S5 AWL Command: FP (Positive Edge)

FP detects a positive (rising) edge — the transition from FALSE to TRUE. The VKE is TRUE for exactly one scan cycle when the monitored signal changes from FALSE to TRUE. FP requires a marker bit (flag) to store the previous state.

Syntax#

U   E 0.0          // Signal to monitor
FP  M 10.0         // Edge memory flag (must be unique to this edge detection)
=   A 4.0          // Output: TRUE for one scan on rising edge

S7 Equivalent#

Code Example — Count Button Presses#

U   E 0.0          // Push button
FP  M 10.0         // Detect rising edge only
ZV  Z 1            // Count up once per press (not continuously)

Without FP, the counter would increment every scan cycle while the button is held down. With FP, it increments exactly once per press.

How It Works#

M 10.0 stores the previous value of the VKE. FP sets VKE to TRUE only when the current VKE is TRUE and M 10.0 (previous state) was FALSE.

Common Errors#

Reusing the same edge memory flag for multiple FP instructions: Each FP must have its own unique marker bit. Sharing M 10.0 between two FP instructions produces unpredictable results.

Related: FN (Negative Edge) — Detects TRUE→FALSE transition#

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

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