Count-up counter (CTU) behavior in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output value of the CTU after 5 rising input pulses?

Consider a Count-Up (CTU) counter with preset value 10 and initial count 0. The input pulse (CU) is triggered 5 times. What is the current count value after these pulses?

PLC Programming

CTU preset = 10
Initial count = 0
Input pulses (CU) = 5 rising edges
Output: Current count value

A 15

B 5

C 10

D 0

💡 Hint

The CTU increments the count by 1 for each rising pulse until it reaches the preset.

The CTU increments the count by 1 for each rising edge of the CU input. After 5 pulses, the count is 5, which is less than the preset 10.

💻 Command Output

intermediate

What happens to the CTU count when the reset (R) input is activated?

A CTU counter has a current count of 7 and preset of 10. The reset input (R) is activated once. What is the count value after reset?

PLC Programming

CTU preset = 10
Current count = 7
Reset input (R) activated once
Output: Current count value

A 0

B 1

C 10

D 7

💡 Hint

Reset input clears the count to zero immediately.

Activating the reset input sets the count value back to zero regardless of the current count.

💻 Command Output

advanced

What is the output of the CTU done bit after 12 input pulses with preset 10?

A CTU counter has preset value 10 and initial count 0. The CU input receives 12 rising pulses. What is the state of the done bit (DN) after these pulses?

PLC Programming

CTU preset = 10
Initial count = 0
Input pulses (CU) = 12 rising edges
Output: Done bit (DN) state (True/False)

A Undefined

B False

C Error

D True

💡 Hint

The done bit is set when count reaches or exceeds preset.

The done bit is set to True when the count reaches or exceeds the preset value. After 12 pulses, count is 12 which is greater than preset 10, so done bit is True.

🧠 Conceptual

advanced

Which statement correctly describes CTU behavior when count exceeds preset?

In a CTU counter, what happens if the count value exceeds the preset value due to input pulses?

A Count stops incrementing and done bit remains set

B Count resets to zero automatically

C Count continues incrementing without limit and done bit stays set

D Count decrements automatically to preset value

💡 Hint

Think about whether CTU limits the count or not after preset is reached.

The CTU count continues to increment beyond the preset value; the done bit remains set once preset is reached or exceeded.

🔧 Debug

expert

Why does this CTU counter never set the done bit despite many input pulses?

Given this CTU logic snippet: CTU preset = 5 Count initialized at 0 CU input pulses rising edge detected Reset input never activated But done bit (DN) never becomes True after 10 pulses. What is the most likely cause?

PLC Programming

CTU counter {
  preset = 5
  count = 0
  CU = rising edge pulses
  R = false
}

Output: done bit (DN) state after 10 pulses

A Count variable is not incremented on CU pulses due to missing rising edge detection

B Preset value is set incorrectly to zero

C Reset input is stuck active, clearing count continuously

D Done bit is wired inversely and never reads True

💡 Hint

Check if the input pulses are properly detected as rising edges.

If the CU input is not detected as rising edges, the count never increments, so done bit never sets.

Practice

1. What does the done bit in a CTU (Count-Up Counter) indicate?

easy

A. The counter has reached its preset value

B. The counter is reset to zero

C. The counter is currently counting down

D. The counter input is inactive

Solution

1. Step 1: Understand the done bit role

   The done bit is a signal that shows when the counter has reached its preset value.

2. Step 2: Relate done bit to counting

   When the count equals the preset, the done bit turns ON to indicate completion.

3. Final Answer:

   The counter has reached its preset value -> Option A

4. Quick Check:

   Done bit = preset reached [OK]

Hint: Done bit means count reached preset value [OK]

Common Mistakes:

• Confusing done bit with reset signal
• Thinking done bit means counting down
• Assuming done bit is always ON

2. Which of the following is the correct syntax to declare a CTU counter named C1 with a preset of 10 in a typical PLC program?

easy

A. CTU C1 PRE 10

B. CTU(C1, 10);

C. CTU C1, PRE:=10;

D. Counter CTU C1 = 10;

Solution

1. Step 1: Identify correct CTU declaration syntax

   Typical PLC syntax uses the format: CTU counterName, PRE:=presetValue;

2. Step 2: Match options to syntax

   CTU C1, PRE:=10; matches the correct syntax with CTU C1, PRE:=10;

3. Final Answer:

   CTU C1, PRE:=10; -> Option C

4. Quick Check:

   CTU declaration uses PRE:= preset [OK]

Hint: Look for PRE:= syntax in CTU declaration [OK]

Common Mistakes:

• Using parentheses incorrectly
• Missing PRE:= keyword
• Using assignment '=' instead of ':='

3. Given a CTU counter with preset 5 and current count 3, what will be the count and done bit after two rising input pulses?

medium

A. Count = 3, Done bit = TRUE

B. Count = 4, Done bit = FALSE

C. Count = 5, Done bit = FALSE

D. Count = 5, Done bit = TRUE

Solution

1. Step 1: Add two pulses to current count

   Starting at 3, two pulses increase count to 5 (3 + 2 = 5).

2. Step 2: Check if count reached preset

   Count equals preset (5), so done bit becomes TRUE.

3. Final Answer:

   Count = 5, Done bit = TRUE -> Option D

4. Quick Check:

   Count + pulses = preset -> done bit TRUE [OK]

Hint: Add pulses to count, compare with preset [OK]

Common Mistakes:

• Not updating count correctly
• Confusing done bit state
• Assuming done bit is TRUE before preset reached

4. A CTU counter is not setting the done bit even after the count reaches the preset. Which of the following is the most likely cause?

medium

A. The preset value is set higher than intended

B. The done bit is wired incorrectly in the program

C. The counter is counting down instead of up

D. The counter is not receiving rising edge pulses

Solution

1. Step 1: Analyze why done bit is not set

   If count reaches preset but done bit stays off, wiring or logic for done bit is likely wrong.

2. Step 2: Evaluate other options

   If pulses were missing or counting down, count wouldn't reach preset. Preset too high means count can't reach it.

3. Final Answer:

   The done bit is wired incorrectly in the program -> Option B

4. Quick Check:

   Done bit wiring error causes no done signal [OK]

Hint: Check done bit wiring if count reaches preset but no done [OK]

Common Mistakes:

• Assuming pulses are missing without checking
• Ignoring done bit wiring
• Confusing counting direction with done bit issue

5. You have a CTU counter with preset 10. The input pulses come irregularly, sometimes multiple pulses at once. How should you modify the program to ensure the count never exceeds the preset and done bit is set correctly?

hard

A. Ignore extra pulses; CTU automatically limits count to preset

B. Use a separate timer to count pulses instead of CTU

C. Add logic to reset the counter when count exceeds preset

D. Increase the preset value to a higher number

Solution

1. Step 1: Understand CTU behavior with irregular pulses

   CTU increments by 1 per rising edge and automatically limits the accumulator to the preset value, preventing overflow.

2. Step 2: Verify done bit sets correctly

   No modification needed; done bit activates when accumulator reaches preset, even with irregular pulses.

3. Final Answer:

   Ignore extra pulses; CTU automatically limits count to preset -> Option A

4. Quick Check:

   CTU ACC ≤ PRE automatically, done sets [OK]

Hint: CTU automatically limits count to preset [OK]

Common Mistakes:

• Assuming CTU allows count to exceed preset
• Adding unnecessary reset logic for overflow
• Using timer or increasing preset unnecessarily