Off-delay timer (TOF) behavior in PLC Programming - Mini Project: Build & Apply

Off-delay timer (TOF) behavior

📖 Scenario: You are programming a simple off-delay timer (TOF) for a machine control system. The timer should keep the output ON for a set delay time after the input signal turns OFF.

🎯 Goal: Build a program that simulates the off-delay timer (TOF) behavior using basic scripting logic. You will create the input signal, set the delay time, implement the timer logic, and display the output state.

📋 What You'll Learn

Create a boolean variable input_signal to represent the input ON/OFF state.

Create an integer variable delay_time to represent the delay time in seconds.

Implement the off-delay timer logic that keeps output_signal ON for delay_time seconds after input_signal turns OFF.

Print the output_signal state after the timer logic.

💡 Why This Matters

🌍 Real World

Off-delay timers are used in machines to keep outputs active for a short time after inputs turn off, such as keeping a motor running briefly after a stop button is pressed.

💼 Career

Understanding timer behavior is essential for PLC programming and automation jobs where precise control of machine timing is required.

1

Create the input signal variable

Create a boolean variable called input_signal and set it to True to represent the input being ON.

PLC Programming

# Create a boolean variable called input_signal and set it to True
# Your code here

Hint

Use input_signal = True to represent the input ON state.

2

Set the delay time

Create an integer variable called delay_time and set it to 5 to represent a 5-second delay.

PLC Programming

input_signal = True
# Create an integer variable called delay_time and set it to 5
# Your code here

Hint

Use delay_time = 5 to set the delay duration.

3

Implement the off-delay timer logic

Create a variable called output_signal. Use a simple logic: if input_signal is True, set output_signal to True. If input_signal is False, keep output_signal True for delay_time seconds before setting it to False. For this step, simulate the timer by setting output_signal to True if input_signal is True or if a variable timer_active is True. Assume timer_active is True for now.

PLC Programming

input_signal = True
delay_time = 5
# Create output_signal and timer_active variables
# Use logic to set output_signal based on input_signal and timer_active
# Your code here

Hint

Use output_signal = input_signal or timer_active to simulate the off-delay timer output.

4

Display the output signal state

Write a print statement to display the text Output signal is ON if output_signal is True, otherwise display Output signal is OFF.

PLC Programming

input_signal = True
delay_time = 5
timer_active = True
output_signal = input_signal or timer_active
# Print the output signal state
# Your code here

Hint

Use an if statement to check output_signal and print the correct message.

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