Off-delay timer (TOF) behavior in PLC Programming

Introduction

An off-delay timer (TOF) helps keep a signal ON for a set time after the input turns OFF. It is useful to delay turning something off smoothly.

Keep a light on for 5 seconds after a switch is turned off.

Delay stopping a motor to cool it down after power is cut.

Hold a valve open briefly after a sensor stops detecting flow.

Keep a fan running for a short time after a machine stops.

Delay turning off an alarm sound after the trigger stops.

Syntax

PLC Programming

TOF(timer_name, input_signal, preset_time)
// timer_name: name of the timer
// input_signal: boolean input to start/stop timer
// preset_time: delay time in milliseconds or seconds

The timer starts counting when the input_signal turns OFF.

The output stays ON during the delay time after input_signal is OFF.

Examples

This timer keeps output ON for 5 seconds after StartButton is released.

PLC Programming

TOF(Timer1, StartButton, 5000)

This timer delays turning off the cooling fan 10 seconds after MotorRunning stops.

PLC Programming

TOF(CoolingTimer, MotorRunning, 10000)

Sample Program

This program simulates an off-delay timer that keeps output ON for 5 seconds after the StartButton is released at 3 seconds.

PLC Programming

BOOL StartButton = TRUE;
BOOL Timer1;

// TOF function simulation
BOOL TOF(BOOL inputSignal, int presetTime) {
    static int offDelayCounter = 0;
    if (inputSignal) {
        offDelayCounter = 0;
        return TRUE;
    } else {
        offDelayCounter += 1000; // increment by 1 second
        if (offDelayCounter < presetTime) {
            return TRUE;
        } else {
            return FALSE;
        }
    }
}

// Simulate timer behavior in steps
for (int time = 0; time <= 7000; time += 1000) {
    if (time == 3000) StartButton = FALSE; // Button released at 3 seconds
    Timer1 = TOF(StartButton, 5000);
    printf("Time: %d ms, StartButton: %s, Timer Output: %s\n", time, StartButton ? "ON" : "OFF", Timer1 ? "ON" : "OFF");
}

Important Notes

The timer output stays ON while the input is ON.

When input turns OFF, the timer output stays ON for the preset delay time.

After the delay, the output turns OFF automatically.

Summary

An off-delay timer keeps output ON after input turns OFF.

It delays turning OFF for a set time.

Useful for smooth shutdowns and safety delays.

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