Comparison instructions (EQ, NE, GT, LT) in PLC Programming - Mini Project: Build & Apply

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

📖 Scenario: You are programming a simple PLC (Programmable Logic Controller) to control a machine. The machine needs to check sensor values and make decisions based on comparisons.For example, the machine should check if a temperature sensor reading equals a setpoint, or if a pressure sensor reading is greater than a safety limit.

🎯 Goal: Build a PLC program that uses comparison instructions EQ (equal), NE (not equal), GT (greater than), and LT (less than) to compare sensor values with setpoints and store the results in boolean variables.

📋 What You'll Learn

Create integer variables for sensor readings and setpoints

Create boolean variables to store comparison results

Use EQ, NE, GT, and LT instructions to compare values

Output the boolean results using print statements

💡 Why This Matters

🌍 Real World

PLC programs often need to compare sensor readings with setpoints to make decisions like turning machines on or off, triggering alarms, or adjusting controls.

💼 Career

Understanding comparison instructions is essential for PLC programmers to write logic that controls industrial machines safely and efficiently.

1

Create sensor and setpoint variables

Create integer variables called temperature and pressure with values 75 and 120 respectively. Also create integer variables called temp_setpoint and pressure_limit with values 75 and 100 respectively.

PLC Programming

# Create integer variables for sensors and setpoints
# Your code here

Hint

Use simple assignment statements to create variables with the exact names and values.

2

Create boolean variables for comparison results

Create boolean variables called temp_equals_setpoint, pressure_not_equal_limit, pressure_greater_than_limit, and temperature_less_than_setpoint. Initialize them all to False.

PLC Programming

temperature = 75
pressure = 120
temp_setpoint = 75
pressure_limit = 100
# Create boolean variables for comparison results
# Your code here

Hint

Use assignment statements to create boolean variables with the exact names and set them to False.

3

Use comparison instructions to update boolean variables

Use the comparison instructions EQ, NE, GT, and LT to update the boolean variables: set temp_equals_setpoint to True if temperature equals temp_setpoint, pressure_not_equal_limit to True if pressure is not equal to pressure_limit, pressure_greater_than_limit to True if pressure is greater than pressure_limit, and temperature_less_than_setpoint to True if temperature is less than temp_setpoint.

PLC Programming

temperature = 75
pressure = 120
temp_setpoint = 75
pressure_limit = 100
temp_equals_setpoint = False
pressure_not_equal_limit = False
pressure_greater_than_limit = False
temperature_less_than_setpoint = False
# Update boolean variables using comparison instructions
# Your code here

Hint

Use Python comparison operators ==, !=, >, and < to set the boolean variables.

4

Print the comparison results

Print the values of temp_equals_setpoint, pressure_not_equal_limit, pressure_greater_than_limit, and temperature_less_than_setpoint each on a new line.

PLC Programming

temperature = 75
pressure = 120
temp_setpoint = 75
pressure_limit = 100
temp_equals_setpoint = (temperature == temp_setpoint)
pressure_not_equal_limit = (pressure != pressure_limit)
pressure_greater_than_limit = (pressure > pressure_limit)
temperature_less_than_setpoint = (temperature < temp_setpoint)
# Print the boolean comparison results
# Your code here

Hint

Use print() to display each boolean variable on its own line.

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