F_TRIG

Falling edge detector. Output Q is TRUE for one scan cycle when the input CLK transitions from TRUE to FALSE.

IEC 61131-3	Section 2.5.2.3.2
Support	Supported

Inputs

Name	Type	Description
CLK	BOOL	Signal to monitor for falling edge

Outputs

Name	Type	Description
Q	BOOL	Edge output (TRUE for one scan on falling edge)

Behavior

The function block stores the previous value of CLK. When CLK transitions from TRUE to FALSE, the output Q is set to TRUE for one scan cycle. On all subsequent scans where CLK remains FALSE, Q is FALSE. When CLK is TRUE, Q is always FALSE.

Example

This example shows that no falling edge is detected when CLK starts FALSE (no transition occurred — the default was already FALSE).

Try it

Source

PROGRAM main
   VAR
      edge1 : F_TRIG;
      detected : BOOL;
   END_VAR

   edge1(CLK := FALSE, Q => detected);
   (* detected is FALSE because CLK did not transition from TRUE to FALSE *)
END_PROGRAM

See Also

• R_TRIG — rising edge detector