Mitsubishi Electric Q26UD(E)HCPU User Manual page 675

(3) Fail-safe measures against failure of the programmable controller
Failure of a CPU module or memory can be detected by the self-diagnostic 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 programmable controllers 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 programmable controller must be
constructed outside the programmable controller.
A system configuration example and fail-safe circuit example are shown below.
*1 The output module for fail-safe purpose must be mounted in the last slot of the system. (Y80 to Y8F in the above
system.)
Internal program
SM412
Y80
0.5s
CPU module
*2 Y80 repeats turning on and then off at 0.5s intervals.
Use a no-contact output module (transistor in the example shown above).
No. of slots 16 points 16 points
Output module for fail-safe purpose 1
No. of slots 16 points 16 points
Y80
Y80
Y81
Y8F
24V
0.5s
0V
*2
Output module
16 points 16 points 16 points
16 points 16 points
On delay timer
T1
1s
Off delay timer
T2
1s
External load
MC
Load
Load
24VDC
T1 T2
MC
APPENDICES
A
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