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(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>
Input
Power
CPU
16
supply
module
points
module
*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.)
YB0
Input
Input
Input
Output
16
16
16
16
points
points
points
points
Internal program
YB0
M9032
YB0
YB1
YBF
24V
0.5s 0.5s
CPU module
Output module
Output
Output
Output
16
16
16
points
points
points
ON delay timer
OFF delay timer
External load
L
to
L
0V
T1
*2
35
Output
Output
Output
Power
16
16
16
supply
points
points
points
module
Output module for fail-safe purpose*1
T1
1s
*3
T
2
1s
MC
- +
24VDC
T
2
MC
Output
16
points
Vacant
YB0
to
YBF
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