC input) must be connected between the - and the midpoint.
The corresponding PLC input is Sink.
The load must be connected between the + and the midpoint.
The corresponding PLC input is Source.
Some sensors have two PNP and NPN outputs (4-wire sensors).