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8. LOADING AND INSTALLATION
8 - 3
MELSEC-A
(2) Fail-safe measures against PLC failures
Problems with the CPU or memory can be detected by the self-diagnosis
function. However, problems with the I/O control area may not be detected by
the CPU.
If such a problem arises, all I/O points turn ON or OFF depending on the
nature of the problem, and it may not be possible to maintain normal
operating conditions and operating safety.
Although Mitsubishi PLCs are manufactured under strict quality control, they
may fail or operate abnormally due to unspecifiable reasons. To prevent the
abnormal operation of the whole system, machine breakdown, and accidents,
build a fail-safe circuit outside the PLC.
The following is an example of a fail-safe circuit.
M9032
Y00
0.5
sec
0.5
sec
Y00
MC
Y00
Y01
Y0F
24 V
0 V
T1
T2
OFF delay timer
1 sec
1 sec
L
L
to
External load
CPU module Output module *1
+ -
MC
T1
T2
to
Internal program
ON delay timer
24 V DC
*1 : Y00 repeats turning ON and then OFF at 0.5 second intervals. Use a no-
contact output module (transistor in the example shown above).