14-12-2012, 03:05 PM
Sinking and Sourcing Concepts
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Introduction
When choosing the type of input or output
module for your system (or
DL05/DL06/DL105 I/O type), it is very
important to have a solid understanding of
sinking and sourcing concepts. Use of
these terms occurs frequently in discussion
of input or output circuits. It is the goal of
this section to make these concepts easy to
understand, so you can make the right
choice the first time when selecting the type
of I/O points for your application. This
section provides short definitions, followed
by general example circuits.
First you will notice that the diagrams on
this page are associated with only DC
circuits and not AC, because of the reference
to (+) and (-) polarities. Therefore,
sinking and sourcing terminology applies
only to DC input and output circuits. Input
and output points that are sinking or
sourcing can conduct current in one direction
only. 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, the
supply and field device can be connected
every time based on an understanding of
sourcing and sinking.
The figure below depicts a sinking input.
To properly connect the external supply, it
must be connected so the input provides a
path to supply common(-). So, 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 is completed. Current flows in the
direction of the arrow when the switch is
closed.
Common terminals and how to use them
In order for a PLC I/O circuit to operate,
current must enter at one terminal and
exit at another. This means at least two
terminals are associated with every I/O
point. In the figure at the right, the input
or output terminal is the main path for the
current. One additional terminal must
provide the return path to the power
supply. Together, the main path and the
return path create a loop, or a complete
circuit for current to flow.
If there was unlimited space and budget
for I/O terminals, then every I/O point
could have two dedicated terminals.
However, providing this level of flexibility
is not practical or even necessary for most
applications. So, most input or output
points on PLCs are in groups that share
the return path (called commons). The
figure at the right shows a group (or
bank) of four input points that share a
common return path. In this way, the four
inputs require only five terminals instead
of eight.