FEDGE

Typing FEDGE evaluates the falling edge of a Boolean expression.
TIP: The IEC 61131-3 standard does not include the FEDGE operator.
FEDGE Operator
Name
FEDGE
Syntax
<edge> := FEDGE ( <boo_expression>, <memo_variable> );
Operands
First operand is any Boolean variable or complex expression
Second operand is an internal Boolean variable used to store the last state of the expression
Return value
TRUE when the expression changes from TRUE to FALSE
FALSE for all other cases
An execution cycle detects one falling edge of an expression with the FEDGE operator. Call the FEDGE operator from a function block instead of the F_TRIG function block.
TIP: Trigger edges for an expression with a variable other than the memory Boolean variable that stores the last state of the expression.
When the expression is a Boolean variable, declare a unique internal variable with same name and the EDGE_prefix for use in the FEDGE expressions for the variable. This internal variable prevents other FEDGE operations from overwriting the memory variable.
Example
(* ST program using FEDGE operator *)

(* this program counts the falling edges of a Boolean input *)

(*Bi120 is an input Boolean variable *)

(* Edge_Bi120 is the memory of the Bi120 variable state *)

If FEDGE (Bi120, Edge_Bi120) Then

  Counter := Counter + 1;

End_if;
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