Loading And Installation; Fail-Safe Circuit Concept - Mitsubishi Q2ACPU User Manual

19.

LOADING AND INSTALLATION

19 LOADING AND INSTALLATION
This chapter describes the loading and installation procedures and precautions to obtain
the maximum system reliability and performance.

19.1 Fail-Safe Circuit Concept

When the PLC is powered ON or OFF, improper outputs may be generated temporarily
depending on the delay time and start-up time differences between the PLC power supply
and the external power supply for the control target (especially, DC).
For example, if the external power supply for a DC output module is powered ON and then
the PLC is powered ON, the DC output module may generate incorrect outputs temporarily
upon the PLC power-ON. To prevent this, it is required to build a circuit by which the PLC
is powered on first.
Also, an external power failure or PLC failure may lead to erroneous operation.
In order to eliminate the possibility of an system error and to ensure fail-safe operation,
create a circuit (emergency stop circuit, protection circuit, interlock circuit, etc.) outside the
PLC for the parts whose faulty operation could cause mechanical damage and/or
accidents.
A system design circuit example based on the above is provided later.
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