Mitsubishi Electric MELSEC-A series User Manual page 40

(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.)
SM412
YB0
*2: Since YB0 turns ON and OFF alternatively at 0.5 second intervals, use a
contactless output module (a transistor is used in the above example).
*3: If an offdelay timer (especially miniature timer) is not available, construct
the failsafe circuit using an ondelay timer shown on the next page.
Input Input Input Output Output
16 16 16 16
points points points points points
Internal program YB0
YB0
YB1
YBF
24V
0.5s 0.5s
0V
CPU module Output module
Output Output
16 16 16
points points
ON delay timer
T1
OFF delay timer *3
T2
External load
L
to
L
T1 T2
*2
26
Output
Output Output Output
Power
points
16 16 16
supply
points points points
module
Output module for fail-safe purpose*1
1s
1s
MC
- +
24VDC
MC
16
Vacant
YB0
to
YBF