Sinking / Sourcing Concepts - Automationdirect.com DirectLOGIC DL205 Series User Manual

Sinking / Sourcing Concepts

Before going further in the study of wiring strategies, you must have a solid understanding of
"sinking" and "sourcing" concepts. Use of these terms occurs frequently in input or output
circuit discussions. It is the goal of this section to make these concepts easy to understand,
further ensuring your success in installation. First the following short definitions are provided,
followed by practical applications.
Sinking = provides a path to supply ground (–)
Sourcing = provides a path to supply source (+)
First you will notice these are only associated with DC circuits and not AC, because of the
reference to (+) and (–) polarities. Therefore, sinking and sourcing terminology only applies
to DC input and output circuits. Input and output points that are sinking only or sourcing
only can conduct current in only one direction. This means it is possible to connect the
external supply and field device to the I/O point with current trying to flow in the wrong
direction, and the circuit will not operate. However, you can successfully connect the supply
and field device every time by understanding "sourcing" and "sinking".
For example, the figure to the right depicts a "sinking"
input. To properly connect the external supply, you
will have to connect it so the input provides a path to
ground (–). Start at the PLC input terminal, follow
through the input sensing circuit, exit at the common
terminal, and connect the supply (–) to the common
terminal. By adding the switch, between the supply (+)
and the input, the circuit has been completed .
Current flows in the direction of the arrow when the
switch is closed.
Apply the circuit principle above to the four possible combinations of input/output
sinking/sourcing types as shown below. The I/O module specifications at the end of this
chapter list the input or output type.
Sinking Input
Input
+
–
Common
Sourcing Input
Common
+
–
Input
Chapter 2: Installation, Wiring and Specifications
Sinking Output
PLC PLC
Input Output
Sensing Switch
Sourcing Output
PLC PLC
Input Output
Sensing Switch
DL205 User Manual, 4th Edition, Rev. B
Input
(sinking)
+
–
Common
Output
Load
+
–
Common
Common
+
–
Output
Load
1
2
3
4
5
6
7
PLC
8
9
Input
Sensing
10
11
12
13
14
A
B
C
D
2–17