Comparison instructions (EQ, NE, GT, LT) in PLC Programming

Introduction

Comparison instructions help decide if one value is equal, not equal, greater, or less than another. This lets the program make choices like a traffic light or a vending machine.

To check if a sensor value matches a target number.

To see if a temperature is higher than a safe limit.

To compare two counters and act when one is bigger.

To detect if a button is pressed or not.

To control a machine step based on a timer value.

Syntax

PLC Programming

EQ(source1, source2)  // Equal
NE(source1, source2)  // Not Equal
GT(source1, source2)  // Greater Than
LT(source1, source2)  // Less Than

Each instruction compares two values and returns TRUE or FALSE.

Use these inside your PLC logic to control outputs or steps.

Examples

Checks if temperature equals 100.

PLC Programming

EQ(temperature, 100)

Checks if sensorValue is not zero.

PLC Programming

NE(sensorValue, 0)

Checks if counter is greater than 10.

PLC Programming

GT(counter, 10)

Checks if timer is less than 5.

PLC Programming

LT(timer, 5)

Sample Program

This program checks if the temperature is above 90. If yes, it sets alarm to TRUE. Otherwise, alarm is FALSE. Then it prints the alarm status.

PLC Programming

VAR
  temperature : INT := 95;
  alarm : BOOL := FALSE;
END_VAR

// Check if temperature is greater than 90
IF GT(temperature, 90) THEN
  alarm := TRUE;
ELSE
  alarm := FALSE;
END_IF

// Output alarm status
// (In real PLC, this might turn on a light)

// Print alarm status (simulated)
PRINT('Alarm status: ' + TO_STRING(alarm));

Important Notes

Comparison instructions always return a boolean: TRUE or FALSE.

Make sure the data types of source1 and source2 match to avoid errors.

Use these instructions to control program flow and outputs clearly.

Summary

Comparison instructions let your PLC decide between options by comparing values.

EQ means equal, NE means not equal, GT means greater than, and LT means less than.

Use them to check conditions and control your machine safely and smartly.

Practice

1. Which comparison instruction checks if two values are exactly the same in PLC programming?

easy

A. EQ

B. NE

C. GT

D. LT

Solution

1. Step 1: Understand the meaning of EQ

   EQ stands for 'equal' and checks if two values are exactly the same.

2. Step 2: Compare with other instructions

   NE means not equal, GT means greater than, and LT means less than, so they do not check equality.

3. Final Answer:

   EQ -> Option A

4. Quick Check:

   EQ means equal [OK]

Hint: EQ means equal, so it checks if values match exactly [OK]

Common Mistakes:

• Confusing EQ with NE
• Thinking GT checks equality
• Mixing LT with EQ

2. Which of the following is the correct syntax to check if variable A is greater than variable B in a PLC program?

easy

A. IF A LT B THEN

B. IF A NE B THEN

C. IF A EQ B THEN

D. IF A GT B THEN

Solution

1. Step 1: Identify the comparison for 'greater than'

   GT means 'greater than', so to check if A is greater than B, use 'A GT B'.

2. Step 2: Check other options

   LT means less than, EQ means equal, NE means not equal, so they are incorrect for this check.

3. Final Answer:

   IF A GT B THEN -> Option D

4. Quick Check:

   GT means greater than [OK]

Hint: GT means greater than, so use it to compare if A > B [OK]

Common Mistakes:

• Using LT instead of GT
• Confusing EQ with GT
• Using NE for greater than check

3. Given the following PLC code snippet:

VAR
  X: INT := 5;
  Y: INT := 10;
  Result: BOOL;
END_VAR

Result := X LT Y;

What will be the value of Result after execution?

medium

A. Syntax Error

B. FALSE

C. TRUE

D. Undefined

Solution

1. Step 1: Understand the comparison LT

   LT means 'less than'. The code checks if X (5) is less than Y (10).

2. Step 2: Evaluate the condition

   Since 5 is less than 10, the condition is true, so Result will be TRUE.

3. Final Answer:

   TRUE -> Option C

4. Quick Check:

   5 LT 10 = TRUE [OK]

Hint: LT means less than; 5 < 10 is true [OK]

Common Mistakes:

• Thinking LT means greater than
• Assuming Result is FALSE
• Confusing syntax with error

4. Identify the error in this PLC comparison instruction:

IF A NEQ B THEN
  // do something
END_IF

medium

A. Missing THEN keyword

B. NEQ is not a valid comparison instruction

C. Comparison should be EQ instead of NEQ

D. IF statement missing parentheses

Solution

1. Step 1: Check the comparison instruction used

   The code uses 'NEQ' which is not a standard PLC comparison instruction.

2. Step 2: Identify correct instruction

   The correct instruction for 'not equal' is 'NE', not 'NEQ'.

3. Final Answer:

   NEQ is not a valid comparison instruction -> Option B

4. Quick Check:

   Use NE for not equal, not NEQ [OK]

Hint: Use NE for not equal, not NEQ [OK]

Common Mistakes:

• Using NEQ instead of NE
• Forgetting THEN keyword
• Adding unnecessary parentheses

5. You want to write a PLC program that turns on a motor only if the temperature is greater than 50 and the pressure is less than 100. Which combination of comparison instructions correctly checks this condition?

hard

A. IF Temperature GT 50 AND Pressure LT 100 THEN Motor := TRUE;

B. IF Temperature LT 50 OR Pressure GT 100 THEN Motor := TRUE;

C. IF Temperature EQ 50 AND Pressure EQ 100 THEN Motor := TRUE;

D. IF Temperature NE 50 AND Pressure NE 100 THEN Motor := TRUE;

Solution

1. Step 1: Translate the condition into comparisons

   The motor should turn on if temperature is greater than 50 (GT 50) and pressure is less than 100 (LT 100).

2. Step 2: Check each option

   IF Temperature GT 50 AND Pressure LT 100 THEN Motor := TRUE; uses GT and LT with AND, matching the condition exactly. Other options use wrong comparisons or OR instead of AND.

3. Final Answer:

   IF Temperature GT 50 AND Pressure LT 100 THEN Motor := TRUE; -> Option A

4. Quick Check:

   GT and LT with AND matches condition [OK]

Hint: Use GT for greater, LT for less, combine with AND for both conditions [OK]

Common Mistakes:

• Using OR instead of AND
• Using EQ instead of GT or LT
• Mixing NE with equality checks