Math instructions (ADD, SUB, MUL, DIV) in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of ADD instruction in PLC

Given the following PLC code snippet using the ADD instruction, what is the value of the variable RESULT after execution?

ADD 5, 3, RESULT

PLC Programming

ADD 5, 3, RESULT

A 15

B 8

C 2

D Error: Invalid operands

💡 Hint

ADD instruction sums the first two numbers and stores in the third.

The ADD instruction adds 5 and 3, resulting in 8 stored in RESULT.

💻 Command Output

intermediate

Output of SUB instruction in PLC

What is the value of RESULT after executing this PLC instruction?

SUB 10, 4, RESULT

PLC Programming

SUB 10, 4, RESULT

A 6

B 14

C -6

D Error: Negative result not allowed

💡 Hint

SUB subtracts the second number from the first.

10 minus 4 equals 6, so RESULT is 6.

💻 Command Output

advanced

Result of MUL instruction with variables

Consider the following PLC instructions:

VAR1 = 7
VAR2 = 6
MUL VAR1, VAR2, RESULT

What is the value of RESULT?

PLC Programming

VAR1 = 7
VAR2 = 6
MUL VAR1, VAR2, RESULT

A 42

B 13

C Error: Undefined variable

D 1

💡 Hint

MUL multiplies the first two operands.

7 multiplied by 6 equals 42, stored in RESULT.

💻 Command Output

advanced

DIV instruction with integer division

What is the value of RESULT after this PLC instruction?

DIV 20, 3, RESULT

PLC Programming

DIV 20, 3, RESULT

A 7

B 6.6667

C 6

D Error: Division by zero

💡 Hint

DIV performs integer division truncating decimals.

20 divided by 3 is 6 with remainder, RESULT stores 6.

🧠 Conceptual

expert

Behavior of DIV instruction with zero divisor

What happens when the DIV instruction is executed with the divisor set to zero, like this?

DIV 10, 0, RESULT

PLC Programming

DIV 10, 0, RESULT

A RESULT is unchanged, no error

B RESULT is set to maximum integer value

C RESULT is set to zero

D Execution halts with a division by zero error

💡 Hint

Division by zero is not allowed in math operations.

Dividing by zero causes a runtime error and stops execution.

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