Assignment and arithmetic in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an assignment in PLC programming?

Assignment stores a value into a variable or memory location so it can be used later in the program.

beginner

How do you add two numbers in a PLC program?

You use an arithmetic instruction like ADD to sum two values and store the result in a variable.

intermediate

Explain the difference between '=' and ':=' in PLC programming.

'=' is often used for comparison, while ':=' is used for assignment to set a variable's value.

intermediate

What happens if you divide a number by zero in a PLC program?

Dividing by zero usually causes an error or exception because it is undefined mathematically.

beginner

Why is it important to use parentheses in arithmetic expressions in PLC programming?

Parentheses control the order of operations, ensuring calculations happen in the correct sequence.

Which symbol is commonly used for assignment in many PLC languages?

A :=

B ==

C =

D =>

The ':=' symbol is typically used to assign a value to a variable in PLC programming.

What is the result of 5 + 3 * 2 in PLC arithmetic without parentheses?

A 16

B 11

C 10

D 13

Multiplication happens before addition, so 3*2=6, then 5+6=11.

Which arithmetic operation is NOT typically supported in basic PLC instructions?

A Addition

B Subtraction

C Division

D Exponentiation

Exponentiation is usually not a basic PLC instruction and may require special function blocks.

What does the instruction 'MOV 10, VAR1' do in a PLC program?

A Adds 10 to VAR1

B Moves VAR1 to 10

C Assigns 10 to VAR1

D Subtracts 10 from VAR1

MOV copies the value 10 into the variable VAR1.

Why should you avoid dividing by zero in PLC programs?

A It results in an error or crash

B It resets the variable to zero

C It doubles the output value

D It causes the program to run faster

Dividing by zero is undefined and usually causes errors or crashes in PLC programs.

Describe how assignment and arithmetic operations work together in a PLC program.

Think about how you calculate and save results in real life.

Explain why order of operations is important when writing arithmetic expressions in PLC programming.

Remember how math class taught you to solve expressions.

Practice

1. In PLC programming, which symbol is used to assign a value to a variable?
speed := 100;

easy

A. The ':=' symbol

B. The '=' symbol

C. The '==' symbol

D. The '->' symbol

Solution

1. Step 1: Understand assignment syntax in PLC

   PLC uses ':=' to assign values to variables, not '=' or '=='.

2. Step 2: Identify correct assignment operator

   The ':=' operator updates the variable with the new value.

3. Final Answer:

   The ':=' symbol -> Option A

4. Quick Check:

   Assignment operator = ':=' [OK]

Hint: Remember ':=' means assign value in PLC [OK]

Common Mistakes:

• Using '=' instead of ':='
• Confusing comparison '==' with assignment
• Using arrow '->' mistakenly
• Forgetting semicolon after assignment

2. Which of the following is the correct syntax to add 5 to a variable count in PLC?

easy

A. count := count + 5;

B. count = count + 5;

C. count += 5;

D. count :=+ 5;

Solution

1. Step 1: Check assignment operator usage

   PLC uses ':=' for assignment, so '=' and '+=' are incorrect here.

2. Step 2: Verify arithmetic addition syntax

   'count := count + 5;' correctly adds 5 to count and assigns it back.

3. Final Answer:

   count := count + 5; -> Option A

4. Quick Check:

   Use ':=' for assignment and '+' for addition [OK]

Hint: Use ':=' for assignment, '+' for addition [OK]

Common Mistakes:

• Using '=' instead of ':='
• Trying '+=' which is not valid in PLC
• Writing ':=+' which is invalid
• Missing semicolon

3. What is the value of result after executing the following PLC code?

temp := 10;
result := temp * 3 + 2;

medium

A. 12

B. 36

C. 30

D. 32

Solution

1. Step 1: Calculate multiplication first

   temp * 3 = 10 * 3 = 30

2. Step 2: Add 2 to the multiplication result

   30 + 2 = 32

3. Final Answer:

   32 -> Option D

4. Quick Check:

   10 * 3 + 2 = 32 [OK]

Hint: Multiply before adding, follow order of operations [OK]

Common Mistakes:

• Adding before multiplying
• Using wrong variable value
• Ignoring operator precedence
• Confusing temp and result variables

4. Identify the error in this PLC code snippet:

speed := 50
speed := speed + 10;

medium

A. Cannot add numbers in PLC

B. Using '=' instead of ':='

C. Missing semicolon after first assignment

D. Variable 'speed' not declared

Solution

1. Step 1: Check statement endings

   PLC statements must end with a semicolon ';'. The first line misses it.

2. Step 2: Confirm other syntax is correct

   Second line uses correct ':=' and addition syntax.

3. Final Answer:

   Missing semicolon after first assignment -> Option C

4. Quick Check:

   Each statement ends with ';' [OK]

Hint: Always end PLC statements with semicolon ';' [OK]

Common Mistakes:

• Forgetting semicolon
• Confusing '=' with ':='
• Assuming addition not allowed
• Ignoring variable declaration rules

5. You want to calculate the average speed from three sensor readings s1, s2, and s3 in PLC. Which code correctly assigns the average to avg_speed?

s1 := 40;
s2 := 50;
s3 := 60;
avg_speed := (s1 + s2 + s3) / 3;

hard

A. avg_speed = (s1 + s2 + s3) / 3;

B. avg_speed := (s1 + s2 + s3) / 3;

C. avg_speed := s1 + s2 + s3 / 3;

D. avg_speed := (s1 + s2 + s3) * 3;

Solution

1. Step 1: Check correct assignment operator

   Use ':=' to assign the calculated value to avg_speed.

2. Step 2: Verify arithmetic expression correctness

   Sum all three readings and divide by 3 inside parentheses for average.

3. Final Answer:

   avg_speed := (s1 + s2 + s3) / 3; -> Option B

4. Quick Check:

   Use ':=' and parentheses for correct average [OK]

Hint: Use parentheses to sum before dividing [OK]

Common Mistakes:

• Using '=' instead of ':='
• Missing parentheses causing wrong division
• Multiplying instead of dividing
• Forgetting semicolon