Mitsubishi Electric MELSEC Q Series User Manual page 5

[Design Precautions]
Configure safety circuits external to the programmable controller to ensure that the entire system
operates safely even when a fault occurs in the external power supply or the programmable
controller. Failure to do so may result in an accident due to an incorrect output or malfunction.
(1) Configure external safety circuits, such as an emergency stop circuit, protection circuit, and
protective interlock circuit for forward/reverse operation or upper/lower limit positioning.
(2) The programmable controller stops its operation upon detection of the following status, and the
output status of the system will be as shown below.
Overcurrent or overvoltage protection of
the power supply module is activated.
The CPU module detects an error such as
a watchdog timer error by the
self-diagnostic function.
All outputs may turn on when an error occurs in the part, such as I/O control part, where the CPU
module cannot detect any error. To ensure safety operation in such a case, provide a safety
mechanism or a fail-safe circuit external to the programmable controller. For a fail-safe circuit
example, refer to Chapter 10 LOADING AND INSTALLATION in the QCPU User's Manual
(Hardware Design, Maintenance and Inspection).
(3) Outputs may remain on or off due to a failure of an output module relay or transistor. Configure
an external circuit for monitoring output signals that could cause a serious accident.
A - 2
DANGER
Status Q series module
All outputs are turned off
All outputs are held or
turned off according to the
parameter setting.
AnS/A series module
All outputs are turned off
All outputs are turned off