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(3) Fail-safe measures against failure of the PLC
Failure of a CPU module or memory can be detected by the self-diagnosis
function. However, failure of I/O control area may not be detected by the
CPU module.
In such cases, all I/O points turn ON or OFF depending on a condition of
problem, and normal operating conditions and operating safety cannot
sometimes be maintained.
Though Mitsubishi PLCs are manufactured under strict quality control,
they may cause failure or abnormal operations due to unspecific reasons.
To prevent the abnormal operation of the whole system, machine
breakdown, and accidents, fail-safe circuitry against failure of the PLC
must be constructed outside the PLC.
Examples of a system and its fail-safe circuitry are described below:
<System example>
Input Input Input Input Output
Power
CPU 16
16
16
16
16
supply
module points points points points points
module
Output
Power Output Output Output 16
16
16 points Vacant
supply 16
Y80
module points points points
Y8F
Output module for fail-safe purpose*1
*1: The output module for fail-safe purpose should be loaded in the last slot
of the system. (Y80 to Y8F in the above system.)
<Fail-safe circuit example>
ON delay timer
Internal program
Y80
T1
1s
OFF delay timer
SM412
Y80
Y80
T2
Y81
External load
L
Y8F
L
1s
MC
24V
0.5s 0.5s
- +
0V
CPU module
24VDC
Output module
*2
T1
T2
MC
*2: Y80 repeats turning ON and then OFF at 0.5 second intervals.
Use a no-contact output module (transistor in the example shown above).
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