Mitsubishi A2USCPU(S1), User Manual

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8 - 4
LOADING AND INSTALLATION 8.
(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, 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:
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.)
Example fail safe circuits
*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.
Power
supply
module
CPU
module
Input
16
points
Output
16
points
Empty
Output module for fail-safe purpose*1
Input
16
points
Input
16
points
Input
16
points
Output
16
points
Output
16
points
Output
16
points
Power
supply
module
Output
16
points
Output
16
points
Output
16
points
Output
16
points
YB0
to
YBF
M9032
T1
1s
T2
1s
L
L
MC
T1
24VDC
YB0
YB1
YBF
24V
0V
YB0
YB0
0.5s 0.5s
MC
CPU module
T2
External load
Internal program
On delay timer
Off delay timer *3
Output module
*2
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