On-delay timer (TON) behavior in PLC Programming - Deep Dive

Overview - On-delay timer (TON) behavior

What is it?

An On-delay timer (TON) is a common function in programmable logic controllers (PLCs) that delays turning on an output after its input turns on. When the input signal activates, the timer starts counting time. Only after the preset time passes does the output turn on. If the input turns off before the time finishes, the timer resets and the output stays off.

Why it matters

TON timers help machines avoid reacting instantly to signals that might be brief or accidental, preventing errors or damage. Without TON timers, machines might start or stop too quickly, causing wear or unsafe conditions. They make automation smoother and safer by adding controlled delays.

Where it fits

Before learning TON timers, you should understand basic PLC inputs and outputs and how digital signals work. After mastering TON timers, you can learn other timer types like off-delay timers (TOF) and retentive timers (RTO), and how to combine timers for complex control.

Mental Model

Core Idea

A TON timer waits for a set time after its input turns on before activating its output, resetting if the input turns off early.

Think of it like...

It's like a slow light switch that only turns the light on if you hold the switch up for a few seconds, ignoring quick flicks.

Input ON ──▶ [Start Timer] ──▶ (Timer counts) ──▶ If time ≥ preset → Output ON
If Input OFF before preset → Timer resets, Output OFF

Build-Up - 7 Steps

1

Foundation

Basic TON Timer Concept

Concept: Introduce the fundamental behavior of the TON timer: delaying output activation after input turns on.

A TON timer has an input, a preset time, and an output. When the input signal goes ON, the timer starts counting. The output stays OFF until the timer reaches the preset time. If the input goes OFF before the timer finishes, the timer resets and output stays OFF.

Result

Output turns ON only after input stays ON continuously for the preset time.

Understanding the delay behavior helps prevent machines from reacting to short or accidental signals.

2

Foundation

Timer Reset Behavior

Concept: Explain how the timer resets if the input turns OFF before the preset time.

If the input signal turns OFF before the timer reaches the preset time, the timer stops counting and resets to zero. The output remains OFF. When the input turns ON again, the timer restarts counting from zero.

Result

Timer never reaches preset if input is interrupted early; output stays OFF.

Knowing the reset behavior prevents misunderstanding why output doesn't activate after brief inputs.

3

Intermediate

Using TON in Ladder Logic

Concept: Show how to implement a TON timer in ladder logic programming.

In ladder logic, a TON timer block has an input coil, a preset time (PT), an elapsed time (ET), and an output coil (Q). When the input coil energizes, ET starts increasing until it equals PT. Once ET ≥ PT, Q turns ON. If input coil de-energizes, ET resets to zero and Q turns OFF.

Result

Ladder logic diagram with TON block delays output coil activation by PT time.

Seeing TON in ladder logic connects theory to practical PLC programming.

4

Intermediate

Elapsed Time (ET) Monitoring

🤔Before reading on: Do you think the elapsed time (ET) resets immediately when input turns off, or does it hold its value?

Concept: Explain the role of elapsed time (ET) and how it behaves during input changes.

ET shows how long the input has been ON. If input turns OFF before PT, ET resets to zero immediately. ET only counts up while input is ON. This allows monitoring how close the timer is to finishing.

Result

ET value reflects current timing progress and resets on input OFF.

Understanding ET helps debug timers and create more complex timing logic.

5

Intermediate

Practical Use Cases for TON

🤔Before reading on: Would you use a TON timer to delay turning OFF a device or turning it ON? Commit to your answer.

Concept: Describe common real-world scenarios where TON timers are useful.

TON timers delay turning ON outputs, such as starting a motor after a safety check or delaying a conveyor belt start. They are not used to delay turning OFF; that is the off-delay timer's job. TON prevents machines from reacting to quick, unwanted signals.

Result

Clear understanding of when to use TON timers in automation.

Knowing correct use cases avoids misapplication and system errors.

6

Advanced

Stacking TON Timers for Complex Delays

🤔Before reading on: Do you think stacking two TON timers in series creates a longer delay equal to the sum of both presets, or just the first timer's delay?

Concept: Show how to combine multiple TON timers to create longer or staged delays.

By connecting the output of one TON timer as the input of another, you can create a longer total delay equal to the sum of their preset times. This allows staged or sequential delays in control processes.

Result

Output activates only after combined preset times of all timers pass.

Understanding stacking expands TON timer flexibility for complex automation.

7

Expert

Common Pitfalls and Internal Timer Behavior

🤔Before reading on: Does the TON timer keep counting internally even if the input turns OFF briefly, or does it reset immediately?

Concept: Reveal internal timer counting behavior and common mistakes in programming TON timers.

Internally, TON timers reset their count immediately when input turns OFF; they do not remember partial counts. A common mistake is expecting the timer to resume counting from where it left off after a brief input OFF. This causes unexpected output delays or failures.

Result

Clear understanding that TON timers require continuous input ON to reach preset time.

Knowing internal reset behavior prevents bugs and helps design robust timing logic.

Under the Hood

The TON timer uses an internal counter that increments as long as the input signal is ON. This counter measures elapsed time against a preset value. If the input signal turns OFF, the counter resets to zero immediately. When the counter reaches the preset time, the timer output activates. The timer relies on the PLC scan cycle to update the counter regularly.

Why designed this way?

TON timers were designed to ensure outputs only activate after a stable input signal, avoiding false triggers from noise or brief signals. Resetting immediately on input OFF simplifies the logic and ensures predictable timing behavior. Alternatives like retentive timers exist for cases needing memory of elapsed time, but TON's simplicity suits most delay needs.

┌─────────────┐
│ Input Signal│───▶ [TON Timer] ──▶ Output
│             │       │
│             │       ▼
│             │   [Counter]
│             │       │
│             │       └─ Resets if Input OFF
└─────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does a TON timer keep counting time if the input turns OFF briefly? Commit to yes or no.

Common Belief: The timer remembers elapsed time even if the input turns OFF briefly and resumes counting when input returns.

Reality: The TON timer resets its elapsed time immediately when input turns OFF; it does not remember partial counts.

Why it matters: Expecting the timer to remember elapsed time leads to timing errors and unexpected output behavior.

Quick: Can a TON timer delay turning OFF an output? Commit to yes or no.

Common Belief: A TON timer can delay turning OFF an output after input turns OFF.

Reality: TON timers only delay turning ON outputs; delaying OFF requires an off-delay timer (TOF).

Why it matters: Using TON to delay OFF causes logic errors and machine malfunctions.

Quick: Does the output turn ON immediately when input turns ON? Commit to yes or no.

Common Belief: The output activates immediately when the input turns ON.

Reality: The output only activates after the preset delay time passes with input continuously ON.

Why it matters: Misunderstanding this causes incorrect expectations and faulty control sequences.

Quick: Is the elapsed time (ET) value useful for debugging? Commit to yes or no.

Common Belief: ET is just an internal value and not useful for monitoring or debugging.

Reality: ET shows how long the input has been ON and helps monitor timer progress and troubleshoot.

Why it matters: Ignoring ET misses a valuable tool for diagnosing timing issues.

Expert Zone

1

TON timers rely on the PLC scan cycle frequency; very short preset times may not be accurate if scan times are long.

2

Stacking multiple TON timers can introduce cumulative scan delays, affecting precise timing in fast systems.

3

Some PLCs implement TON timers with retentive options or special flags, which can change reset behavior subtly.

When NOT to use

Do not use TON timers when you need the timer to remember elapsed time after input turns OFF; use retentive timers (RTO) instead. For delaying output turn-off, use off-delay timers (TOF). For very precise timing, consider hardware timers or real-time clocks.

Production Patterns

TON timers are widely used for motor start delays, safety interlocks, and debouncing noisy inputs. In production, they are often combined with sensors and safety checks to ensure machines start only after stable conditions. Advanced systems use TON timers in sequence or parallel to create complex timing profiles.

Connections

Finite State Machines (FSM)

TON timers act as timed transitions within FSMs controlling machine states.

Understanding TON timers helps grasp how time delays control state changes in automation logic.

Human Reaction Time

TON timers mimic human delay in reacting to signals, adding intentional wait periods.

Knowing human reaction delays helps design machine delays that feel natural and safe.

Traffic Light Control Systems

TON timers implement the delay phases in traffic lights, controlling how long lights stay green or red.

Seeing TON timers in traffic control shows their real-world impact on safety and flow.

Common Pitfalls

#1 Expecting the timer to continue counting after input briefly turns OFF.

Wrong approach:

If input OFF briefly:
timer_elapsed_time += scan_time
if timer_elapsed_time >= preset:
output = ON

Correct approach:

If input OFF:
timer_elapsed_time = 0
If input ON:
timer_elapsed_time += scan_time
if timer_elapsed_time >= preset:
output = ON

Root cause:

Misunderstanding that TON timers reset elapsed time immediately on input OFF.

#2 Using TON timer to delay turning OFF an output.

Wrong approach:

TON input = output coil
Preset time = delay
Output coil turns OFF after delay

Correct approach:

Use off-delay timer (TOF) for delaying output turn OFF, not TON.

Root cause:

Confusing TON and TOF timer functions and their intended use.

#3 Setting preset time shorter than PLC scan time, expecting precise delay.

Wrong approach:

Preset time = 1 ms
PLC scan time = 10 ms
Expect output after 1 ms

Correct approach:

Set preset time ≥ PLC scan time for reliable timing, e.g., 20 ms if scan is 10 ms.

Root cause:

Ignoring PLC scan cycle impact on timer accuracy.

Key Takeaways

An On-delay timer (TON) delays output activation until its input has been ON continuously for a preset time.

If the input turns OFF before the preset time, the timer resets immediately and output stays OFF.

TON timers are used to prevent machines from reacting to brief or accidental signals, improving safety and reliability.

Understanding elapsed time (ET) helps monitor timer progress and debug timing issues.

TON timers should not be used to delay output turn-off; use off-delay timers (TOF) for that purpose.

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