Fail-Safe Circuit Concept - Mitsubishi Electric MELSEC-Q Series User Manual

4.2

Fail-Safe Circuit Concept

To prevent the programmable controller from providing false output or
input, make up the circuit and program so that the programmable
controller system will be started up in the following procedure.
(1) Turn on the power to the programmable controller.
(2) Turn on the external power supply used for the process control.
(3) Turn on the START switch.
(4) Turn on the power to the output devices by using a program.
(5) Confirm that all external power supplies are turned on, and then,
an I/O control program should be executed.
For example, if the processing external supply power of a DC output
module is switched on and then the programmable controller is powered
on, the DC output module may provide false output instantaneously.
Though Mitsubishi programmable controllers are manufactured under
strict quality control, they may cause failure or abnormal operations due
to unspecific reasons. To prevent the abnormal operation of the whole
system, machine breakdown, and accidents (e.g. emergency stop,
protective and interlock circuits), fail-safe circuitry against failure of the
programmable controller must be constructed outside the
programmable controller.
The following page gives an example of system designing that conforms
to the explanation mentioned above and an example of fail-safe
measures when the programmable controller causes a failure.
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