F_TRIG — Falling-edge trigger
Pin layout
| Direction | Name | Type | Description |
|---|---|---|---|
| Input | CLK | BOOL | Signal to edge-detect |
| Output | Q | BOOL | TRUE for one scan after CLK falls TRUE→FALSE |
Semantics
Q := NOT CLK AND _previousCLK
_previousCLK := CLK
Internal _previousCLK starts as FALSE. So the first call
where CLK = TRUE does not fire Q (no previous TRUE),
but the next call where CLK = FALSE while _previousCLK = TRUE fires Q for one scan.
Example
VAR
edgeFall : F_TRIG;
END_VAR
edgeFall(CLK := xButton);
IF edgeFall.Q THEN
DoOnRelease();
END_IF;
Symmetric to R_TRIG — same usage pattern, only the edge direction differs.
IEC reference
IEC 61131-3 third edition (2013), Annex F.5.2.
matiec conformance
Implemented per the standard.