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(2) Fail-safe measures to cover the possibility of PC failure
Problems with a CPU or memory can be detected by the self diagnosis function.
However, problems with I/O control area may not be detected by the CPU.
In such cases, all I/O points turn ON or OFF depending on a condition of
problem, and normal operating conditions and operating safety cannot
sometimes be maintained.
Though Mitsubishi PCs are manufactured under strict quality control, they may
cause failure or abnormal operations due to unspecified reasons. To prevent
abnormal operations of whole system, machine breakdown, and accidents, build
a fail-safe circuit outside the PC.
The following gives an example of a fail-safe circuit.
Y00
0.5 sec.
*1 Since Y00 alternates between ON and OFF at 0.5 second intervals, use a contactless output
module (in the above example this is a transistor).
*2 If an OFF delay timer (especially a miniature timer) is not available, use ON delay timers to
make a fail-safe circuit as shown below.
Y00
Internal program
Y00
M9032
Y00
Y01
Y0F
24V
0.5 sec.
CPU module
Output unit
A fail-safe circuit built with ON delay timers
Y00
Internal program
M9032
Y00
Y01
0.5 sec. 0.5 sec.
Y0F
24V
CPU module
Output module
ON delay timer
OFF delay timer
External load
L
L
0V
T1
*1
M1
M1
M2
T2
External load
to
– +
0V
T1
29
T1
1 sec.
T2
1 sec.
MC
+
24 VDC
T2
MC
On-delay timer
T1
1s
*1
M1
On-delay timer
T2
1s
M2
MC
L
to
L
24VDC
M2
MC

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