F_TRIG / F_TRIG_S - Falling edge detector

This edge detection function block detects a falling edge. If a falling edge is detected at the input CLK, the output Q changes from FALSE to TRUE. Q remains TRUE until the next call of the function block. If the function block is called for the first time, Q is FALSE until the first falling edge is detected.

The function block is available as standard function block F_TRIG and safety-related function block F_TRIG_S.

F_TRIG

Parameter	Data types	Description
CLK	BOOL	Detects a falling edge
Q	BOOL	If a falling edge is detected, Q changes from FALSE to TRUE

F_TRIG_S

Parameter	Data types	Description
CLK	SAFEBOOL	Detects a falling edge
Q	SAFEBOOL	If a falling edge is detected, Q changes from FALSE to TRUE

NOTE:

Function blocks have to be instantiated. Like variables, instances have to be declared before they can be inserted in a code body. Instances must be unique within the POU. In the following example, the instance name 'F_TRIG_S_1' is used for the F_TRIG_S FB.

Example for a safety-related function block declaration F_TRIG_S

Variables declarations in this example

NOTE:

If you want to use the standard function block F_TRIG in your code worksheet, you have to select the data type 'F_TRIG' for the function block instance in the local variables worksheet. Accordingly, the data type 'BOOL' must be used instead of 'SAFEBOOL'.