Timers & CountersBeginnerIEC 61131-3

TOF Timer in PLC: Off-Delay Timing and Examples

Quick answer
A TOF timer delays an output turning off. Q becomes TRUE immediately while IN is TRUE. When IN changes to FALSE, ET increases and Q stays TRUE for PT; Q becomes FALSE when ET reaches PT. If IN returns TRUE during the delay, the off-delay is cancelled.

The TOF (Timer Off-Delay) function block provides a delayed turn-off behavior. When the input (IN) goes TRUE, the output (Q) immediately goes TRUE. When IN transitions from TRUE to FALSE, the timer begins counting. Q remains TRUE until the elapsed time (ET) reaches the preset time (PT), at which point Q goes FALSE. If IN goes TRUE again before the timer expires, the timer resets and Q stays TRUE. TOF timers are commonly used for keeping outputs active for a period after an input condition is removed, such as cooling fan run-on, conveyor coast-down delays, and light-off timers.

Reviewed 2026-07-25

Parameters

Inputs

Name	Type	Description
IN	BOOL	Timer input - output follows immediately when TRUE
PT	TIME	Preset off-delay time value

Outputs

Name	Type	Description
Q	BOOL	Timer output - stays TRUE during off-delay
ET	TIME	Elapsed time since IN went FALSE

How TOF off-delay timer works scan by scan

Read the waveforms and state table together. The diagram shows the sequence; the table states what the program should observe at each execution.

TOF off-delay timing diagram

Q follows the rising input immediately. Timing begins on the falling edge of IN, and Q stays high until the preset interval PT expires.

Blue is the input, green is the boolean output, and amber is elapsed time or current count. Transitions are shown at task executions; real timing resolution depends on the controller and task.

State table

Moment	Input	Internal state	Output	Engineering meaning
Input active	IN = TRUE	ET = T#0s	Q = TRUE	There is no delay on the leading edge.
Input removed	IN changes TRUE → FALSE	ET starts increasing	Q = TRUE	The output is held on for the off-delay interval.
Preset reached	IN remains FALSE	ET = PT	Q = FALSE	The commanded run-on period is complete.
Input returns during timing	IN changes FALSE → TRUE	ET resets to T#0s	Q stays TRUE	The pending turn-off is cancelled without an output dropout.

Ladder and Structured Text examples

Cooling-fan run-on after a motor stops

|----[ Motor_Running ]----[ TOF FanRunOn ]----|
|                            PT: T#30s         |
|----[ FanRunOn.Q ]----------( Cooling_Fan )---|

FanRunOn.Q is TRUE while the motor runs and for 30 seconds after Motor_Running becomes FALSE. Confirm that this is process run-on logic, not a safety stop path.

IEC-style Structured Text call

VAR
    FanRunOn    : TOF;
    MotorRunning : BOOL;
    CoolingFan   : BOOL;
END_VAR

FanRunOn(IN := MotorRunning, PT := T#30s);
CoolingFan := FanRunOn.Q;

The output assignment reads the function-block state after the instance call. If MotorRunning returns TRUE before 30 seconds, ET resets and CoolingFan remains TRUE.

Vendor compatibility

A shared mnemonic does not guarantee an identical interface or state machine. Confirm the instruction help for the controller, firmware, language and library version used on the project.

Platform	Instruction	What maps cleanly	Caveat to verify
Siemens S7-1200/1500	TOF with IEC_TIMER instance data	IN falling starts PT; Q remains set during the interval	ET display and instance-retention details must be checked in the selected CPU and TIA Portal version.
Rockwell Logix	TOF in Ladder; TOFR in FBD/ST	DN stays set while the off-delay times after a true-to-false rung transition	The Logix TIMER/FBD_TIMER structures use PRE, ACC and status bits rather than the IEC interface shown here.
CODESYS / IEC libraries	TOF	Common IN, PT, Q and ET interface	A PT below the task period may not clear Q until the next task cycle.

Failure modes and edge cases to test

IN starts FALSE after download or restart

Do not assume Q is initially TRUE. Validate startup behavior and initialize the surrounding command logic.

IN returns TRUE just before PT

The pending off transition is cancelled; Q remains TRUE and ET returns to zero.

PT is zero or below the task period

The visible Q transition is task-resolution limited and may occur on the next execution.

TOF is used in a safety circuit

A standard timer is not a certified safety function. Use the approved safety block and validated architecture.

Evidence and review status

The IEC-style IN, PT, Q and ET behavior was checked against current Siemens and Schneider Electric documentation. Controller-family execution, timing resolution and safety-library behavior remain version specific.

Technical review date: 2026-07-25

Critical behavior

Q becomes true immediately when IN is true; TOF delays only the falling output transition.
When IN falls, ET increases and Q remains true until the preset time has elapsed.
If IN returns true before PT expires, the pending off transition is cancelled and ET resets.
A standard TOF is process logic, not a certified safety delay or emergency-stop function.

Verification checklist

1Prove the immediate Q response when IN rises.
2Measure Q while IN remains false for just under and just over PT.
3Return IN true during the delay and confirm there is no output dropout.
4Test startup, task-resolution and zero-preset behavior on the target controller.

Worked test

Verify a 30-second cooling-fan run-on

Call FanRunOn(IN := MotorRunning, PT := T#30s) and drive CoolingFan from FanRunOn.Q.

1Start the motor and confirm CoolingFan turns on without a 30-second delay.
2Stop the motor and confirm CoolingFan remains on while ET increases.
3Restart the motor at 20 seconds and confirm the pending off transition is cancelled.
4Stop again, leave IN false for the full interval and record the Q falling transition.

Expected result: CoolingFan is on whenever MotorRunning is true and stays on for one complete 30-second interval after the final falling edge.

Technical sources

The target controller's version-specific instruction help remains authoritative.

TOF: Generate off-delay
Siemens STEP 7 V21 documentation — Instance, Q and ET behavior
TOF / TOF_S off-delay function block
Schneider Electric — IEC-style interface and task-cycle caveat
IEC 61131-3:2025 publication record
IEC

Platform-Specific Implementation

Siemens (TIA Portal)

Use IEC_Timer (TOF) in TIA Portal. Legacy: SFB 5 (TOF_TIME). Behaves identically to IEC standard.

Allen-Bradley (Studio 5000)

Use TOF instruction in Studio 5000. Timer data type is TIMER. Access .DN, .TT, .ACC bits.

CODESYS

Standard TOF in Standard library. Fully IEC 61131-3 compliant.

Common Applications

Cooling fan run-on after motor stops
Conveyor coast-down delay
Light-off timer (stairwell lights)
Lubrication pump run-on
Exhaust fan delay

Common Mistakes to Avoid

Confusing TOF with TON behavior
Forgetting that Q goes TRUE immediately when IN is TRUE
Not understanding that the timer only counts when IN is FALSE

Frequently Asked Questions

What is TOF (Timer Off-Delay) in PLC programming?

Output stays TRUE for preset time after input goes FALSE. The TOF (Timer Off-Delay) function block provides a delayed turn-off behavior. When the input (IN) goes TRUE, the output (Q) immediately goes TRUE.

What are common applications of TOF (Timer Off-Delay)?

TOF (Timer Off-Delay) is commonly used for: Cooling fan run-on after motor stops, Conveyor coast-down delay, Light-off timer (stairwell lights), Lubrication pump run-on, Exhaust fan delay.

How do I use TOF (Timer Off-Delay) in different PLC platforms?

Siemens: Use IEC_Timer (TOF) in TIA Portal. Legacy: SFB 5 (TOF_TIME). Behaves identically to IEC standard. Allen-Bradley: Use TOF instruction in Studio 5000. Timer data type is TIMER. Access .DN, .TT, .ACC bits. CODESYS: Standard TOF in Standard library. Fully IEC 61131-3 compliant.