Assignment and arithmetic in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of arithmetic assignment in PLC structured text

What is the value of variable Result after executing this PLC structured text code?

VAR
A : INT := 5;
B : INT := 3;
Result : INT;
END_VAR

Result := A + B * 2;

PLC Programming

VAR
  A : INT := 5;
  B : INT := 3;
  Result : INT;
END_VAR

Result := A + B * 2;

A 11

B 16

C 13

D 10

💡 Hint

Remember the order of operations: multiplication happens before addition.

The expression is evaluated as 5 + (3 * 2) = 5 + 6 = 11.

💻 Command Output

intermediate

Result of combined assignment and subtraction

What will be the value of Counter after this code runs?

VAR
Counter : INT := 10;
END_VAR

Counter := Counter - 4 + 2;

PLC Programming

VAR
  Counter : INT := 10;
END_VAR

Counter := Counter - 4 + 2;

A 4

B 6

C 8

D 12

💡 Hint

Calculate subtraction and addition from left to right.

Counter = 10 - 4 + 2 = 6 + 2 = 8.

📝 Syntax

advanced

Identify the syntax error in arithmetic assignment

Which option contains a syntax error in the assignment statement in PLC structured text?

A Result := (A - B) / 2;

B Result := (A + B) * 2;

C Result := A - B / 2;

D Result := A + * B;

💡 Hint

Look for invalid operator placement.

Option D has an invalid operator sequence '+ *' which is not allowed.

💻 Command Output

advanced

Value of variable after mixed arithmetic and assignment

Given the code below, what is the final value of Output?

VAR
X : INT := 4;
Y : INT := 3;
Output : INT;
END_VAR

Output := X * Y - X / Y;

PLC Programming

VAR
  X : INT := 4;
  Y : INT := 3;
  Output : INT;
END_VAR

Output := X * Y - X / Y;

A 10

B 11

C 9

D 8

💡 Hint

Remember integer division truncates decimals.

X * Y = 4 * 3 = 12; X / Y = 4 / 3 = 1 (integer division); Output = 12 - 1 = 11.

💻 Command Output

expert

Final value after sequential arithmetic assignments

What is the final value of Value after running this code?

VAR
Value : INT := 2;
END_VAR

Value := Value + 3;
Value := Value * 2;
Value := Value - 4;

PLC Programming

VAR
  Value : INT := 2;
END_VAR

Value := Value + 3;
Value := Value * 2;
Value := Value - 4;

A 6

B 8

C 10

D 4

💡 Hint

Calculate each assignment step by step.

Step 1: Value = 2 + 3 = 5; Step 2: Value = 5 * 2 = 10; Step 3: Value = 10 - 4 = 6.

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