Mitsubishi 13J851, Руководство пользователя

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(2) Fail-safe measures to cover the possibility of PLC failure
Although a CPU module detects failures of PLC CPU module and memory
by using the self-diagnostics function, it may not detect those of I/O control
area.
In such cases, all I/O points may turn ON or OFF depending on the failure,
or the control target may malfunction or the safety may not be ensured.
Mitsubishi PLCs are manufactured under strict quality control, however,
build a fail-safe circuit outside the PLC in order that the PLC failure due to
unspecified reasons will not result in the machinery breakdown or
accidents.
The system example including fail safe circuit and fail safe circuit example
are provided below.
<System example>
Input
16
points
Power
supply
module
Output module for fail-safe purpose*1
CPU
module
Input
16
points
Input
16
points
Input
16
points
Output
16
points
Output
16
points
Vacant
Output
16
points
Output
16
points
Output
16
points
YBF
YB0
Power
supply
module
Output
16
points
Output
16
points
Output
16
points
to
*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.)
<Fail-safe circuit example>
Internal program
SM412
T1
ON delay timer
1s
T2
OFF delay timer
1s
L
L
MC
T2 T1
+ -
24VDC
YB0
YB1
YBF
24V
0V
YB0
YB0
External load
0.5s 0.5s
CPU module Output module
*2
MC
to
*2: Y80 turns ON and then 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.