Counter preset and reset in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a preset counter after input pulses

Given a counter with a preset value of 5 and an initial count of 0, what will be the counter value after 3 input pulses without reset?

PLC Programming

Preset = 5
Count = 0
Input pulses = 3
Reset = False

For each pulse:
  Count = Count + 1
  If Count >= Preset:
    Counter Done = True
  Else:
    Counter Done = False

Output Count

A 5

B 3

C 0

D 8

💡 Hint

Count increases by one for each pulse until it reaches the preset.

The counter increments by 1 for each pulse. After 3 pulses, the count is 3, which is less than the preset 5, so the count remains 3.

💻 Command Output

intermediate

Counter value after reset is triggered

A counter has a preset of 10 and current count of 7. If a reset signal is activated, what will be the counter value immediately after reset?

PLC Programming

Preset = 10
Count = 7
Reset = True

If Reset == True:
  Count = 0

Output Count

A 7

B 10

C 1

D 0

💡 Hint

Reset sets the counter back to zero.

When reset is true, the counter value is set to zero regardless of previous count.

🧠 Conceptual

advanced

Effect of preset value on counter done signal

If a counter has a preset value of 8 and counts input pulses, at which count value does the 'done' signal become true?

A When count equals 8

B When count is less than 8

C When count is greater than 8

D When count equals 0

💡 Hint

The done signal activates exactly when the count reaches the preset.

The done signal becomes true when the count matches the preset value, indicating the counter reached its target.

💻 Command Output

advanced

Counter behavior with preset and multiple resets

A counter has a preset of 4. It receives 2 pulses, then a reset, then 3 pulses. What is the final count?

PLC Programming

Preset = 4
Count = 0

# 2 pulses
Count += 2

# Reset
Count = 0

# 3 pulses
Count += 3

Output Count

A 3

B 5

C 0

D 4

💡 Hint

Reset clears the count before the last pulses.

After 2 pulses, count is 2. Reset sets count to 0. Then 3 pulses increase count to 3.

🚀 Application

expert

Determining counter output after complex pulse and reset sequence

A counter with preset 6 starts at 0. It receives pulses in this order: 3 pulses, reset, 2 pulses, reset, 6 pulses. What is the final count and is the done signal true or false?

PLC Programming

Preset = 6
Count = 0
Done = False

# 3 pulses
Count += 3

# Reset
Count = 0

# 2 pulses
Count += 2

# Reset
Count = 0

# 6 pulses
Count += 6

Done = (Count >= Preset)

Output Count and Done

A Count = 6, Done = False

B Count = 5, Done = False

C Count = 6, Done = True

D Count = 0, Done = False

💡 Hint

Reset clears count before next pulses; done is true if count >= preset.

After final 6 pulses, count equals preset 6, so done is true.

Practice

1. What does the preset value in a PLC counter do?

easy

A. It sets the target count the counter must reach.

B. It resets the counter to zero.

C. It increments the counter by one.

D. It pauses the counter from counting.

Solution

1. Step 1: Understand the role of preset in counters

   The preset value defines the goal number the counter counts up to before triggering an action.

2. Step 2: Differentiate preset from reset and increment

   Reset clears the count, increment adds to count, but preset sets the target number.

3. Final Answer:

   It sets the target count the counter must reach. -> Option A

4. Quick Check:

   Preset = target count [OK]

Hint: Preset is the goal number, reset clears count [OK]

Common Mistakes:

• Confusing preset with reset
• Thinking preset increments count
• Assuming preset pauses counting

2. Which of the following is the correct way to reset a counter named CNT1 in ladder logic?

easy

A. Use RES CNT1 instruction.

B. Use PRE CNT1 instruction.

C. Use INC CNT1 instruction.

D. Use SET CNT1 instruction.

Solution

1. Step 1: Identify reset instruction syntax

   In PLC ladder logic, RES is the standard instruction to reset a counter.

2. Step 2: Confirm other instructions meanings

   PRE sets preset, INC increments, SET sets bits, not reset counter.

3. Final Answer:

   Use RES CNT1 instruction. -> Option A

4. Quick Check:

   Reset = RES instruction [OK]

Hint: Reset uses RES instruction, not PRE or INC [OK]

Common Mistakes:

• Using PRE instead of RES to reset
• Confusing increment with reset
• Using SET which is for bits, not counters

3. Given a counter with preset 5 and current count 3, what will be the count after two increments?

medium

A. 4

B. 5

C. 3

D. 6

Solution

1. Step 1: Calculate count after increments

   Starting count is 3, two increments add 2, so 3 + 2 = 5.

2. Step 2: Compare count to preset

   The count reaches the preset value 5 after two increments.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   3 + 2 = 5 [OK]

Hint: Add increments to current count, compare to preset [OK]

Common Mistakes:

• Not adding increments correctly
• Confusing preset with current count
• Assuming count resets automatically

4. A PLC counter is not resetting when the reset button is pressed. Which of these is the most likely cause?

medium

A. The preset value is set too high.

B. The increment instruction is used instead of reset.

C. The counter is already at zero.

D. The reset instruction is missing or not connected to the reset button.

Solution

1. Step 1: Check reset wiring and instruction

   If the reset button does not trigger reset, likely the reset instruction is missing or not linked to the button.

2. Step 2: Eliminate other causes

   Preset value or current count does not prevent reset; increment instruction wrong but would not cause no reset on button press.

3. Final Answer:

   The reset instruction is missing or not connected to the reset button. -> Option D

4. Quick Check:

   Reset fails if instruction missing or unlinked [OK]

Hint: Reset needs instruction linked to button [OK]

Common Mistakes:

• Blaming preset for reset failure
• Ignoring wiring or logic connection
• Confusing increment with reset

5. You want a counter to count up to 10, then reset automatically and start counting again from zero. Which ladder logic setup achieves this?

hard

A. Set preset to 0, increment counter indefinitely.

B. Set preset to 10, reset counter manually with a button.

C. Set preset to 10, use counter done bit to trigger reset instruction.

D. Use reset instruction before counting starts only.

Solution

1. Step 1: Understand automatic reset logic

   To reset automatically, use the counter's done bit (which is true when count reaches preset) to trigger the reset instruction.

2. Step 2: Evaluate other options

   Manual reset requires user action, preset 0 is invalid for counting, resetting only before counting won't loop.

3. Final Answer:

   Set preset to 10, use counter done bit to trigger reset instruction. -> Option C

4. Quick Check:

   Done bit triggers reset for auto count loop [OK]

Hint: Use done bit to auto reset counter [OK]

Common Mistakes:

• Relying on manual reset for auto counting
• Setting preset to zero
• Resetting only once before counting