Fail-Safe Measures For Programmable Controller Failure - Mitsubishi Electric MELSEC iQ-R Series Configuration Manual

Fail-safe measures for programmable controller failure

A CPU module and a remote head module can detect hardware failures of themselves and their memory by the self-
diagnostic function. However, failures which occur in a part, such as an I/O control part, may not be detected.
In this case, depending on the failure, all input or output points may turn on or off, or normal operation and safety of the
control-target device may not be ensured.
Even though Mitsubishi programmable controllers are manufactured under strict quality control, they may fail due to some
reasons. Provide fail-safe circuits external to the programmable controller so that no machine is damaged and no accident
occurs.
A system example and its fail-safe circuit example are shown below.
*1 Mount an output module for fail-safe purpose in the last slot of the system. In the example above, Y80 to Y8F are assigned to the output
module.
Internal program
SM412
Y80
Y80
0.5s 0.5s
CPU module
*2 Since Y80 turns on and off at 0.5 second intervals, use a contactless output module. (In the example above, a transistor output module
is used.)
APPX
192
Appendix 8 General Safety Requirements
Y80
Load
Y81
Load
Y8F
24V
0V
*2
Output module
T1
On delay timer
T1
1s
Off delay timer
T2
1s
MC
24VDC
T2
MC
*1