Mitsubishi Electric MELSEC L Series Manual page 4

[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) 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) Machine OPR (Original Point Return) of the positioning function is controlled by two kinds of
data: an OPR direction and an OPR speed. Deceleration starts when the near-point watchdog
signal turns on. If an incorrect OPR direction is set, motion control may continue without
deceleration. To prevent machine damage caused by this, configure an interlock circuit external
to the programmable controller.
(3) When the CPU module detects an error during control by the positioning function, the motion
slows down and stops.
(4) 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 MELSEC-L CPU Module User's Manual (Hardware Design,
Maintenance and Inspection).
(5) 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.
● Configure a circuit so that the external power supply is turned off first and then the programmable
controller. If the programmable controller is turned off 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. Incorrect output or malfunction due to a communication failure may result in an
accident.
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WARNING