Counter preset and reset in PLC Programming

Introduction

Counters help count events in machines. Preset sets a target count. Reset clears the count back to zero.

Counting items on a conveyor belt until a set number is reached.

Tracking how many times a button is pressed before action.

Resetting a count after a machine cycle completes.

Stopping a process when a count limit is met.

Starting a new count after maintenance or error.

Syntax

PLC Programming

COUNTER_NAME: CTU (CU, R, PV, CV, Q);
Where:
CU = Count Up input (boolean)
R = Reset input (boolean)
PV = Preset Value (integer)
CV = Current Value (integer, output)
Q = Output bit (true when CV >= PV)

CU input increases the count by 1 on each rising edge.

R input resets the count to zero immediately.

Examples

Counts up when StartButton is pressed. Resets when ResetButton is pressed. Stops counting when 10 is reached.

PLC Programming

Counter1: CTU(CU:=StartButton, R:=ResetButton, PV:=10, CV=>Counter1Value, Q=>Counter1Done);

Counts sensor triggers up to 5. Resets on emergency stop.

PLC Programming

Counter2: CTU(CU:=SensorTrigger, R:=EmergencyStop, PV:=5, CV=>CountVal, Q=>DoneFlag);

Sample Program

This program simulates a counter that increments when StartButton is pressed. It stops counting after reaching 3. ResetButton clears the count and done flag.

PLC Programming

PROGRAM CounterExample
VAR
  StartButton: BOOL := FALSE;
  ResetButton: BOOL := FALSE;
  CounterValue: INT := 0;
  CounterDone: BOOL := FALSE;
END_VAR

// Simulate pressing StartButton 3 times
StartButton := TRUE;
IF StartButton THEN
  IF NOT CounterDone THEN
    CounterValue := CounterValue + 1;
    IF CounterValue >= 3 THEN
      CounterDone := TRUE;
    END_IF
  END_IF
END_IF
StartButton := FALSE;

// Reset the counter
ResetButton := TRUE;
IF ResetButton THEN
  CounterValue := 0;
  CounterDone := FALSE;
END_IF
ResetButton := FALSE;

// Output current count and done status
END_PROGRAM

Important Notes

Preset value (PV) is the target count to reach before output Q turns true.

Resetting clears the current count immediately, so counting can start fresh.

Count Up (CU) input usually triggers on a rising edge to avoid multiple counts.

Summary

Counters track how many times something happens.

Preset sets the goal number to count to.

Reset clears the count to start over.

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