Comparison instructions (EQ, NE, GT, LT) in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EQ instruction do in PLC programming?

The EQ (Equal) instruction compares two values and returns true if they are exactly the same.

beginner

How does the NE instruction work in PLC programming?

The NE (Not Equal) instruction checks if two values are different. It returns true if the values are not the same.

beginner

What is the purpose of the GT instruction?

GT (Greater Than) compares two values and returns true if the first value is larger than the second.

beginner

Explain the LT instruction in simple terms.

LT (Less Than) checks if the first value is smaller than the second value. It returns true if it is.

beginner

Why are comparison instructions important in PLC programming?

They help the PLC make decisions by comparing values, like checking if a sensor reading meets a condition to start or stop a machine.

Which instruction returns true when two values are exactly the same?

A NE

B EQ

C GT

D LT

EQ checks if two values are equal and returns true if they match.

What does the NE instruction check for?

A If values are not equal

B If first value is greater

C If values are equal

D If first value is less

NE returns true when the two values are different.

If you want to check if a temperature is higher than 100, which instruction would you use?

A EQ

B NE

C GT

D LT

GT checks if the first value is greater than the second.

Which instruction returns true if the first value is smaller than the second?

A EQ

B NE

C GT

D LT

LT returns true when the first value is less than the second.

Why do PLCs use comparison instructions?

A To compare values and make decisions

B To perform math calculations

C To store data

D To control motor speed directly

Comparison instructions help PLCs decide actions based on value comparisons.

Describe how the EQ and NE instructions differ in PLC programming.

Think about when you want to know if two things match or don't match.

Explain a real-life example where GT and LT instructions might be used in a PLC program.

Imagine checking if a tank is too full or a machine is overheating.

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