Mitsubishi Electric Q02HCPU-A - Page 151

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LOADING AND INSTALLATION9.
(2) 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:
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 the failsafe circuit
using an on delay timer shown on the next page.
Power
supply
module
CPU
module
Input
16
points
Output
16
points
Empty
Output module for fail-safe purpose*1
Input
16
points
Input
16
points
Input
16
points
Output
16
points
Output
16
points
Output
16
points
Power
supply
module
Output
16
points
Output
16
points
Output
16
points
Output
16
points
YB0
to
YBF
M9032
T1
1s
T2
1s
L
L
MC
T1
24VDC
YB0
YB1
YBF
24V
0V
YB0
YB0
0.5s0.5s
MC
CPU module
T2
External load
Internal program
On delay timer
Off delay timer *3
Output module
*2
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to
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