Mitsubishi Electric MELSEC A Series User Manual page 3

[DESIGN PRECAUTIONS]
(2) When the PLC detects the following problems, it will stop calculation
and turn off all output in the case of (a). In the case of (b), it will stop
calculation and hold or turn off all output according to the parameter
setting.
Note that the AnS series module will turn off the output in either of
cases (a) and (b).
(a) The power supply module has over current
protection equipment and over voltage protection equipment.
(b) The PLC CPUs self-diagnosis functions, such as the watch dog
timer error, detect problems.
In addition, all output will be turned on when there are problems that
the PLC CPU cannot detect, such as in the I/O controller. Build a fail
safe circuit exterior to the PLC that will make sure the equipment
operates safely at such times. See section 9.1 of this manual for
example fail safe circuits.
(3) Output could be left on or off when there is trouble in the outputs
module relay or transistor. So build an external monitoring circuit that
will monitor any single outputs that could cause serious trouble.
When overcurrent which exceeds the rating or caused by short-circuited
load flows in the output module for a long time, it may cause smoke or
fire. To prevent this, configure an external safety circuit, such as fuse.
Build a circuit that turns on the external power supply when the PLC main
module power is turned on. If the external power supply is turned on
first, it could result in erroneous output or erroneous operation.
When a data link results in a communication error, the faulty station
changes in operating status depending on the used data link type.
(1) For the data link data, the data prior to the communication error will be
held.
(2) The MELSECNET (II,/B,/10) remote I/O station will turn all output off.
(3) The MELSECNET/MINI-S3 remote I/O station will hold the output or
turn all output off depending on the E.C. remote setting.
Refer to the data link manuals regarding the method for setting the
communication problem station and the operation status when there are
communication problem.
DANGER
A-2