Off-delay timer (TOF) behavior in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the off-delay timer when the input turns off.

PLC Programming

if input_signal == False:
    timer_start = [1]

Drag options to blanks, or click blank then click option'

ATrue

BFalse

CNone

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting timer_start to False stops the timer instead of starting it.

Using None or 0 does not correctly start the timer.

The off-delay timer starts timing when the input signal turns off, so setting timer_start to True activates the timer.

2fill in blank

medium

Complete the code to reset the off-delay timer when the input is on.

PLC Programming

if input_signal == True:
    timer_reset = [1]

Drag options to blanks, or click blank then click option'

ATrue

BNone

CFalse

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting timer_reset to True keeps the timer running.

Using None or 1 does not properly reset the timer.

When the input signal is on, the off-delay timer should reset, so timer_reset must be set to False.

3fill in blank

hard

Fix the error in the timer countdown logic.

PLC Programming

if timer_running:
    timer_value [1] 1

Drag options to blanks, or click blank then click option'

A-=

B+=

C*=

D/=

Attempts:

3 left

💡 Hint

Common Mistakes

Using += increases the timer value instead of decreasing it.

Using *= or /= changes the timer value incorrectly.

The timer counts down by subtracting 1 from the timer value each cycle when running.

4fill in blank

hard

Fill both blanks to check if the timer has expired and reset it.

PLC Programming

if timer_value [1] 0:
    timer_running = [2]

Drag options to blanks, or click blank then click option'

A<=

B==

C>

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' misses cases where timer_value is negative.

Setting timer_running to True keeps the timer running after expiry.

The timer expires when its value is less than or equal to zero, and then the timer_running flag should be set to False to stop it.

5fill in blank

hard

Fill all three blanks to implement the off-delay timer output logic.

PLC Programming

if input_signal == True:
    output_signal = [1]
elif timer_running == True and timer_value [2] 0:
    output_signal = [3]

Drag options to blanks, or click blank then click option'

ATrue

B>

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Setting output to False when timer is still running.

Using incorrect comparison operators for timer_value.

When input is on, output is True. When timer is running and timer_value is greater than zero, output remains True.

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