On-delay timer (TON) behavior in PLC Programming - Step-by-Step Execution

Concept Flow - On-delay timer (TON) behavior

Input Signal OFF

↓

Timer NOT Running

↓

Input Signal ON?

No→Stay Idle

Yes↓

Start Timer

↓

Timer Accumulates Time

↓

Timer Done?

No→Keep Timing

Yes↓

Output ON

↓

Input OFF?

Yes→Reset Timer & Output

No↓

Maintain Output ON

The on-delay timer waits for the input to be ON continuously for a set time before turning the output ON.

Execution Sample

PLC Programming

TON(IN=input_signal, PT=3s, Q=output, ET=elapsed_time)

// input_signal changes from OFF to ON at time 0
// Timer counts up to 3 seconds
// Output turns ON after 3 seconds ON input

This simulates an on-delay timer that turns output ON after input stays ON for 3 seconds.

Execution Table

Time (s)	Input Signal	Timer Running	Elapsed Time (ET)	Output (Q)	Action
0	OFF	No	0	OFF	Input OFF, timer not running, output OFF
1	ON	Yes	1	OFF	Input ON, timer started, ET=1s, output still OFF
2	ON	Yes	2	OFF	Input ON, timer running, ET=2s, output still OFF
3	ON	Yes	3	ON	ET reached PT=3s, output turns ON
4	ON	Yes	3	ON	Input still ON, output remains ON
5	OFF	No	0	OFF	Input OFF, timer reset, output OFF

💡 At 5s, input goes OFF, timer resets and output turns OFF

Variable Tracker

Variable	Start (0s)	After 1s	After 2s	After 3s	After 4s	After 5s
input_signal	OFF	ON	ON	ON	ON	OFF
timer_running	No	Yes	Yes	Yes	Yes	No
elapsed_time	0	1	2	3	3	0
output	OFF	OFF	OFF	ON	ON	OFF

Key Moments - 3 Insights

Why does the output stay OFF during the first 2 seconds even though the input is ON?

Because the timer needs to accumulate the full preset time (3 seconds) before the output turns ON, as shown in rows 2 and 3 of the execution_table.

What happens to the timer and output when the input signal goes OFF at 5 seconds?

The timer resets elapsed time to 0 and stops running, and the output turns OFF immediately, as seen in the last row of the execution_table.

Does the output turn ON immediately when the input turns ON?

No, the output only turns ON after the input has been ON continuously for the preset time (3 seconds), demonstrated between 1s and 3s in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at time 2 seconds. What is the output (Q)?

A ON

B OFF

C Undefined

D Flashing

💡 Hint

Check the 'Output (Q)' column at time 2 seconds in the execution_table.

At which time does the timer reach the preset time and output turns ON?

A 1 second

B 2 seconds

C 3 seconds

D 4 seconds

💡 Hint

Look at when 'Elapsed Time (ET)' equals the preset time and output changes to ON in the execution_table.

If the input signal stayed ON for only 2 seconds and then turned OFF, what would happen to the output?

A Output stays OFF

B Output turns ON after 2 seconds

C Output turns ON immediately when input turns ON

D Output toggles ON and OFF

💡 Hint

Refer to the behavior before the timer reaches preset time in the execution_table and variable_tracker.

Concept Snapshot

On-delay timer (TON) waits for input to be ON continuously for a preset time (PT).
Timer starts counting elapsed time (ET) when input is ON.
Output (Q) turns ON only after ET >= PT.
If input goes OFF before PT, timer resets and output stays OFF.
Used to delay actions until input is stable ON.

Full Transcript

An on-delay timer (TON) in PLC programming waits for an input signal to be ON continuously for a set time before turning its output ON. When the input is OFF, the timer is not running and output is OFF. When the input turns ON, the timer starts counting elapsed time. If the input stays ON for the full preset time, the output turns ON. If the input goes OFF before the preset time, the timer resets and output remains OFF. This behavior ensures the output only activates after a stable input signal, preventing false triggers from brief input changes.

Practice

1. What does an On-delay timer (TON) do in a PLC program?

easy

A. It delays turning off the output after the input is deactivated.

B. It immediately turns on the output when the input is activated.

C. It delays turning on the output after the input is activated.

D. It toggles the output on and off repeatedly.

Solution

1. Step 1: Understand the function of an On-delay timer

   An On-delay timer waits for a set time after the input signal turns ON before activating the output.

2. Step 2: Compare options with this behavior

   Only It delays turning on the output after the input is activated. correctly describes this delay before output activation.

3. Final Answer:

   It delays turning on the output after the input is activated. -> Option C

4. Quick Check:

   On-delay timer delays output ON = C [OK]

Hint: On-delay means wait before output turns ON [OK]

Common Mistakes:

• Confusing on-delay with off-delay timer
• Thinking output turns on immediately
• Mixing delay before turning off output

2. Which of the following is the correct syntax to declare an On-delay timer named Timer1 with a preset time of 5 seconds in a typical PLC ladder logic?

easy

A. TON Timer1 PT:=T#5s;

B. TON Timer1 PRE 5;

C. TON Timer1, 5s;

D. TIMER TON Timer1 5;

Solution

1. Step 1: Identify standard TON timer syntax

   In IEC 61131-3 style, TON timers use PT (preset time) with time format like T#5s.

2. Step 2: Match options to this syntax

   TON Timer1 PT:=T#5s; uses PT:=T#5s which is correct syntax for preset time 5 seconds.

3. Final Answer:

   TON Timer1 PT:=T#5s; -> Option A

4. Quick Check:

   Preset time uses PT:=T#5s format [OK]

Hint: Look for PT:=T#5s for preset time in TON [OK]

Common Mistakes:

• Using incorrect time format like '5s' without T#
• Missing PT:= keyword for preset time
• Confusing TON declaration syntax

3. Given the following PLC code snippet using an On-delay timer TON Timer1 with a preset time of 3 seconds:

Timer1(IN:=InputSignal, PT:=T#3s);

// Assume InputSignal turns ON at time 0s
// What is the value of Timer1.Q at 2 seconds?

medium

A. TRUE

B. FALSE

C. Undefined

D. Timer resets

Solution

1. Step 1: Understand TON behavior with preset 3 seconds

   The output Q turns TRUE only after input is ON continuously for 3 seconds.

2. Step 2: Check time at 2 seconds

   At 2 seconds, input has been ON less than 3 seconds, so Q remains FALSE.

3. Final Answer:

   FALSE -> Option B

4. Quick Check:

   Q turns TRUE after 3s delay, at 2s Q = FALSE [OK]

Hint: Q turns TRUE only after preset time elapsed [OK]

Common Mistakes:

• Assuming Q is TRUE immediately when input turns ON
• Confusing preset time with elapsed time
• Thinking Q toggles before preset time

4. You have this PLC TON timer code snippet:

TON Timer1;
Timer1(IN:=StartButton, PT:=T#4s);

// Output Q is not turning ON even after 5 seconds of StartButton pressed.
// What is the likely error?

medium

A. Preset time PT is set too low.

B. Timer1 is missing a reset instruction.

C. Timer1.Q output is not used correctly.

D. StartButton input is not wired or always FALSE.

Solution

1. Step 1: Analyze why Q never turns ON after 5 seconds

   If input StartButton never becomes TRUE, timer never starts counting.

2. Step 2: Check other options

   Preset time 4s is enough; missing reset won't stop Q turning ON; output usage unrelated to timer counting.

3. Final Answer:

   StartButton input is not wired or always FALSE. -> Option D

4. Quick Check:

   Input must be TRUE to start timer [OK]

Hint: Check input signal wiring if timer never starts [OK]

Common Mistakes:

• Assuming timer counts without input TRUE
• Thinking reset is mandatory to start timer
• Ignoring input signal state

5. You want to create a safety light that turns ON only if a door remains open for more than 10 seconds continuously. Which PLC On-delay timer setup correctly achieves this behavior?

hard

A. Use a TON timer with input = DoorOpen sensor, PT = 10s, output controls light.

B. Use a TON timer with input = NOT DoorOpen, PT = 10s, output controls light.

C. Use a TOF (off-delay) timer with input = DoorOpen, PT = 10s, output controls light.

D. Use a TON timer with input = DoorOpen sensor, PT = 5s, output controls light.

Solution

1. Step 1: Identify correct input and delay for safety light

   The light should turn ON only after door is open for 10 seconds, so input must be DoorOpen sensor TRUE and PT = 10s.

2. Step 2: Evaluate options

   Use a TON timer with input = DoorOpen sensor, PT = 10s, output controls light. matches this exactly. Use a TON timer with input = NOT DoorOpen, PT = 10s, output controls light. uses NOT DoorOpen which is wrong. Use a TOF (off-delay) timer with input = DoorOpen, PT = 10s, output controls light. uses off-delay timer which delays turning OFF, not ON. Use a TON timer with input = DoorOpen sensor, PT = 5s, output controls light. uses 5s delay which is too short.

3. Final Answer:

   Use a TON timer with input = DoorOpen sensor, PT = 10s, output controls light. -> Option A

4. Quick Check:

   Delay ON after door open 10s = Use a TON timer with input = DoorOpen sensor, PT = 10s, output controls light. [OK]

Hint: Input = DoorOpen, PT = 10s for delayed ON light [OK]

Common Mistakes:

• Using off-delay timer instead of on-delay
• Using inverted input signal
• Setting delay time too short