On-delay timer (TON) behavior in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding TON Timer Basic Behavior

In a PLC program, an On-delay timer (TON) starts timing when its input is TRUE. What happens to the timer's output (Q) during the timing period before the preset time is reached?

A The output Q remains TRUE from the moment the input becomes TRUE.

B The output Q immediately becomes TRUE as soon as the input is TRUE.

C The output Q remains FALSE until the preset time elapses.

D The output Q toggles between TRUE and FALSE during the timing period.

💡 Hint

Think about what 'on-delay' means: the output changes only after a delay.

The TON timer output Q stays FALSE during the timing period and only becomes TRUE after the preset time has elapsed while the input remains TRUE.

💻 Command Output

intermediate

Output State After Input Reset

A TON timer has a preset time of 5 seconds. The input (IN) is TRUE for 3 seconds, then goes FALSE. What is the output Q immediately after the input goes FALSE at 3 seconds?

A Q is TRUE because the timer reached the preset time.

B Q is FALSE because the timer did not reach the preset time.

C Q remains TRUE for 2 more seconds after input goes FALSE.

D Q toggles between TRUE and FALSE every second.

💡 Hint

The timer output only becomes TRUE after the preset time is reached while input is TRUE.

Since the input was TRUE for only 3 seconds, less than the 5-second preset, the output Q remains FALSE when input goes FALSE.

📝 Syntax

advanced

Correct TON Timer Ladder Logic Syntax

Which of the following ladder logic snippets correctly represents a TON timer with input IN, preset time PT of 10 seconds, and output Q?

A TON Timer1(IN:=IN, PT:=T#10s, Q=>Q);

B TON Timer1(IN=IN, PT=10s, Q=Q);

C TON Timer1(IN, PT=10, Q);

D TON Timer1(IN=IN, PT=10, Q=Q)

💡 Hint

Look for correct syntax with assignment operators and time format.

Option A uses the correct syntax with := for input assignment, T#10s for time format, and => for output assignment, which is standard in IEC 61131-3 structured text.

🔧 Debug

advanced

Identifying TON Timer Behavior Bug

A programmer notices that the TON timer output Q never becomes TRUE, even though the input IN stays TRUE for longer than the preset time. Which of the following is the most likely cause?

A The output Q is wired incorrectly in the hardware.

B The timer preset time PT is set to zero or a very small value.

C The input IN is toggling rapidly between TRUE and FALSE.

D The timer is not being called or updated in the program cyclically.

💡 Hint

Consider how timers work in PLC scan cycles.

TON timers require the program to call and update them cyclically. If the timer function is not called every scan, it will not count time and output Q will never become TRUE.

🚀 Application

expert

Calculating TON Timer Output After Multiple Input Cycles

A TON timer has a preset time of 4 seconds. The input IN is TRUE for 3 seconds, then FALSE for 2 seconds, then TRUE again for 5 seconds. What is the state of output Q at the end of the second TRUE period?

A Q is TRUE because the second TRUE period exceeds the preset time.

B Q is FALSE because the timer resets when input goes FALSE.

C Q is TRUE because the total TRUE time (3+5=8s) exceeds the preset time.

D Q toggles between TRUE and FALSE during the second TRUE period.

💡 Hint

Remember that TON timers reset timing when input goes FALSE.

The timer resets when input goes FALSE, so the first 3 seconds do not count towards the second timing. The second TRUE period of 5 seconds exceeds the 4-second preset, so output Q is TRUE.

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