Math instructions (ADD, SUB, MUL, DIV) in PLC Programming

Introduction

Math instructions help a PLC do basic calculations like adding, subtracting, multiplying, and dividing numbers. This lets machines make decisions based on numbers.

Calculate total items produced by adding counts.

Find the difference between target and actual temperature.

Multiply speed by time to get distance traveled.

Divide total cost by number of units to find cost per unit.

Syntax

PLC Programming

ADD source1, source2, destination
SUB source1, source2, destination
MUL source1, source2, destination
DIV source1, source2, destination

Each instruction takes two source values and stores the result in a destination.

Sources and destination are usually registers or memory locations.

Examples

Adds 5 and 3, stores 8 in register D0.

PLC Programming

ADD 5, 3, D0

Subtracts 2 from the value in D1, stores result in D2.

PLC Programming

SUB D1, 2, D2

Multiplies value in D3 by 4, stores result in D4.

PLC Programming

MUL D3, 4, D4

Divides value in D5 by 2, stores result in D6.

PLC Programming

DIV D5, 2, D6

Sample Program

This program adds 10 and 20, then subtracts 5, multiplies by 3, and divides by 5. It moves results to output registers.

PLC Programming

ADD 10, 20, D0
SUB D0, 5, D1
MUL D1, 3, D2
DIV D2, 5, D3
// Output values
MOV D0, OUT0
MOV D1, OUT1
MOV D2, OUT2
MOV D3, OUT3

Important Notes

Division by zero must be avoided to prevent errors.

Results may be truncated if using integer registers.

Check your PLC manual for exact syntax as it can vary.

Summary

Math instructions perform basic calculations in PLCs.

They use two inputs and store the result in a destination.

Use them to automate calculations in machines easily.

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