Chapter 8: Installation and Wiring
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Do-more T1H Series PLC User Manual, 1st Edition, Rev. C
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I/O “Common” Terminal Concepts
In order for an I/O circuit to operate, current
must enter at one terminal and exit at another.
Therefore, 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..
If there was unlimited space and budget for
I/O terminals, every I/O point could have two
dedicated terminals 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 points are in groups
which share the return path (called commons).
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.
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 Terminator I/O input and output modules
group their I/O points into banks that share a
common return path. The best indication of I/O
common grouping is on the wiring label, such
as the one shown to the right. The miniature
schematic shows two banks of circuits with four
output points in each. The common terminal for
each is labeled COM 0 and COM 1, respectively.
In this wiring label example, the positive terminal
of a DC supply connects to the common
terminals. Some symbols you will see on the
wiring labels, and their meanings are:
+
–
I/O
Circuit
(I/O Point)
Return Path
Field
Device
Main Path
+
–
Input
Sensing
Input 4
Common
Input 3
Input 2
Input 1
L
AC supply
AC or DC supply
Input Switch
Output Load
DC supply
+
–
