Timer preset and accumulated values in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Timer Accumulated Value After Start

Consider a timer in a PLC program with a preset value of 5000 ms. The timer starts and runs for 3000 ms before being stopped. What is the accumulated value of the timer at this point?

A 5000

B 8000

C 0

D 3000

💡 Hint

The accumulated value increases as the timer runs until it reaches the preset.

The accumulated value counts up from zero as the timer runs. After 3000 ms, it will be 3000, less than the preset of 5000 ms.

💻 Command Output

intermediate

Timer Output Status After Accumulated Value Reaches Preset

A timer has a preset of 10000 ms. When the accumulated value reaches 10000 ms, what is the typical output status of the timer?

A Output is ON (true)

B Output is OFF (false)

C Output toggles ON and OFF rapidly

D Output resets to zero

💡 Hint

The timer output usually activates when the accumulated value meets the preset.

When the accumulated value equals or exceeds the preset, the timer output turns ON to signal the timer has finished.

📝 Syntax

advanced

Identify the Correct Timer Reset Syntax

Which of the following code snippets correctly resets the accumulated value of a timer named T1 in a PLC program?

A T1.ACC = 0;

B T1.ACC := 0;

C T1.PRE = 0;

D RESET(T1);

💡 Hint

In many PLC languages, ':=' is used for assignment, not '='.

The correct way to reset the accumulated value is to assign zero using ':=' operator, as in 'T1.ACC := 0;'.

🔧 Debug

advanced

Debug Timer Accumulated Value Not Resetting

A timer's accumulated value does not reset after the reset command is issued. Which of the following is the most likely cause?

A The preset value is set to zero

B The reset command is placed before the timer starts

C The reset command uses '=' instead of ':=' for assignment

D The timer output is not connected

💡 Hint

Check the assignment operator used in the reset command.

Using '=' instead of ':=' for assignment causes the reset command to fail, so the accumulated value does not reset.

🚀 Application

expert

Calculate Total Time After Multiple Timer Cycles

A timer has a preset of 2000 ms. It runs three cycles, each time accumulating to the preset and then being reset immediately. What is the total accumulated time after the third cycle completes?

A 6000 ms

B 2000 ms

C 0 ms

D 8000 ms

💡 Hint

Add the preset time for each completed cycle.

Each cycle accumulates 2000 ms before reset. After three cycles, total time is 3 x 2000 = 6000 ms.

Practice

1. What does the preset value of a timer represent in PLC programming?

easy

A. The number of times the timer has been reset

B. The current time counted by the timer

C. The total time the timer should count before activating

D. The delay before the timer starts counting

Solution

1. Step 1: Understand timer preset meaning

   The preset value is the target time set for the timer to reach before it triggers.

2. Step 2: Differentiate from accumulated value

   The accumulated value shows elapsed time, not the target time.

3. Final Answer:

   The total time the timer should count before activating -> Option C

4. Quick Check:

   Preset = target time [OK]

Hint: Preset is the goal time, accumulated is progress [OK]

Common Mistakes:

• Confusing preset with accumulated value
• Thinking preset changes during timing
• Assuming preset is a delay before start

2. Which of the following is the correct way to set a timer's preset value to 5000 milliseconds in a typical PLC timer instruction?

easy

A. TIMER.PRESET = 5000

B. TIMER.ACC = 5000

C. TIMER.SET = 5000

D. TIMER.TIME = 5000

Solution

1. Step 1: Identify correct property for preset

   In PLC timers, the preset value is usually set with the property named 'PRESET'.

2. Step 2: Check other options

   'ACC' is for accumulated time, 'SET' and 'TIME' are not standard properties for preset.

3. Final Answer:

   TIMER.PRESET = 5000 -> Option A

4. Quick Check:

   Preset set with .PRESET property [OK]

Hint: Preset uses .PRESET, accumulated uses .ACC [OK]

Common Mistakes:

• Using .ACC to set preset
• Using non-standard property names
• Confusing preset with timer start command

3. Given the following timer status after running:
TIMER.PRESET = 10000
TIMER.ACC = 7500
What is the timer's current state?

medium

A. Timer has finished counting and is done

B. Timer is paused at 7500 milliseconds

C. Timer is reset and not started

D. Timer is still counting and not done

Solution

1. Step 1: Compare accumulated and preset values

   The accumulated value (7500) is less than the preset (10000), so the timer has not reached its target.

2. Step 2: Determine timer state

   Since accumulated is less than preset, the timer is still counting and not done.

3. Final Answer:

   Timer is still counting and not done -> Option D

4. Quick Check:

   ACC < PRESET means timer running [OK]

Hint: If ACC < PRESET, timer is running [OK]

Common Mistakes:

• Assuming timer is done when ACC < PRESET
• Confusing paused with running
• Thinking reset means ACC is non-zero

4. A timer's accumulated value never increases even though the timer is enabled. Which is the most likely cause?

medium

A. The preset value is set to zero

B. The timer is not enabled or triggered properly

C. The accumulated value is manually reset continuously

D. The timer's accumulated value is greater than preset

Solution

1. Step 1: Understand timer behavior when enabled

   The accumulated value increases only when the timer is enabled and counting.

2. Step 2: Analyze causes for no increase

   If the timer is not enabled or triggered, the accumulated value stays the same.

3. Final Answer:

   The timer is not enabled or triggered properly -> Option B

4. Quick Check:

   Timer must be enabled to accumulate time [OK]

Hint: Timer must be enabled to increase ACC [OK]

Common Mistakes:

• Assuming preset zero stops accumulation
• Ignoring timer enable condition
• Thinking ACC resets automatically

5. You want a process to run exactly 15 seconds after a start signal. Which timer setup ensures the process starts only after 15 seconds have fully elapsed?

hard

A. Set TIMER.PRESET = 15000 and start process when TIMER.ACC >= TIMER.PRESET

B. Set TIMER.PRESET = 15000 and start process immediately when timer starts

C. Set TIMER.PRESET = 15000 and start process when TIMER.ACC = 0

D. Set TIMER.PRESET = 15000 and start process when TIMER.ACC < TIMER.PRESET

Solution

1. Step 1: Understand timing requirement

   The process must start only after 15 seconds have fully passed.

2. Step 2: Use timer values to control start

   Starting the process when accumulated time is equal or greater than preset ensures 15 seconds elapsed.

3. Final Answer:

   Set TIMER.PRESET = 15000 and start process when TIMER.ACC >= TIMER.PRESET -> Option A

4. Quick Check:

   Start after ACC >= PRESET for exact delay [OK]

Hint: Start process only when ACC reaches preset [OK]

Common Mistakes:

• Starting process immediately on timer start
• Starting before accumulated time reaches preset
• Using ACC = 0 as start condition