Mitsubishi Electric MELSEC iQ-F Series User Manual page 4

[DESIGN PRECAUTIONS]
WARNING
● Make sure to set up the following safety circuits outside the PLC to ensure safe system operation
even during external power supply problems or PLC failure. Otherwise, malfunctions may cause
serious accidents.
- Most importantly, set up the following: an emergency stop circuit, a protection circuit, an interlock
circuit for opposite movements (such as normal vs. reverse rotation), and an interlock circuit (to
prevent damage to the equipment at the upper and lower positioning limits).
- Note that when the CPU module detects an error, such as a watchdog timer error, during self-
diagnosis, all outputs are turned off. Also, when an error that cannot be detected by the CPU
module occurs in an input/output control block, output control may be disabled. External circuits
and mechanisms should be designed to ensure safe machinery operation in such a case.
- Note that when an error occurs in a relay, transistor or triac of an output circuit, the output might
stay on or off. For output signals that may lead to serious accidents, external circuits and
mechanisms should be designed to ensure safe machinery operation in such a case.
● In an output circuit, when a load current exceeding the current rating 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.
● For the operating status of each station after a communication failure of the network, refer to relevant
manuals for the network. Incorrect output or malfunction may result in an accident.
● Construct an interlock circuit in the program so that the whole system always operates on the safe
side before executing the control (for data change) of the PLC in operation.
Read the manual thoroughly and ensure complete safety before executing other controls (for program
change, parameter change, forcible output and operation status change) of the PLC in operation.
Otherwise, the machine may be damaged and accidents may occur due to erroneous operations.
● Especially, in the case of a control from an external device to a remote programmable controller,
immediate action cannot be taken for a problem on the programmable controller due to a
communication failure. Determine the handling method as a system when communication failure
occurs along with configuration of interlock circuit on a program, by considering the external
equipment and CPU module.
● Do not write any data to the "system area" and "write-protect area" of the buffer memory in the
intelligent function module. Executing data writing to the "system area" or "write-protect area" may
cause malfunction of the programmable controller alarm. For the "system area" or "write-protect
area", refer to  Page 136 Buffer Memory.
● If a communication cable is disconnected, the network may be unstable, resulting in a communication
failure of multiple stations. Construct an interlock circuit in the program so that the system always
operates on the safe side even if communications fail. Incorrect output or malfunction may result in an
accident.
2