Off-delay timer (TOF) behavior in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an Off-delay timer (TOF) do in PLC programming?

An Off-delay timer (TOF) keeps its output ON for a set time after the input turns OFF, then switches the output OFF once the time expires.

beginner

In an Off-delay timer, when does the timing start?

The timing starts when the input signal to the timer turns OFF.

intermediate

What happens to the output of a TOF timer if the input turns ON again before the delay finishes?

The timer resets and the output stays ON without counting down the delay time.

intermediate

How is the output of a TOF timer different from an On-delay timer (TON)?

TOF output stays ON after input turns OFF for a delay, while TON output turns ON only after input stays ON for a delay.

beginner

Why are Off-delay timers useful in automation?

They help keep devices running briefly after a stop signal, preventing abrupt stops and allowing safe shutdowns.

When does the Off-delay timer (TOF) start counting down?

A When the input turns OFF

B When the input turns ON

C Immediately at power-up

D When the output turns OFF

The TOF timer starts its delay countdown when the input signal changes from ON to OFF.

What is the output state of a TOF timer while the input is ON?

A OFF

B ON

C Flashing

D Undefined

While the input is ON, the TOF timer output remains ON continuously.

If the input to a TOF timer turns ON again during the delay, what happens?

A The timer continues counting down

B The timer stops permanently

C The timer resets and output stays ON

D The output turns OFF immediately

If input turns ON again before delay ends, the TOF timer resets and output stays ON without delay.

Which timer type keeps output ON after input turns OFF for a delay?

A Off-delay timer (TOF)

B On-delay timer (TON)

C Retentive timer

D Pulse timer

The Off-delay timer (TOF) keeps output ON for a set time after input turns OFF.

What is a practical use of an Off-delay timer in automation?

A To start a motor after a delay

B To count items on a conveyor

C To measure temperature

D To keep a fan running briefly after a machine stops

Off-delay timers keep devices like fans running briefly after stop signals for safe shutdown.

Explain how an Off-delay timer (TOF) works in simple terms.

Think about what happens when you switch something off but it keeps running for a little while.

Describe a real-life example where an Off-delay timer would be useful in automation.

Imagine a fan that cools a machine even after it stops working.

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