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This short video shows you the concept of sinking and sourcing plc inputs.

When a PNP sensor is wired into a power source, it delivers a positive voltage signal to a PLC input that is grounded. In simpler terms, the PNP sensor sends current toward the PLC input, which then routes it to ground. This makes the PNP sensor a source of current, while the PLC input module acts as the sink. Typically, PNP sensors are referred to as sourcing devices because they output +24V signals and are designed to work with sinking PLC input modules, which are internally connected to the system's negative terminal (ground).

On the other hand, an NPN sensor functions by providing a path to ground (negative voltage). To complete the circuit, it relies on a positive voltage, which is supplied by the PLC input module. In this configuration, the PLC input sends current to the NPN sensor, making the PLC the source and the NPN sensor the sink, as it channels the current to ground. NPN sensors are often labeled as sinking devices and usually operate with -24V inputs. They must therefore be paired with sourcing PLC input modules capable of delivering a positive voltage, typically around +24VDC.