R_TRIG — Rising-edge trigger

Pin layout

Direction	Name	Type	Description
Input	`CLK`	`BOOL`	Signal to edge-detect
Output	`Q`	`BOOL`	TRUE for exactly one scan after CLK rises FALSE→TRUE

Syntax

VAR
    edgeRise : R_TRIG;
END_VAR

edgeRise(CLK := xButton);

IF edgeRise.Q THEN
    iCounter := iCounter + 1;
END_IF;

Semantics

Per Annex F.5.1, evaluated on each call:

Q := CLK AND NOT _previousCLK.
_previousCLK := CLK.

The internal _previousCLK starts as FALSE on instance creation. So the first call where CLK = TRUE produces a rising edge regardless of how CLK looked outside the program — useful for one-shot startup actions.

Why edge detection at all?

PLC scan cycles often see the same level signal TRUE for many consecutive cycles. If you want “do X once when the button is pressed”, you can’t just write IF xButton THEN DoX(); END_IF; because that fires DoX() every scan while the button is held down.

R_TRIG turns the level into an event:

edgeRise(CLK := xButton);
IF edgeRise.Q THEN
    DoX();   (* fires exactly once per press *)
END_IF;

IEC reference

IEC 61131-3 third edition (2013), Annex F.5.1.

matiec conformance

Implemented per the standard. The instance-must-live-across- cycles requirement (and therefore the FB nature) is the most common LLM trap — R_TRIG(CLK := x) directly is a compile error; only myEdge(CLK := x) works.