Count-up counter (CTU) behavior in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a Count-up counter (CTU) do in PLC programming?

A CTU increases its count by one each time it receives a rising edge (transition from 0 to 1) on its input signal until it reaches a preset value.

beginner

What happens when the CTU's current count reaches the preset value?

The CTU sets its output bit (done bit) to true, indicating the count target has been reached.

intermediate

How can you reset a CTU counter to zero?

By activating the reset input (if available) or using a separate reset instruction in the PLC program to clear the current count.

intermediate

Why does a CTU only count on a rising edge of the input signal?

Counting on the rising edge prevents multiple counts from a single continuous input signal, ensuring accurate increments only when the signal changes from off to on.

beginner

What is the difference between the current count and the preset value in a CTU?

The current count is the number of times the input has triggered the counter, while the preset value is the target number the counter must reach to activate its done output.

What triggers the CTU to increase its count?

A Continuous high input signal

B A falling edge on the input signal

C A rising edge on the input signal

D Manual reset

The CTU counts only when the input signal changes from 0 to 1, which is called a rising edge.

What does the CTU output bit indicate?

A The counter is reset

B The current count is zero

C The input signal is low

D The current count has reached the preset value

The output bit (done bit) is set when the current count equals or exceeds the preset value.

How can you clear the count of a CTU?

A By activating the reset input or reset instruction

B By turning off the input signal

C By increasing the preset value

D By disconnecting the power

The count is cleared by using the reset input or a reset command in the PLC program.

If the input signal stays high continuously, how many counts will the CTU register?

A Multiple counts

B One count

C Zero counts

D Counts equal to preset value

The CTU counts only on the rising edge, so a continuous high signal counts as one increment.

What is the role of the preset value in a CTU?

A It defines the target count to activate the output

B It resets the counter

C It stores the current count

D It controls the input signal

The preset value is the target number the counter must reach to set the done output bit.

Explain how a Count-up counter (CTU) works in a PLC program.

Think about what happens when the input signal changes and how the counter knows when to stop counting.

Describe the difference between the current count and the preset value in CTU behavior.

Consider what each value represents in the counting process.

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