Mitsubishi MELSEC-A A1SCPUC24-R2, User Manual

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35
(2) Fail-safe measures to cover the possibility of programmable
controller failure
Problems with a CPU module and memory can be detected by the
self diagnostics function. However, problems with 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 the
problem, and normal operation and safety cannot be maintained.
Though Mitsubishi programmable controllers are manufactured
under strict quality control, they may fail or malfunction due to
unspecified reasons. To prevent the whole system failure, machine
breakdown, and accidents, build a fail-safe circuit outside the
programmable controller.
Examples of a system and its fail-safe circuitry are described
below:
<System example>
*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.)
Vacant
Output module for fail-safe purpose*1
Power
supply
module
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Input
16
points
Input
16
points
Input
16
points
Input
16
points
CPU
module
Power
supply
module
YB0
to
YBF
to
MC
*2
Output module CPU module
0.5s 0.5s
External load
YB0
YB0
0V
24V
YBF
YB1
YB0
24VDC
- +
T1 2 T
MC
L
L
1s
*3 OFF delay timer
2 T
1s
ON delay timer
T1
M9032
Internal program