I/O "Common" Terminal Concepts - Automationdirect.com DirectLOGIC DL205 Series User Manual

Chapter 2: Installation, Wiring and Specifications

I/O "Common" Terminal Concepts

1
In order for a PLC I/O circuit to operate,
current must enter at one terminal and exit
2
at another. Therefore, at least two terminals
are associated with every I/O point. In the
figure to the right, the Input or Output
3
terminal is the main path for the current.
One additional terminal must provide the
4
return path to the power supply.
5
If there was unlimited space and budget for
I/O terminals, every I/O point could have
two dedicated terminals as the figure above
6
shows. However, providing this level of
flexibility is not practical or even necessary
7
for most applications. So, most Input or
Output points on PLCs are in groups which
8
share the return path (called commons). The
figure to the right shows a group (or bank) of
four input points which share a common
9
return path. In this way, the four inputs
require only five terminals instead of eight.
10
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
11
sometimes necessary on commons.
Most DL205 input and output modules group their I/O
12
points into banks that share a common return path.
The best indication of I/O common grouping is on the
13
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
14
"CB", respectively.
In the wiring label example, the positive terminal of a
A
DC supply connects to the common terminals. Some
symbols you will see on the wiring labels, and their
B meanings are:
AC supply
C
D
2–18
DL205 User Manual, 4th Edition, Rev. B
DC supply AC or DC supply
– +
Input Switch Output Load
+
–
+
–
L
Field Main Path
Device
(I/O Point)
Return Path
PLC
Input
Sensing
Input 1
Input 2
Input 3
Input 4
Common
IN 24
VDC
A
0 4
1 5
2 6
B 3 7
D2–16ND3–2
20-28VDC
8mA
CLASS 2
CA
0
4
1
5
2
6
3
7
NC
CB
0
4
1
5
2
6
3
7
D2-16ND3-2
PLC
I/O
Circuit