Provided by standard library.

The function block F_TRIG a falling edge.

FUNCTION_BLOCK F_TRIG

VAR_INPUT

CLK: BOOL;

END_VAR

VAR_OUTPUT

Q: BOOL;

END_VAR

VAR

M: BOOL := FALSE;

END_VAR

Q := NOT CLK AND NOT M;

M := NOT CLK;

The output Q and the help variable M will remain FALSE as long as the input variable CLK returns TRUE. As soon as CLK returns FALSE, Q will first return TRUE, then M will be set to TRUE. This means each time the function is called up, Q will return FALSE until CLK has a rising followed by a falling edge.

Example in Function Block Diagram

Example in Structured Text

FTRIGInst(CLK:= VarBOOL1);

VarBOOL2 := FTRIGInst.Q;