Mitsubishi Q2ACPU, User Manual

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19 - 5
19. LOADING AND INSTALLATION
(2) Fail-safe measures for PLC failure
While failure of a CPU module and its memory can be detected by the self-
diagnostics function, an error occurred in I/O control area may not be detected by the
CPU module.
In such a case, depending on the condition of the failure, all device points could turn
ON or OFF resulting in a situation where normal operations of the control target and
safety cannot be ensured.
Though Mitsubishi PLCs are manufactured under strict quality control, create a fail-
safe circuit outside the PLC to prevent mechanical damage and accidents in the case
of a PLC failure occurred due to any cause.
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.)
<Fail safe circuit example>
*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 a fail-safe circuit
using an on delay timer shown on the next page.
power
supply
module
power
supply
module
CPU
module
Input
16
points
Input
16
points
Input
16
points
Input
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
Output
16
points
YB0
YBF
Empty
Output module for fail-
safe purpose *1
SM412
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