Timers & CountersBeginnerIEC 61131-3

TON Timer in PLC: On-Delay Timing and Examples

Quick answer
A TON timer delays an output turning on. When IN remains TRUE, ET increases toward PT; Q becomes TRUE when ET reaches PT. If IN becomes FALSE, a standard non-retentive TON resets Q and ET. The timer instance must be executed regularly for its state to update.

The TON (Timer On-Delay) is one of the most commonly used function blocks in PLC programming. It starts counting when the input (IN) transitions from FALSE to TRUE, and sets the output (Q) to TRUE after the preset time (PT) has elapsed. While the input remains TRUE, the elapsed time (ET) continues to increment until it reaches PT. When the input goes FALSE, both Q and ET are immediately reset to their initial values. TON timers are essential for implementing time delays, debouncing inputs, sequence control, and motor start delays in industrial automation systems.

Reviewed 2026-07-25

Parameters

Inputs

Name	Type	Description
IN	BOOL	Timer enable input - starts timing when TRUE
PT	TIME	Preset time value (e.g., T#5s for 5 seconds)

Outputs

Name	Type	Description
Q	BOOL	Timer done output - TRUE when ET >= PT
ET	TIME	Elapsed time - current timer value

How TON on-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.

TON on-delay timing diagram

IN rises first. ET accumulates until the preset interval PT has elapsed, then Q rises. IN falling resets both ET and Q.

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
Before enable	IN = FALSE	ET = T#0s	Q = FALSE	The timer is reset and ready for a new continuous interval.
Enable begins	IN changes FALSE → TRUE	ET starts increasing	Q = FALSE	The on-delay interval has started but is not complete.
Preset reached	IN remains TRUE	ET = PT	Q = TRUE	The condition persisted for the complete delay.
Enable removed	IN = FALSE	ET resets to T#0s	Q = FALSE	A standard TON does not retain partial or completed time.

Ladder and Structured Text examples

Motor permissive after a five-second delay

|----[ Start_Request ]----[ TON StartDelay ]----|
|                            PT: T#5s           |
|----[ StartDelay.Q ]--------( Motor_Permissive )|

StartDelay must stay enabled for the complete five seconds. Use StartDelay.Q as the delayed permissive; do not drive the motor directly from the timer-enable rung.

IEC-style Structured Text call

VAR
    StartDelay      : TON;
    StartRequest    : BOOL;
    MotorPermissive : BOOL;
END_VAR

StartDelay(IN := StartRequest, PT := T#5s);
MotorPermissive := StartDelay.Q;

Declare one persistent instance and call it on each intended task execution. A temporary instance or conditional call can lose or freeze the state needed to measure elapsed time.

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	TON with IEC_TIMER instance data	Same on-delay intent and IN/PT/Q/ET model	The instance data and task call determine state updates; verify DB retentivity separately from normal TON behavior.
Rockwell Logix	TON in Ladder; TONR in FBD/ST is a different instruction	PRE/ACC and DN represent the preset, elapsed value and done state	Logix tags, status bits and execution rules are controller-family specific. Do not infer IEC parameter names from the mnemonic alone.
CODESYS / IEC libraries	TON	Common IN, PT, Q and ET interface	Timer resolution and what happens when an instance is not called depend on the runtime and task configuration.

Failure modes and edge cases to test

IN drops one scan before PT

Q remains FALSE and the standard TON resets ET; the next TRUE interval starts from zero.

PT is shorter than or close to the task interval

Q can only be observed on a task execution. Measure the effective delay on the target runtime.

The timer call is inside conditional logic

Outputs may stop updating while the call is skipped. Prefer a regular call with a controlled IN input.

The same instance is reused

Two logical conditions overwrite one state machine. Give each independent delay its own timer instance.

Evidence and review status

TON behavior and standard IN, PT, Q and ET interface were checked against current CODESYS documentation. Rockwell timer structures and time bases are vendor-specific.

Technical review date: 2026-07-25

Critical behavior

A rising IN starts timing; a falling IN resets a non-retentive TON.
Q becomes true when IN remains true and the elapsed time reaches PT.
The block must be called for its state to update.
A preset close to task period or timer resolution needs platform-specific timing tests.

Verification checklist

1Test false input, timing input, done state and reset state.
2Interrupt IN before PT and verify ET and Q reset.
3Confirm time literal syntax and time base in the target IDE.
4Check that one timer instance is not reused for unrelated conditions.

Worked test

Test a five-second start delay

Call StartDelay(IN := EnableRequest, PT := T#5s).

1Hold EnableRequest false and record Q and ET.
2Set EnableRequest true for 4 seconds.
3Continue until at least 5 seconds.
4Set EnableRequest false.

Expected result: Q stays false before 5 seconds, becomes true after the continuous delay and resets with ET when EnableRequest goes false.

Technical sources

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

TON Standard function block
CODESYS Development
TON: Generate on-delay
Siemens STEP 7 V20 documentation — S7-1200/1500 IN, PT, Q and ET behavior

Platform-Specific Implementation

Siemens (TIA Portal)

In TIA Portal, use IEC_Timer (TON) or legacy SFB 4 (TON_TIME). Access via 'Timer operations' in the instruction tree. Data type is IEC_TIMER for S7-1200/1500.

Allen-Bradley (Studio 5000)

Use TON instruction in Studio 5000/RSLogix 5000. Preset (PRE) is in milliseconds by default. Timer data type is TIMER. Access .DN (done), .TT (timing), .ACC (accumulated).

CODESYS

Standard IEC TON available in Standard library under Timers. Time literals use T# prefix (e.g., T#5s, T#100ms). Fully IEC 61131-3 compliant.

Common Applications

Motor start delay
Debouncing inputs
Sequence timing
Watchdog timers
Pump priming delay

Common Mistakes to Avoid

Forgetting that ET resets when IN goes FALSE
Using TON when TOF is more appropriate for off-delay
Not accounting for scan time in very short timer presets
Assuming timer continues after IN goes FALSE

Frequently Asked Questions

What is TON (Timer On-Delay) in PLC programming?

Starts timing on rising edge, output activates after preset delay. The TON (Timer On-Delay) is one of the most commonly used function blocks in PLC programming. It starts counting when the input (IN) transitions from FALSE to TRUE, and sets the output (Q) to TRUE after the preset time (PT) has elapsed.

What are common applications of TON (Timer On-Delay)?

TON (Timer On-Delay) is commonly used for: Motor start delay, Debouncing inputs, Sequence timing, Watchdog timers, Pump priming delay.

How do I use TON (Timer On-Delay) in different PLC platforms?

Siemens: In TIA Portal, use IEC_Timer (TON) or legacy SFB 4 (TON_TIME). Access via 'Timer operations' in the instruction tree. Data type is IEC_TIMER for S7-1200/1500. Allen-Bradley: Use TON instruction in Studio 5000/RSLogix 5000. Preset (PRE) is in milliseconds by default. Timer data type is TIMER. Access .DN (done), .TT (timing), .ACC (accumulated). CODESYS: Standard IEC TON available in Standard library under Timers. Time literals use T# prefix (e.g., T#5s, T#100ms). Fully IEC 61131-3 compliant.