Confused about source and sink types in PLCs? Here's a simplified explanation:

Confused about source and sink types in PLCs? Here's a simplified explanation:

Source vs Sink: For transistor circuits, source (PNP) means the IO circuit supplies current, while sink (NPN) means it absorbs current.

DO (Digital Output):

PNP (source) outputs are common in Siemens 300/400 series and European PLCs. They have an internal power source, so current flows out through DO, allowing direct relay driving without external power.

NPN (sink) outputs are typical in Japanese, Taiwanese, Siemens 200 series, and most domestic PLCs. They require external wiring to a 24V power source, with current flowing from external devices into the output module.

DI (Digital Input):

The distinction depends on current flow direction relative to the DI terminal.

Generally, if DI's COM is connected to 24V and input is valid at 0V, current flows out (source). If COM is connected to 0V and input is valid at 24V, current flows in (sink).

Special Note on Japanese PLCs: Some (like Mitsubishi) define DI based on acceptable output types. A DI that can accept a source DO is called a "source input"; one that can accept a sink DO is a "sink input".

DIDO Pairing: Source DI can connect to sink DO, and vice versa. Same-type DI/DO connections may need pull-up resistors or other measures to prevent reverse polarity.

Siemens PLCs:

Sink Logic: Signal ON when current flows out of the input terminal.

Source Logic: Signal ON when current flows into the input terminal.

With a positive power supply, a low voltage (0V) at the input terminal during an "ON" signal indicates sink logic; a high voltage (typically 24V) indicates source logic.

Source inputs are high-level valid (current flows in), while sink inputs are low-level valid (current flows out).

Mitsubishi FX3U:

Source: Current flows out, with PNP transistor output characteristics.

Sink: Current flows in, with NPN transistor output characteristics.

Regional Preferences:

S7-200 PLCs can handle both sink and source types, while S7-300 PLCs are typically source. Western Europe often uses source types with PNP switches for high-level inputs. Japan and South Korea prefer sink types, using NPN switches for low-level inputs.

Source outputs are positive DC poles, sink outputs are negative. Siemens PLCs usually have both, but source is more common. Mitsubishi PLCs support both inputs, but sink is more common, corresponding to NPN-type proximity switches.