Mitsubishi Electric MELCEC Q Series User Manual page 151

9. LOADING AND INSTALLATION
(2)
System example
Fail safe circuit example
Fail-safe measures for programmable controller 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 programmable controllers are manufactured under strict quality
control, create a fail-safe circuit outside the programmable controller to prevent
mechanical damage and accidents in the case of a programmable controller failure
occurred due to any cause.
Examples of a system and its fail-safe circuitry are described below:
Power Input Input Input
CPU
supply 16 16 16
module
module 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
M9032
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 the failsafe circuit
using an on delay timer shown on the next page.
Input Output Output Output Output
16 16 16 16 16
points points points points
YB0
External load
YB1
to
YBF
24V
0V
Output module
*2
9 - 19
Output Output Output
Power
supply 16 16 16
points points points
module
Output module for fail-safe purpose*1
On delay timer
T1
1s
Off delay timer *3
T2
1s
MC
L
to
L
- +
24VDC
T2
T1
MC
Output
16
points
Empty
YB0
to
YBF