(4) When the programmable controller detects an abnor mal condition, it stops the operation and all
outputs are:
• Turned off if the overcurrent or overvoltage prot ection of the power supp ly module is activated.
• Held or turned off according to the parameter setting if the self-diagnos tic 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, re fer to "General Safety Requirements" in 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 moni toring 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.