In PLC signal input wiring, we often hear about source-type input wiring and sinking input wiring. Many people are confused about what source-type wiring and sinking wiring actually mean. Today, I will explain what source-type wiring and sinking wiring are.
Source and sink types generally refer to transistor circuits. As the names suggest, sink refers to the signal leaking out, while source refers to the signal flowing in. Since the judgment is based on whether the signal flows in or out, there needs to be a reference point to determine whether the current flows in or out from this reference point. Different PLCs use different reference points.
In the wiring process of Mitsubishi PLC signal input, input point X is used as the reference point. Whether the signal flows in or out from this input point (point X) determines whether it is a source-type connection or a sink-type connection. A signal flowing in from point X is called a source-type connection, and a signal flowing out from point X is called a sink-type connection.
In Siemens PLCs, the common terminal M of the input terminals is used as the reference point. When a signal flows into the common terminal (M) of the input signal terminals, it is called a source input, and when a signal flows out of the common terminal (M) of the input signal terminals, it is called a sink input.
This is why it's called a source connection in Mitsubishi PLCs, but a sink connection in Siemens PLCs.
In the wiring process of Siemens PLCs, if the signal input terminal needs to be connected as a source input, the common terminal M needs to be connected to a 24V voltage terminal; this connection method is also called a common anode connection. If the signal input terminal needs to be connected as a sink input, the common terminal M needs to be connected to a 0V voltage terminal; this connection method is also called a common cathode connection. See the diagram:
In PLC signal input, we typically use either PNP or NPN output type inductive switches. The difference between the two lies in the type of output signal, as shown in the figure:
For NPN type output sensors, when there is a signal output, the signal output line (black) is connected to the negative power supply line (blue), so the output signal is low level. According to the circuit principle, when the output signal of an NPN type sensor is connected to the input point of a PLC, the other common terminal M must be connected to a high level (i.e., the 24V power supply terminal). Therefore, when an NPN type sensor is connected to the input terminal of a PLC, the PLC input terminal connection method should use the source type connection method.
For PNP type output sensors, when there is a signal output, the signal output line (black) is connected to the positive power supply line (brown), so the output is high level. When connected to the input signal terminal of the PLC, the common terminal M must be connected to a low level (i.e., 0V of the power supply). Therefore, it should be connected in a sinking connection.
