Provided by standard library.

The function block R_TRIG detects a rising edge.

FUNCTION_BLOCK R_TRIG

VAR_INPUT

 CLK : BOOL;

END_VAR

VAR_OUTPUT

 Q : BOOL;

END_VAR

VAR

 M : BOOL := FALSE;

END_VAR

 Q := CLK AND NOT M;

 M := CLK;

 

The output Q and the help variable M will remain FALSE as long as the input variable CLK is FALSE. As soon as CLK returns TRUE, 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 falling edge followed by an rising edge.