TOF — Timer-Off-Delay

Pin layout

Direction	Name	Type	Description
Input	`IN`	`BOOL`	Drives Q TRUE on rising edge; starts the off-delay on falling edge
Input	`PT`	`TIME`	How long Q stays TRUE after IN returns to FALSE
Output	`Q`	`BOOL`	TRUE while IN is TRUE OR within PT after IN’s last falling edge
Output	`ET`	`TIME`	Elapsed since IN’s last falling edge (0 while IN is TRUE)

Semantics

IN goes FALSE → TRUE  : Q := TRUE; ET := T#0s
IN stays TRUE         : Q stays TRUE; ET stays at 0
IN goes TRUE → FALSE  : start counting ET
ET reaches PT          : Q := FALSE
IN goes FALSE → TRUE again before PT : Q stays TRUE; ET reset

TOF is the symmetric opposite of TON: TON delays the rising edge of Q, TOF delays the falling edge.

Common use: drag-out time on a pump after a sensor signal disappears, debounce of a falling level, “stay armed for X seconds after the trigger releases”.

Example

VAR
    releaseDelay : TOF;
END_VAR

releaseDelay(IN := xLevelOk, PT := T#5s);

(* Q is TRUE while xLevelOk is TRUE, AND for 5s after it goes FALSE *)
xPump := releaseDelay.Q;

IEC reference

IEC 61131-3 third edition (2013), Annex F.7.2.

matiec conformance

Implemented per the standard.