S5 AWL Command: FN (Negative Edge)

FN detects a negative (falling) edge — the transition from TRUE to FALSE. The VKE is TRUE for exactly one scan cycle when the monitored signal changes from TRUE to FALSE.

Syntax#

U   E 0.0          // Signal to monitor
FN  M 10.1         // Edge memory flag (unique per FN)
=   A 4.1          // Output: TRUE for one scan on falling edge

S7 Equivalent#

Code Example — Detect Sensor Release#

U   E 0.0          // Sensor (TRUE while part is present)
FN  M 10.1         // Detect when part leaves sensor
=   M 20.0         // "Part has passed" signal — TRUE for one scan

Typical Use#

Detecting when a signal turns off: button release, sensor clearing, motor stopping, valve closing. Essential for sequencing — "do this when X finishes" rather than "do this while X is active."

Related: FP (Positive Edge) — Detects FALSE→TRUE transition#

Part of the S5 AWL Command Reference. Maintained by PLCcheck.ai.

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