Mitsubishi A1SJH(S8) User Manual page 109

8. LOADING AND INSTALLATION
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System example
Example fail safe circuits
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, there is a possibility of setting all points to ON or OFF, or a situation
may develop where normal operations and safety of the controlled subject cannot be
assured, depending on the condition of the failure.
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:
Power Input Input Input
CPU
supply 16 16 16
module
module
points points 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.)
Internal program
M9032
YB0
YB0
0.5s 0.5s
CPU module
*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 off delay timer (especially miniature timer) is not available, construct the failsafe circuit
using an on delay timer shown on the next page.
Input Output Output Output Output
16 16 16 16 16
points points points points points
YB0
External load
YB1
to
YBF
24V
0V
Output module
*2
8 - 4
Power Output Output Output
16 16
supply
points points points
module
Output module for fail-safe purpose*1
On delay timer
T1
1s
Off delay timer *3
T2
1s
MC
L
to
L
- +
24VDC
T2
T1
MC
Output
16
16 points
Empty
YB0
to
YBF