1. SINKING AND SOURCING
Two terms most frequently mentioned when discussing connections to inputs or outputs are “sinking” and “sourcing“. These two concepts are very important in connecting a PLC correct
The most brief definition of these two concepts would be:
SINKING = Common GND line (-) SOURCING = Common VCC line (+)

2. First thing that catches one’s eye are “+” and “-” supply, DC supply
First thing that catches one’s eye are “+” and “-” supply, DC supply. Inputs and outputs which are either sinking or sourcing can conduct electricity only in one direction, so they are only supplied with direct current. According to what we’ve said thus far, each input or output has its own return line, so 5 inputs would need 10 screw terminals on PLC controller housing. Instead, we use a system of connecting several inputs to one return line as in the following picture. These common lines are usually marked “COMM” on the PLC controller housing.

3. Sinking and Sourcing are terms used to define the control of direct current flow in a load. A sinking digital I/O (input/output) provides a grounded connection to the load, whereas a sourcing digital I/O provides a voltage source to the load. Consider a simple circuit that consists of one digital input connected to a digital output. The circuit needs a voltage source, a ground, and a load. A sourcing digital I/O provides the voltage needed for the circuit. A sinking digital I/O provides the ground needed in the circuit. The digital input provides the load required for the circuit to work.

4. By applying the circuit principles to the four possible combinations of input/output sinking/sourcing types, there are four circuits, as shown above. The common terminal is the terminal that serves as the common return path for all I/O points in the bank.