Mitsubishi Q2ACPU User Manual page 294

19. LOADING AND INSTALLATION
WARNING
Create a safety circuit outside the PLC to ensure the whole system will operate
safely even if an external power failure or a PLC failure occurs.
Otherwise, incorrect output or malfunction may cause an accident.
(1) When creating an emergency stop circuit, a protection circuit or an interlock
circuit for incompatible actions such as forward/reverse rotation or for damage
prevention such as the upper/lower limit setting in positioning, create it outside
the PLC.
(2) When the PLC detects the following error conditions, it stops the operation and
turn off all the outputs.
• The overcurrent or overvoltage protector of the power supply module is
activated.
• The PLC CPU detects an error such as a watchdog timer error by the self-
diagnostics function.
In the case of an error undetectable by the PLC CPU, such as an I/O control
part error, all the outputs may turn on. In order to make all machines operate
safely in such a case, set up a fail-safe circuit or a specific mechanism outside
the PLC.
(3) Depending on the failure of the output module's relay or transistor, the output
status may remain ON or OFF incorrectly. For output signals that may lead to a
serious accident, create an external monitoring circuit.
If load current more than the rating or overcurrent due to a short circuit in the load
has flowed in the output module for a long time, it may cause a fire and smoke.
Provide an external safety device such as a fuse.
Design a circuit so that the external power will be supplied after power-up of the
PLC. Activating the external power supply prior to the PLC may result in an accident
due to incorrect output or malfunction.
For the operation status of each station at a communication error in data link, refer
to the respective data link manual.
Otherwise, incorrect output or malfunction may cause an accident.
When controlling a running PLC (data modification) by connecting a peripheral
device to the CPU module or a PC to a special function module, create an interlock
circuit on sequence programs so that the whole system functions safely all the time.
Also, before performing any other controls (e.g. program modification, operating
status change (status control)), read the manual carefully and ensure the safety. In
these controls, especially the one from an external device to a PLC in a remote
location, some PLC side problem may not be resolved immediately due to failure of
data communications.
To prevent this, create an interlock circuit on sequence programs and establish
corrective procedures for communication failure between the external device and
the PLC CPU.
When setting up the system, do not allow any empty slot on the base unit. If any slot
is left empty, be sure to use a blank cover (AG60) or a dummy module (AG62) for it.
When using the extension base unit, A52B, A55B or A58B, attach the included
dustproof cover to the module in slot 0. This must be done because some internal
parts of the module may be flied during a short circuit test or when an overcurrent or
overvoltage is accidentally applied to the external I/O section.
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