Installation, Wiring,
and Specifications
2–16
Installation, Wiring, and Specifications
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 to 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.
+
–
I/O
Circuit
PLC
(I/O Point)
Return Path
Field
Device
Main Path
If we had unlimited space and budget for
I/O terminals, then every I/O point could
have two dedicated terminals just as the
figure above shows. However, providing
this level of flexibility is not practical or
even necessary for most applications. So,
most Input or Output point groups on
PLCs share the return path among two or
more I/O points. The figure to the right
shows a group (or bank) of 4 input points
which share a common return path. In this
way, the four inputs require only five
terminals instead of eight.
+
–
Input
Sensing
PLC
Input 4
Common
Input 3
Input 2
Input 1
Note: In the circuit above, the current in the common path is 4 times any channel’s
input current when all inputs are energized. This is especially important in output
circuits, where heavier gauge wire is sometimes necessary on commons.
Most DL05 input and output circuits are
grouped into banks that share a common
return path. The best indication of I/O
common grouping is on the wiring label.
The I/O common grouping bar, labeled at
the right, occurs in the section of wiring
label below it. It indicates X0, X1, X2, and
X3 share the common terminal located to
the left of X1.
The following complete label shows two banks of four inputs and two banks of three
outputs. One common is provided for each bank.
The following label is for DC output versions. One common is provided for all of the
outputs and the terminal on the bottom right accepts power for the output stage.
I/O “Common”
Terminal Concepts
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