Mitsubishi Q2ACPU User Manual page 297

19. LOADING AND INSTALLATION
(2)
<System example>
power
CPU
supply
module
module
<Fail safe circuit example>
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:
Input Input Input Input Output
16 16 16 16
points points 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
SM412
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 a fail-safe circuit
using an on delay timer shown on the next page.
Output Output Output
16 16
16 16
points points points
YB0
OFF delay timer *3
External load
YB1 L
YBF L
24V
0V
Output
T1
*2
module
19 - 5
power
Output Output
Output
supply
16 16 16
module
points points
points
Output module for fail-
safe purpose *1
ON delay timer
T1
1s
T2
1s
MC
24VDC
T2
MC
Output
16
points
Empty
YB0
YBF