Mitsubishi Melsec-Q Series User Manual page 219

(3) Fail-safe measures against failure of the programmable controller
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 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.
Figure 10.3 and Figure 10.4 are a system example and its fail-safe circuit example.
*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.)
No. of slots 16 points 16 points 16 points 16 points 16 points
Output module for fail-safe purpose 1
No. of slots 16 points 16 points 16 points 16 points
Figure 10.3 System example
CHAPTER10 LOADING AND INSTALLATION
9
10
3
4
5
6
7
8
10 - 5