Counter preset and reset in PLC Programming - Mini Project: Build & Apply

Counter Preset and Reset in PLC Programming

📖 Scenario: You are programming a simple conveyor belt system controlled by a PLC (Programmable Logic Controller). The system counts the number of items passing a sensor. You need to preset the counter to a starting value and reset it when needed.

🎯 Goal: Build a PLC program that presets a counter to 10, increments the counter when an item passes, and resets the counter when a reset button is pressed.

📋 What You'll Learn

Create a counter variable called Counter with a preset value of 10

Create a boolean variable called ItemDetected to simulate item passing

Create a boolean variable called ResetButton to simulate reset input

Increment Counter by 1 when ItemDetected is true

Reset Counter to 0 when ResetButton is true

Print the current value of Counter

💡 Why This Matters

🌍 Real World

Counters are used in industrial automation to track items, operations, or events. Presetting and resetting counters is essential for starting and controlling processes.

💼 Career

Understanding counter preset and reset logic is fundamental for PLC programmers and automation engineers working with manufacturing and process control systems.

1

Create the counter variable with preset value

Create an integer variable called Counter and set it to the preset value 10.

PLC Programming

# Your code here

Hint

Use a simple assignment to set Counter to 10.

2

Create input variables for item detection and reset button

Create two boolean variables called ItemDetected and ResetButton. Set both to False initially.

PLC Programming

Counter = 10
# Your code here

Hint

Use False to represent buttons not pressed or no item detected.

3

Add logic to increment and reset the counter

Write code to increment Counter by 1 when ItemDetected is True. Also, reset Counter to 0 when ResetButton is True.

PLC Programming

Counter = 10
ItemDetected = False
ResetButton = False
# Your code here

Hint

Use if and elif statements to check the inputs and update Counter.

4

Print the current counter value

Write a print statement to display the current value of Counter.

PLC Programming

Counter = 10
ItemDetected = False
ResetButton = False

if ResetButton:
    Counter = 0
elif ItemDetected:
    Counter += 1
# Your code here

Hint

Use print(Counter) to show the counter value.

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