Sinking and sourcing are two fundamental concepts in PLC (Programmable Logic Controller) wiring and digital input/output (I/O) configurations. They refer to different ways of connecting input and output devices to a PLC system.

1. Sinking (NPN - Negative-Positive-Negative): In a sinking configuration, the current flows from the positive voltage supply through the load (input or output device) and then to the PLC's input or output terminal. The load is connected to the common ground, which is typically the 0V reference of the PLC. When the input is active (e.g., a sensor is triggered or a switch is closed), it allows current to flow through the load, completing the circuit.

For input devices, sinking configuration is common for sensors like NPN (N-channel, negative logic) proximity sensors and switches. For output devices, sinking configuration is used with NPN (N-channel) transistors, where the current is drawn to the 0V common terminal when the output is active.

2. Sourcing (PNP - Positive-Negative-Positive): In a sourcing configuration, the current flows from the PLC's positive voltage supply through the load and then to the common ground (typically the 0V reference of the PLC). When the input is active, or the output is active, it allows current to flow through the load, completing the circuit.

For input devices, sourcing configuration is common for PNP (P-channel, positive logic) proximity sensors and switches. For output devices, sourcing configuration is used with PNP (P-channel) transistors, where the current is sourced from the positive voltage supply when the output is active.

It's essential to understand the type of devices you are working with and the PLC's input/output specifications to ensure correct wiring. Mismatching the configuration can lead to incorrect operation and potential damage to the devices or the PLC. Many PLC systems can be configured to accept both sinking and sourcing configurations, making them versatile for various applications.