Mitsubishi Electric Q06HCPU - Page 675

673
APPENDICES
A
Appendix 9 General Safety Requirements
(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.)
*2 Y80 repeats turning on and then off at 0.5s intervals.
Use a no-contact output module (transistor in the example shown above).
Extension cable
Y80 to Y8F
No. of slots
Output module for fail-safe purpose 1
16 points 16 points 16 points 16 points16 points
No. of slots 16 points 16 points 16 points16 points
Input
Output
Output
Empty
Output
Output Output
Input Input Input
SM412
T1
1s
T2
1s
Load
MC
T2T1
24VDC
Y80
Y81
Y8F
24V
0V
Y80
Y80
0.5s0.5s
MC
Internal program
CPU module
Output module
On delay timer
Off delay timer
External load
*2
Load