Why Comparison instructions (EQ, NE, GT, LT) in PLC Programming? - Purpose & Use Cases

The Big Idea

What if your machine could instantly know when something is wrong without you watching every second?

The Scenario

Imagine you are controlling a factory machine manually. You have to check if a temperature is exactly 100 degrees, or if a pressure is higher than a safe limit, by constantly watching dials and writing notes.

The Problem

This manual checking is slow and tiring. You might miss a critical change or make mistakes writing down values. It's hard to react quickly and keep the machine safe.

The Solution

Comparison instructions like EQ (equal), NE (not equal), GT (greater than), and LT (less than) let the PLC automatically compare values. The machine can instantly decide what to do next without human delay or error.

Before vs After

✗ Before

if temperature == 100 then start_cooling
if pressure > 50 then stop_machine

✓ After

EQ temperature, 100 -> start_cooling
GT pressure, 50 -> stop_machine

What It Enables

With these instructions, your PLC can make fast, accurate decisions to keep machines running safely and efficiently.

Real Life Example

In a bottling plant, the PLC uses LT to check if the bottle count is less than the target and NE to detect if a sensor is not triggered, so it can stop the line immediately if something goes wrong.

Key Takeaways

Manual checks are slow and error-prone.

Comparison instructions automate decision-making in PLCs.

This leads to safer, faster, and more reliable machine control.

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