Chapter 8: Installation and Wiring
Do-more T1H Series PLC User Manual, 1st Edition, Rev. C
8-27
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 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 Terminator I/O system 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.
By applying 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.
+
–
Input
Sensing
Input
Common
(sinking)
+
–
Input
Sensing
Load
Sinking Input
Sinking Output
Sourcing Input
Sourcing Output
Input
Common
+
–
Output
Switch
Output
Common
+
–
Input
Sensing
Load
Input
Common
+
–
Output
Switch
Output
Common
