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1 -4
PLC sequentially executes
the stored program and
gets new output results
(has not sent to external
terminals yet)
X0
Y0
M100
X100
X1
X3
M505
X10
Y126
Y0
Y1
First
step
Last step
Input processing (Reading the
status of all external input terminals)
Output processing (Output the resulting
signals to external output terminals)
C
ycl
ic e
xe
cut
ion
Besides the time scan difference mentioned above, the other difference between the conventional and PLC Ladder
Diagram is “Reverse flow” characteristic. As shown in the diagram below, if X0, X1, X4 and X6 are ON, and the remaining
elements are OFF. In a conventional Ladder Diagram circuit, a reverse flow route for output Y0 can be defined by the
dashed line and Y0 will be ON. While for PLC, Y0 is OFF because the PLC Ladder Diagram scans from left to right, if X3 is
off then CPU believes node “a” is OFF, although X4 and node “b” are all ON, since the PLC scan reaches X3 first. In other
words, the PLC ladder can only allow left to right signal flow while conventional ladder can flow bi-directional.
Reverse flow of conventional Ladder diagram
X2
X5
X0
X3
X1
X4
X6
a
b
Y0
X1
Y0
X2
X0
X3
X5
X4
X6
a
b