Mitsubishi Electric MELSEC-Q Series User Manual page 80

(2) Fail-safe measures to cover the possibility of programmable
controller failure
Although a CPU module detects failures of programmable
controller CPU and memory by using the self-diagnostics function,
it may not detect those of I/O control area.
In such cases, all I/O points may turn on or off depending on the
failure, or the control target may malfunction or the safety may not
be ensured.
Mitsubishi programmable controllers are manufactured under strict
quality control, however, build a fail-safe circuit outside the
programmable controller in order that the programmable controller
failure due to unspecified reasons will not result in the machinery
breakdown or accidents.
The system example including fail safe circuit and fail safe circuit
example are provided below.
<System example>
Power
Input
CPU
supply
16
module
module
points
*1 The output module for fail-safe purpose should be mounted on the last slot of the system.
(YB0 to YBF in the above system.)
Input Input Input Output Output
16 16 16 16
points points points points points
Extension cable
Output Output
16 16 16
points points
46
Output
Power Output Output Output
supply 16 16 16
module points points points
Output module for fail-safe purpose*1
16
points
Vacant
YB0
to
YBF