Double Output (Double Coil) Operation And Countermeasures - Mitsubishi Electric MELSEC FX Series Structured Programming Manual

FXCPU Structured Programming Manual
(Application Functions)
1.3.2

Double output (double coil) operation and countermeasures

This subsection explains the double output (double coil) operation and countermeasures.
1. Operation of double outputs
When a coil (output variable) is used twice (double coils) in another program block to be executed or in the
same program block, the PLC gives priority to the latter coil.
Suppose that the same coil Y003 is used in two positions as
shown in the right figure.
For example, suppose that X001 is ON and X002 is OFF.
In the first coil Y003, the image memory is set to ON and the
output Y004 is also set to ON because the input X001 is ON.
In the second coil Y003, however, the image memory is set to
OFF because the input X002 is OFF.
As a result, the actual output to the outside is "Y003: OFF, Y004:
ON".
2. Countermeasures against double outputs
Double outputs (double coils) do not cause an illegal input error (program error), but the operation is
complicated as described above.
Change the program as shown in the example below.
A B
C E
D
The SET and RST instructions or jump instruction can be used instead, or a same output coil can be
programmed at each state using step ladder instructions STL and RET.
When you use the step ladder instructions STL and RET, note that the PLC regards it as double coils if you
program, inside the state, an output coil located outside the RET instruction from another program block or
the STL instruction.
Y000
Ignored
Y000
Or
1.3 Cautions on Creation of Fundamental Programs
Input processing
X001=ON X002=OFF
X001
Y003
Second
X002
Output processing
Y003=OFF Y004=ON
Y000
A B
C E
D
M100
A B
M101
C E
D
Y000
M100
M101
1 Outline
First
Y003
Y004
Y003
A
17
1
2
3
4
5
6