What is Sinking and Sourcing?

Sinking and Sourcing:

PLC has input and output lines through which it connects to a directing system. The inputs can be keys, switches, sensors, while the outputs are directed to different devices, from simple signalling lights to complex communication modules.

This is a very important part of the history of PLC controllers because it directly influences what can be connected and how it can be connected to the inputs or outputs of the controller. The two terms that are most frequently mentioned when analyzing connections to inputs or outputs are “sink” and “source”. These two concepts are very important to connect a PLC correctly with an external environment. The shortest definition of these two concepts would be:


SINKING = Common GND line (-)
SOURCING = Common VCC line (+)


The first thing that draws attention is the supply “+” and “-“, the supply of CC. The incoming and outgoing sinking or originating inputs and outputs can conduct electricity only in one direction, so they are only supplied with direct current.

According to what we have said so far, each input or output has its own return line, so 5 inputs would need 10 screw terminals in the PLC controller housing. Instead, we use a multi-input connection system to a return line as in the following image. These common lines are usually marked with “COM” on the PLC controller housing.



Input line:

The following images show some examples of sensor outputs and their connection to a PLC controller. The sensor output actually marks the size of a signal given by a sensor on its output when this sensor is active. In one case, this is + V (supply voltage, usually 12 or 24V) and otherwise a GND (0V). Another thing that is worth mentioning is that collapse-provisioning and provisioning are always used: collapse of collapse, and not the matching of supply or provisioning of origin.


Output lines:

The output lines of the PLC controller can usually be:
-transistors in PNP connection
-transistors in NPN connection

The next two images show a realistic way of how a PLC manages external devices. It should be noted that a major difference between these two images is the position of the “output charging device”. By “output charging device” we refer to some relay, signalling light or similar.

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