Math instructions (ADD, SUB, MUL, DIV) in PLC Programming - Mini Project: Build & Apply

Basic Math Operations with PLC Instructions

📖 Scenario: You are programming a simple PLC (Programmable Logic Controller) to perform basic math operations on two numbers. These operations include addition, subtraction, multiplication, and division. This is common in industrial automation where calculations are needed for controlling machines.

🎯 Goal: Build a PLC program that stores two numbers, selects a math operation, performs the operation using the correct PLC math instruction, and outputs the result.

📋 What You'll Learn

Create two integer variables named num1 and num2 with exact values 12 and 4 respectively.

Create a variable named operation to hold the math operation as a string: 'ADD', 'SUB', 'MUL', or 'DIV'.

Use a CASE or IF structure to select the correct math instruction based on operation.

Perform the math operation using the PLC instructions ADD, SUB, MUL, or DIV.

Store the result in a variable named result.

Output the result.

💡 Why This Matters

🌍 Real World

PLC programs often need to perform math calculations to control machines, adjust speeds, or calculate totals in manufacturing.

💼 Career

Understanding basic math instructions in PLC programming is essential for automation engineers and technicians working with industrial control systems.

1

Create the initial variables

Create two integer variables called num1 and num2 with values 12 and 4 respectively.

PLC Programming

# Define num1 and num2 with values 12 and 4
# Your code here

Hint

Use the assignment operator := to assign values to variables.

2

Add the operation selector

Create a string variable called operation and set it to 'ADD'.

PLC Programming

num1 := 12;
num2 := 4;
# Define operation variable with value 'ADD'
# Your code here

Hint

Use single quotes for string values in PLC programming.

3

Perform the math operation

Use a CASE statement on operation to perform the correct math instruction: ADD, SUB, MUL, or DIV. Store the result in a variable called result.

PLC Programming

num1 := 12;
num2 := 4;
operation := 'ADD';
# Use CASE on operation to set result using ADD, SUB, MUL, DIV
# Your code here

Hint

Use CASE to select the operation and assign result accordingly.

4

Output the result

Write a statement to output the value of result.

PLC Programming

num1 := 12;
num2 := 4;
operation := 'ADD';

CASE operation OF
  'ADD':
    result := num1 + num2;
  'SUB':
    result := num1 - num2;
  'MUL':
    result := num1 * num2;
  'DIV':
    result := num1 / num2;
ELSE
  result := 0;
END_CASE;

# Output the result
# Your code here

Hint

Use the PLC output function (e.g., WriteOutput(result);) to display the result.

Practice

1. What does the ADD instruction do in PLC programming?

easy

A. It multiplies two numbers.

B. It subtracts one number from another.

C. It adds two numbers and stores the result.

D. It divides one number by another.

Solution

1. Step 1: Understand the ADD instruction

   The ADD instruction takes two input values and calculates their sum.

2. Step 2: Identify the result storage

   The result of the addition is stored in a destination variable or register.

3. Final Answer:

   It adds two numbers and stores the result. -> Option C

4. Quick Check:

   ADD = sum [OK]

Hint: ADD means sum two values and save result [OK]

Common Mistakes:

• Confusing ADD with SUB
• Thinking ADD multiplies
• Assuming ADD divides

2. Which of the following is the correct syntax for a SUB instruction in PLC?

easy

A. SUB source1, source2, destination

B. SUB destination, source1, source2

C. SUB source1; source2; destination

D. SUB(source1, source2, destination)

Solution

1. Step 1: Recall the standard syntax format

   The SUB instruction syntax is: SUB source1, source2, destination.

2. Step 2: Match options with syntax

   SUB source1, source2, destination matches the correct order and separators (commas).

3. Final Answer:

   SUB source1, source2, destination -> Option A

4. Quick Check:

   SUB syntax uses commas and order: source1, source2, destination [OK]

Hint: Syntax uses commas: SUB source1, source2, destination [OK]

Common Mistakes:

• Swapping destination and sources
• Using semicolons instead of commas
• Adding parentheses incorrectly

3. Given the following PLC instructions:

ADD 5, 3, D1
MUL D1, 2, D2
SUB D2, 4, D3
DIV D3, 2, D4

What is the final value stored in D4?

medium

A. 4

B. 6

C. 2

D. 1

Solution

1. Step 1: Calculate ADD and MUL results

   ADD 5 + 3 = 8 stored in D1; MUL 8 * 2 = 16 stored in D2.

2. Step 2: Calculate SUB and DIV results

   SUB 16 - 4 = 12 stored in D3; DIV 12 / 2 = 6 stored in D4.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   5+3=8, 8*2=16, 16-4=12, 12/2=6 [OK]

Hint: Calculate step-by-step and store intermediate results [OK]

Common Mistakes:

• Mixing order of operations
• Forgetting to store intermediate results
• Dividing before subtracting

4. Identify the error in this PLC math instruction:

DIV 10, 0, D1

medium

A. Incorrect syntax for DIV instruction.

B. Operands must be swapped.

C. Destination variable missing.

D. Division by zero error.

Solution

1. Step 1: Analyze the DIV instruction inputs

   The instruction tries to divide 10 by 0, which is mathematically undefined.

2. Step 2: Understand PLC behavior on division by zero

   Most PLCs will raise an error or fault when dividing by zero.

3. Final Answer:

   Division by zero error. -> Option D

4. Quick Check:

   DIV by zero causes error [OK]

Hint: Never divide by zero in PLC math instructions [OK]

Common Mistakes:

• Ignoring zero divisor
• Assuming syntax error instead
• Swapping operands incorrectly

5. You want to calculate the average of three sensor readings stored in S1, S2, and S3 using PLC math instructions. Which sequence correctly computes the average and stores it in AVG?

hard

A. ADD S1, S2, T1
ADD T1, S3, T2
DIV T2, 3, AVG

B. ADD S1, S2, T1
DIV T1, 3, T2
ADD T2, S3, AVG

C. DIV S1, 3, T1
DIV S2, 3, T2
DIV S3, 3, T3
ADD T1, T2, T4
ADD T4, T3, AVG

D. SUB S1, S2, T1
ADD T1, S3, T2
DIV T2, 3, AVG

Solution

1. Step 1: Sum the three sensor readings

   First ADD S1 and S2 into T1, then ADD T1 and S3 into T2 to get total sum.

2. Step 2: Divide the total sum by 3

   Use DIV to divide T2 by 3 and store the result in AVG, which gives the average.

3. Final Answer:

   ADD S1, S2, T1 ADD T1, S3, T2 DIV T2, 3, AVG -> Option A

4. Quick Check:

   Sum all then divide by count = average [OK]

Hint: Sum all values first, then divide by count [OK]

Common Mistakes:

• Dividing before summing
• Using SUB instead of ADD
• Adding after dividing only two values