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LADDER LOGIC • FX5U / GX WORKS3

How do I latch and reset an alarm in FX5U ladder?

SET the alarm memory when the fault occurs, and RST it only when the reset command is present and the original fault is clear.

Practical explanation

A latched alarm preserves evidence after a brief fault disappears.

Separate alarm acknowledgement from permission to restart the machine.

Check in this order

  1. 1Choose a memory device with the required retention behavior.
  2. 2Prevent reset while the fault remains active.
  3. 3Display the original cause, not only a common alarm.

Ladder instruction example

LD X10
SET M100
LD X11
ANI X10
RST M100

Example only—allocate devices and interlocks for your own project.

Devices and registers

X10: fault condition
X11: reset command
M100: alarm memory

Safety: Never use an ordinary PLC memory bit as the sole storage or reset element for a safety trip.

Last reviewed: 2 October 2026

Official source: MELSEC iQ-F FX5 Programming/Application manuals

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