Comparison instructions (EQ, NE, GT, LT) in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of EQ instruction in PLC ladder logic

Given the following PLC ladder logic snippet using the EQ (equal) instruction, what will be the value of the output coil EQ_Output after execution?

Inputs:
- Input1 = 10
- Input2 = 10

Logic:
EQ_Output = EQ(Input1, Input2)

PLC Programming

Input1 = 10
Input2 = 10
EQ_Output = (Input1 == Input2)

A EQ_Output is FALSE

B EQ_Output causes a runtime error

C EQ_Output is undefined

D EQ_Output is TRUE

💡 Hint

EQ checks if two values are exactly the same.

Since Input1 and Input2 are both 10, the EQ instruction returns TRUE, setting EQ_Output to TRUE.

💻 Command Output

intermediate

Result of NE (not equal) instruction

What will be the output of the NE instruction when comparing these two values?

InputA = 5
InputB = 8

NE_Output = NE(InputA, InputB)

PLC Programming

InputA = 5
InputB = 8
NE_Output = (InputA != InputB)

A NE_Output is TRUE

B NE_Output is FALSE

C NE_Output is NULL

D NE_Output causes a syntax error

💡 Hint

NE returns TRUE if values are different.

Since 5 is not equal to 8, NE returns TRUE, so NE_Output is TRUE.

💻 Command Output

advanced

Output of GT (greater than) instruction with negative numbers

Consider the GT instruction comparing two signed integers:

Value1 = -3
Value2 = -7

GT_Output = GT(Value1, Value2)

What is the value of GT_Output?

PLC Programming

Value1 = -3
Value2 = -7
GT_Output = (Value1 > Value2)

A GT_Output causes a type error

B GT_Output is FALSE

C GT_Output is TRUE

D GT_Output is NULL

💡 Hint

Remember that -3 is greater than -7 on the number line.

Since -3 is greater than -7, the GT instruction returns TRUE.

💻 Command Output

advanced

LT (less than) instruction with equal values

What will the LT instruction output be when comparing two equal values?

Var1 = 12
Var2 = 12

LT_Output = LT(Var1, Var2)

PLC Programming

Var1 = 12
Var2 = 12
LT_Output = (Var1 < Var2)

A LT_Output is FALSE

B LT_Output causes a runtime error

C LT_Output is TRUE

D LT_Output is TRUE if Var1 is less or equal

💡 Hint

LT only returns TRUE if the first value is strictly less than the second.

Since Var1 equals Var2, LT returns FALSE because 12 is not less than 12.

🧠 Conceptual

expert

Understanding combined comparison instructions in PLC

In a PLC program, you want to check if a temperature value is between 20 and 30 degrees inclusive using comparison instructions EQ, NE, GT, and LT.

Which combination of instructions correctly implements this check?

Options:

A Check if (GE(Temp, 20) AND LE(Temp, 30))

B Check if (GT(Temp, 19) AND LT(Temp, 31))

C Check if (EQ(Temp, 20) OR EQ(Temp, 30))

D Check if (NE(Temp, 20) AND NE(Temp, 30))

💡 Hint

Use GT and LT with values just outside the range to include boundaries.

Since GE and LE instructions are not given, using GT(Temp, 19) and LT(Temp, 31) includes 20 and 30 because Temp > 19 means Temp >= 20 and Temp < 31 means Temp <= 30.

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