Math instructions (ADD, SUB, MUL, DIV) in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add two numbers and store the result.

PLC Programming

[1] IN1 , IN2 , OUT

Drag options to blanks, or click blank then click option'

AADD

BSUB

CDIV

DMUL

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUB instead of ADD will subtract values.

Using MUL or DIV will multiply or divide instead of adding.

The ADD instruction adds the values of IN1 and IN2 and stores the result in OUT.

2fill in blank

medium

Complete the code to subtract IN2 from IN1 and store the result.

PLC Programming

[1] IN1 , IN2 , OUT

Drag options to blanks, or click blank then click option'

AADD

BDIV

CMUL

DSUB

Attempts:

3 left

💡 Hint

Common Mistakes

Using ADD will add instead of subtract.

Using MUL or DIV will multiply or divide instead of subtracting.

The SUB instruction subtracts IN2 from IN1 and stores the result in OUT.

3fill in blank

hard

Fix the error in the code to multiply IN1 and IN2 correctly.

PLC Programming

[1] IN1 , IN2 , OUT

Drag options to blanks, or click blank then click option'

AMUL

BSUB

CADD

DDIV

Attempts:

3 left

💡 Hint

Common Mistakes

Using ADD or SUB will not multiply values.

Using DIV will divide instead of multiply.

The MUL instruction multiplies IN1 and IN2 and stores the result in OUT.

4fill in blank

hard

Fill both blanks to divide IN1 by IN2 and store the result.

PLC Programming

[1] IN1 , IN2 , [2]

Drag options to blanks, or click blank then click option'

ADIV

BIN1

COUT

DIN2

Attempts:

3 left

💡 Hint

Common Mistakes

Using ADD, SUB, or MUL instead of DIV.

Storing the result in IN1 or IN2 instead of OUT.

The DIV instruction divides IN1 by IN2 and stores the result in OUT.

5fill in blank

hard

Fill all three blanks to create a program that adds IN1 and IN2, multiplies the sum by IN3, and stores the final result.

PLC Programming

[1] IN1 , IN2 , TEMP
MUL [2] , [3] , OUT

Drag options to blanks, or click blank then click option'

AADD

BTEMP

CIN3

DOUT

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up TEMP and OUT variables.

Using wrong instructions for operations.

First, ADD sums IN1 and IN2 storing in TEMP. Then MUL multiplies TEMP by IN3 storing in OUT.

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