Counter preset and reset in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a counter preset in PLC programming?

A counter preset is a set value that the counter counts up to or down from. When the counter reaches this preset value, it triggers an action or output.

beginner

How does a counter reset work in PLC programming?

A counter reset clears the current count value back to zero or a defined start point, allowing the counting process to begin again from the start.

intermediate

Why is it important to use both preset and reset in a counter?

Using both preset and reset allows precise control: the preset defines the target count, and the reset clears the count to start fresh, ensuring accurate counting cycles.

beginner

In a real-life scenario, how would a counter preset and reset be used?

Imagine a factory line counting boxes. The preset sets how many boxes to count before stopping. The reset clears the count after the batch is done, ready for the next batch.

intermediate

What happens if you do not reset a counter after it reaches its preset?

If you don't reset the counter, it will not start counting from zero again, which can cause incorrect counts and errors in the process control.

What does the preset value in a PLC counter represent?

A The target count to reach

B The current count value

C The reset signal

D The timer duration

The preset value is the target number the counter counts up to or down from.

What is the main purpose of resetting a counter?

A To increase the count faster

B To pause the counting

C To set the target count

D To clear the count and start over

Resetting clears the current count so counting can start fresh.

If a counter preset is set to 10, what happens when the count reaches 10?

A The counter stops counting forever

B The counter resets automatically

C The counter triggers an output or event

D The counter ignores the preset

Reaching the preset usually triggers an output or event in the PLC program.

Why should you reset a counter after it reaches its preset?

A To avoid incorrect counting in the next cycle

B To increase the preset value

C To stop the machine permanently

D To speed up the counting

Resetting ensures the counter starts fresh for the next counting cycle.

Which of these is NOT a function of a counter in PLC programming?

A Counting events

B Timing delays

C Triggering outputs at preset

D Resetting count

Timing delays are handled by timers, not counters.

Explain how counter preset and reset work together in a PLC program.

Think about how counting starts and stops in a process.

Describe a simple real-world example where you would use a counter preset and reset.

Imagine counting boxes on a conveyor belt.

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