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[Design Precautions]
WARNING
● 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) Emergency stop circuits, protection circuits, and protective interlock circuits for conflicting
operations (such as forward/reverse rotations or upper/lower limit positioning) must be configured
external to the programmable controller.
(2) When the programmable controller detects an abnormal condition, it stops the operation and all
outputs are:
• Turned off if the overcurrent or overvoltage protection of the power supply module is activated.
• Held or turned off according to the parameter setting if the self-diagnostic function of the CPU
module detects an error such as a watchdog timer error.
Also, all outputs may be turned on if an error occurs in a part, such as an 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 the "GENERAL SAFETY REQUIREMENTS" chapter in the Safety
Guidelines included with the CPU module or head module.
(3) Outputs may remain on or off due to a failure of a component such as a transistor in an output
circuit. Configure an external circuit for monitoring output signals that could cause a serious
accident.
● In an output circuit, when a load current exceeding the rated current or an overcurrent caused by a
load short-circuit flows for a long time, it may cause smoke and fire. To prevent this, configure an
external safety circuit, such as a fuse.
● Configure a circuit so that the programmable controller is turned on first and then the external power
supply. If the external power supply is turned on first, an accident may occur due to an incorrect output
or malfunction.
● For the operating status of each station after a communication failure, refer to relevant manuals for
each network. Failure to do so may result in an accident due to an incorrect output or malfunction.
● When changing data from a peripheral device connected to the CPU module to the running
programmable controller, configure an interlock circuit in the program to ensure that the entire system
will always operate safely. For other forms of control (such as program modification or operating
status change) of a running programmable controller, read the relevant manuals carefully and ensure
that the operation is safe before proceeding. Especially, when a remote programmable controller is
controlled by an external device, immediate action cannot be taken if a problem occurs in the
programmable controller due to a communication failure. To prevent this, configure an interlock circuit
in the program, and determine corrective actions to be taken between the external device and CPU
module in case of a communication failure.