Timer preset and accumulated values in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the preset value in a PLC timer?

The preset value is the time duration set for the timer to run before it completes. It acts like a target time the timer counts up to.

beginner

What does the accumulated value represent in a PLC timer?

The accumulated value shows how much time has already passed since the timer started. It increases until it reaches the preset value.

beginner

How does the accumulated value behave when a PLC timer is reset?

When the timer is reset, the accumulated value goes back to zero, starting the count fresh.

intermediate

Why is it important to set the correct preset value in a timer?

Setting the correct preset ensures the timer runs for the desired time, controlling processes accurately and avoiding errors.

beginner

If a timer's accumulated value equals its preset value, what happens next?

The timer finishes counting, and its output or done bit turns ON, signaling the timer has completed.

What does the preset value in a PLC timer specify?

A The current time counted by the timer

B The delay before the timer starts

C The number of times the timer has started

D The total time the timer should count before finishing

The preset value is the target time the timer counts up to before completing.

What happens to the accumulated value when the timer is reset?

A It decreases slowly

B It resets to zero

C It stays the same

D It doubles

Resetting the timer sets the accumulated value back to zero.

When does the timer's done bit turn ON?

A When the accumulated value equals the preset value

B When the timer starts

C When the timer is reset

D When the accumulated value is zero

The done bit turns ON when the timer reaches its preset time.

Which value shows how much time has passed since the timer started?

A Reset value

B Preset value

C Accumulated value

D Start value

The accumulated value tracks elapsed time during the timer run.

Why should you carefully choose the preset value?

A To control process timing accurately

B To reset the timer automatically

C To make the timer run faster

D To avoid using the accumulated value

Correct preset values ensure the timer controls processes at the right time.

Explain the difference between the preset and accumulated values in a PLC timer.

Think of preset as the goal and accumulated as the progress.

Describe what happens to the accumulated value and done bit when a timer reaches its preset value.

Consider what signals the timer has finished.

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