Off-delay timer (TOF) behavior in PLC Programming - Step-by-Step Execution

Concept Flow - Off-delay timer (TOF) behavior

Input Signal ON

↓

Timer OFF? NO

↓

Output ON

↓

Input Signal OFF

↓

Start TOF Timer

↓

Timer Running?

↓

Output ON

The TOF timer keeps the output ON for a set delay after the input turns OFF, then turns output OFF.

Execution Sample

PLC Programming

Input = True
Output = True
Start TOF timer when Input goes False
Output stays True during timer
Output turns False after delay

Simulates TOF timer behavior: output stays ON after input OFF for delay, then turns OFF.

Execution Table

Step	Input	Timer State	Timer Elapsed (s)	Output	Action
1	True	Stopped	0	True	Input ON, Output ON, Timer stopped
2	True	Stopped	0	True	Input still ON, no timer
3	False	Running	0	True	Input OFF, start timer, Output stays ON
4	False	Running	1	True	Timer running, Output ON
5	False	Running	2	True	Timer running, Output ON
6	False	Running	3	True	Timer running, Output ON
7	False	Stopped	3	False	Timer done, Output OFF
8	False	Stopped	3	False	Input OFF, Output OFF, Timer stopped

💡 Timer elapsed reaches preset delay (3s), output turns OFF, timer stops.

Variable Tracker

Variable	Start	After Step 3	After Step 6	Final
Input	True	False	False	False
Timer State	Stopped	Running	Running	Stopped
Timer Elapsed (s)	0	0	3	3
Output	True	True	True	False

Key Moments - 2 Insights

Why does the output stay ON even after the input turns OFF at step 3?

Because the TOF timer starts running and keeps the output ON during the preset delay, as shown in execution_table rows 3 to 6.

When exactly does the output turn OFF?

At step 7, when the timer elapsed reaches the preset delay (3 seconds), the timer stops and output turns OFF, as seen in execution_table row 7.

Visual Quiz - 3 Questions

Look at the execution table, what is the timer state at step 4?

A Stopped

B Paused

C Running

D Reset

💡 Hint

Check the 'Timer State' column at step 4 in the execution_table.

At which step does the output finally turn OFF?

A Step 5

B Step 7

C Step 6

D Step 8

💡 Hint

Look at the 'Output' column changes in the execution_table.

If the preset delay was 5 seconds instead of 3, what would happen at step 7?

A Timer would still be running and output ON

B Output would turn OFF

C Input would turn ON

D Timer would reset

💡 Hint

Refer to how timer elapsed compares to preset delay in execution_table and variable_tracker.

Concept Snapshot

Off-delay timer (TOF):
- Output stays ON while input is ON.
- When input turns OFF, timer starts.
- Output remains ON during timer delay.
- After delay, output turns OFF.
- Timer resets if input turns ON again.

Full Transcript

An off-delay timer (TOF) keeps the output ON while the input signal is ON. When the input turns OFF, the timer starts counting. During this delay, the output remains ON. Once the timer reaches the preset delay time, the output turns OFF and the timer stops. If the input turns ON again before the delay ends, the timer resets and output stays ON. This behavior is shown step-by-step in the execution table and variable tracker.

Practice

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

easy

A. Delays turning the output ON after the input turns ON

B. Turns the output ON immediately when the input turns ON

C. Keeps the output ON for a set time after the input turns OFF

D. Turns the output OFF immediately when the input turns OFF

Solution

1. Step 1: Understand TOF basic function

   An Off-delay timer keeps the output ON even after the input signal goes OFF, but only for a preset delay time.

2. Step 2: Compare options with TOF behavior

   Keeps the output ON for a set time after the input turns OFF matches this behavior exactly, while others describe different timer types or immediate actions.

3. Final Answer:

   Keeps the output ON for a set time after the input turns OFF -> Option C

4. Quick Check:

   TOF delays OFF after input OFF = D [OK]

Hint: Remember TOF delays turning OFF, not ON [OK]

Common Mistakes:

• Confusing TOF with On-delay timer (TON)
• Thinking output turns OFF immediately
• Mixing input and output timing

2. Which of the following is the correct syntax to declare an Off-delay timer (TOF) named T1 with a preset time of 5 seconds in a typical PLC ladder logic?

easy

A. TOF T1, 5s

B. TIMER_OFF T1, 5

C. TON T1, 5s

D. OFF_DELAY T1, 5000

Solution

1. Step 1: Identify correct timer type keyword

   TOF is the standard abbreviation for Off-delay timer in many PLC languages.

2. Step 2: Check syntax format

   TOF T1, 5s uses TOF with timer name and preset time in seconds, matching typical syntax.

3. Final Answer:

   <code>TOF T1, 5s</code> -> Option A

4. Quick Check:

   TOF keyword with timer and time = C [OK]

Hint: Look for TOF keyword and time unit seconds [OK]

Common Mistakes:

• Using TON instead of TOF
• Wrong time format or units
• Using non-standard timer names

3. Given the following PLC code snippet using an Off-delay timer (TOF):

INPUT = ON (for 10s)
TOF T1, 5s
OUTPUT = T1.Q
Then INPUT turns OFF at time 10s.
What is the OUTPUT state at time 12s?

medium

A. Undefined

B. OFF

C. Flashing ON/OFF

D. ON

Solution

1. Step 1: Understand TOF timing

   When INPUT turns OFF at 10s, TOF keeps OUTPUT ON for 5 seconds delay.

2. Step 2: Calculate OUTPUT state at 12s

   12s is 2 seconds after INPUT OFF, which is less than 5s delay, so OUTPUT remains ON.

3. Final Answer:

   ON -> Option D

4. Quick Check:

   TOF delay not expired at 12s = ON [OK]

Hint: Check if delay time passed after input OFF [OK]

Common Mistakes:

• Assuming OUTPUT turns OFF immediately
• Confusing ON-delay with OFF-delay
• Ignoring preset delay time

4. You wrote this PLC code using an Off-delay timer (TOF):

TOF T1, 3s
OUTPUT = T1.Q
INPUT turns OFF at 5s
But OUTPUT turns OFF immediately at 5s instead of delaying.
What is the most likely error?

medium

A. TOF timer is used instead of TON timer

B. Preset time for TOF is set to zero or missing

C. OUTPUT is assigned incorrectly to INPUT instead of T1.Q

D. INPUT signal never turned OFF

Solution

1. Step 1: Analyze timer preset time effect

   If preset time is zero or missing, TOF will not delay turning OFF output.

2. Step 2: Match symptom with cause

   Immediate OFF at input OFF means no delay, likely due to zero preset time.

3. Final Answer:

   Preset time for TOF is set to zero or missing -> Option B

4. Quick Check:

   Zero preset time causes no delay [OK]

Hint: Check if preset time is properly set [OK]

Common Mistakes:

• Confusing TOF with TON timer
• Assigning output to wrong variable
• Ignoring input signal changes

5. In a safety system, you want a motor to keep running for 10 seconds after the stop button is released to allow smooth shutdown. Which PLC timer setup using Off-delay timer (TOF) is correct?

1) TOF MotorTimer, 10s
2) TON MotorTimer, 10s
3) TOF MotorTimer, 0s
4) TON MotorTimer, 0s

hard

A. Option 1 only

B. Option 2 only

C. Options 1 and 3

D. Options 2 and 4

Solution

1. Step 1: Identify timer type for delayed OFF

   TOF timer delays turning OFF output after input OFF, perfect for smooth shutdown.

2. Step 2: Check preset time correctness

   Only option 1 uses TOF with 10s delay, matching requirement.

3. Final Answer:

   Option 1 only -> Option A

4. Quick Check:

   TOF with 10s delay = smooth shutdown [OK]

Hint: Use TOF with correct delay time for shutdown [OK]

Common Mistakes:

• Using TON instead of TOF for delayed OFF
• Setting delay time to zero
• Selecting multiple options incorrectly