Assignment and arithmetic in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign the value 10 to the variable 'counter'.

PLC Programming

counter [1] 10;

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

A:=

B==

C+=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' which is a comparison operator, not assignment.

Using '=' which is common in other languages but not in PLC.

In PLC programming, ':=' is used for assignment to set the value of a variable.

2fill in blank

medium

Complete the code to add 5 to the variable 'total'.

PLC Programming

total [1] total + 5;

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

A=

B:=

C==

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+=' which is not standard in PLC languages.

Using '==' which is for comparison, not assignment.

Use ':=' to assign the new value of total after adding 5.

3fill in blank

hard

Fix the error in the code to multiply 'speed' by 2 and assign it back.

PLC Programming

speed [1] speed * 2;

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

A==

B+=

C=

D:=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or '==' which are not valid assignment operators in PLC.

Using '+=' which is not supported.

Use ':=' to assign the result of the multiplication back to 'speed'.

4fill in blank

hard

Fill both blanks to subtract 3 from 'count' and assign it back.

PLC Programming

count [1] count [2] 3;

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

A:=

B+

C-

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of ':=' for assignment.

Using '+' instead of '-' for subtraction.

Use ':=' for assignment and '-' for subtraction.

5fill in blank

hard

Fill all three blanks to create a new variable 'result' that stores the sum of 'a' and 'b'.

PLC Programming

result [1] [2] [3];

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

A:=

Ba

C+

Db

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of ':=' for assignment.

Separating '+' and 'b' incorrectly.

Assign 'result' using ':=' to the sum of 'a' and 'b'.

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