Do-more H2 Series PLC User Manual, 1st Edition, Rev. F - H2-DM-M
8–22
Chapter 8: Installation and Wiring
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2
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9
10
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A
B
C
D
I/O “Common” Terminal Concepts
In order for a PLC 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. An additional
terminal must provide the return path to the
power supply.
If there were 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 on PLCs are in groups which share the
return path (called commons). The figure
to the right shows a group (or bank) of four
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 Do-more 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. There are two circuit banks with
eight input points in each. The common terminal for each is
labeled “CA” and “CB”, respectively.
In the 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:
L
AC supply
AC or DC supply
Input Switch
Output Load
DC supply
+
–
+
–
I/O
Circuit
PLC
(I/O Point)
Return Path
Field
Device
Main Path
+
–
Input
Sensing
PLC
Input 4
Common
Input 3
Input 2
Input 1
IN
A
B
D2-16ND3-2
24
VDC
20-28VDC
8mA
0
1
2
3
NC
0
1
2
3
CA
4
5
6
7
CB
4
5
6
7
0
1
2
3
4
5
6
7
Содержание Do-more H2 PLC Series
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