CASE statement in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the CASE statement with the variable 'command'.

PLC Programming

CASE [1] OF
  1: Start;
  2: Stop;
  ELSE Idle;
END_CASE;

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

Astate

Bstatus

Cmode

Dcommand

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not defined like 'status' or 'mode'.

Leaving the CASE statement without a controlling variable.

The CASE statement must use the variable 'command' to decide which action to take.

2fill in blank

medium

Complete the code to handle the case when 'command' equals 3.

PLC Programming

CASE command OF
  1: Start;
  2: Stop;
  [1]: Pause;
  ELSE Idle;
END_CASE;

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

A3

B0

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number not matching the Pause command.

Confusing the ELSE case with a specific case.

The case for 'Pause' corresponds to when 'command' equals 3.

3fill in blank

hard

Fix the error in the CASE statement by completing the missing keyword.

PLC Programming

CASE command [1]
  1: Start;
  2: Stop;
  ELSE Idle;
END_CASE;

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

AOF

BIS

CTHEN

DDO

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'IS', 'THEN', or 'DO' instead of 'OF'.

Omitting the keyword entirely.

The correct syntax requires the keyword 'OF' after the CASE variable.

4fill in blank

hard

Fill both blanks to complete the CASE statement that handles commands 1 and 2.

PLC Programming

CASE command [1]
  [2]: Start;
  2: Stop;
  ELSE Idle;
END_CASE;

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

AOF

B1

C3

DIS

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'IS' instead of 'OF'.

Using wrong case numbers like 3 for Start.

The CASE statement needs 'OF' after the variable and '1' to handle the Start command.

5fill in blank

hard

Fill all three blanks to create a CASE statement that returns 'Running' for 1, 'Stopped' for 2, and 'Paused' for 3.

PLC Programming

CASE [1] [2]
  1: [3];
  2: Stopped;
  3: Paused;
  ELSE Idle;
END_CASE;

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

Astatus

BOF

CRunning

Dcommand

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names like 'status'.

Using wrong keywords instead of 'OF'.

Putting wrong action names for case 1.

The CASE statement uses 'command' as variable, 'OF' as keyword, and 'Running' as the action for case 1.

Practice

1. What is the main purpose of a CASE statement in PLC programming?

easy

A. To stop the program execution immediately

B. To repeat a set of instructions multiple times

C. To select one action from many based on a variable's value

D. To declare variables for use in the program

Solution

1. Step 1: Understand the role of CASE statement

   The CASE statement evaluates a variable and chooses one matching action from multiple options.

2. Step 2: Compare with other options

   Repeating instructions is done by loops, declaring variables is separate, and stopping execution is a different command.

3. Final Answer:

   To select one action from many based on a variable's value -> Option C

4. Quick Check:

   CASE statement = Select action by value [OK]

Hint: CASE chooses action based on value, like a traffic signal [OK]

Common Mistakes:

• Confusing CASE with loops
• Thinking CASE declares variables
• Assuming CASE stops program

2. Which of the following is the correct syntax to start a CASE statement in PLC structured text?

easy

A. CASE variable OF

B. SWITCH(variable) {

C. IF variable THEN

D. SELECT variable FROM

Solution

1. Step 1: Recall PLC structured text CASE syntax

   The correct syntax to start a CASE statement is CASE variable OF.

2. Step 2: Eliminate other options

   SWITCH(variable) { is JavaScript style, C is IF statement, D is invalid syntax.

3. Final Answer:

   CASE variable OF -> Option A

4. Quick Check:

   PLC CASE starts with 'CASE variable OF' [OK]

Hint: PLC CASE always uses 'CASE variable OF' to start [OK]

Common Mistakes:

• Using SWITCH instead of CASE
• Confusing IF with CASE
• Writing SELECT instead of CASE

3. Given the following PLC code snippet, what will be the value of output after execution?

CASE input OF
  1: output := 'One';
  2: output := 'Two';
  3: output := 'Three';
ELSE
  output := 'Other';
END_CASE;

If input is 2?

medium

A. 'One'

B. 'Two'

C. 'Three'

D. 'Other'

Solution

1. Step 1: Match input value with CASE options

   The input is 2, so the CASE matches the line 2: output := 'Two';.

2. Step 2: Determine output value

   Since input is 2, output is assigned the string 'Two'.

3. Final Answer:

   'Two' -> Option B

4. Quick Check:

   Input 2 matches case 2, output = 'Two' [OK]

Hint: Match input to CASE label to find output [OK]

Common Mistakes:

• Choosing ELSE output for matching case
• Confusing input 2 with 1 or 3
• Ignoring ELSE clause

4. Identify the error in this CASE statement snippet:

CASE status OF
  0: action := 'Stop';
  1: action := 'Start';
  2 action := 'Pause';
ELSE
  action := 'Unknown';
END_CASE;

medium

A. Missing colon after 2 in third case

B. ELSE clause is not allowed in CASE

C. END_CASE should be ENDCASE

D. Variable 'action' is not declared

Solution

1. Step 1: Check syntax of each CASE line

   The third case line is missing a colon after 2; it should be 2: action := 'Pause';.

2. Step 2: Verify other parts

   ELSE clause is valid, END_CASE is correct syntax, and variable declaration is not shown but not an error here.

3. Final Answer:

   Missing colon after 2 in third case -> Option A

4. Quick Check:

   Each case label needs a colon ':' [OK]

Hint: Each case label must end with ':' [OK]

Common Mistakes:

• Omitting colon after case value
• Thinking ELSE is invalid
• Confusing END_CASE syntax

5. You want to use a CASE statement to control a motor speed setting based on an integer input from 0 to 4, where 0 means 'Stop', 1 means 'Slow', 2 means 'Medium', 3 means 'Fast', and any other value means 'Error'. Which of the following CASE statements correctly implements this logic?

hard

A. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
END_CASE;

B. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
4: motor_speed := 'Error';
END_CASE;

C. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
4: motor_speed := 'Error';
ELSE motor_speed := 'Error';
END_CASE;

D. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
ELSE motor_speed := 'Error';
END_CASE;

Solution

1. Step 1: Check the required mapping

   Values 0 to 3 have specific motor speeds; any other value should set 'Error'.

2. Step 2: Analyze each option

   CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; ELSE motor_speed := 'Error'; END_CASE; uses ELSE for all other values, correctly handling unexpected inputs. CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; 4: motor_speed := 'Error'; END_CASE; treats 4 as 'Error' but misses ELSE for other values. CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; 4: motor_speed := 'Error'; ELSE motor_speed := 'Error'; END_CASE; incorrectly uses a colon after ELSE ('ELSE:'), which is invalid syntax. CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; END_CASE; lacks handling for values outside 0-3.

3. Final Answer:

   CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; ELSE motor_speed := 'Error'; END_CASE -> Option D

4. Quick Check:

   Use ELSE for unexpected values in CASE [OK]

Hint: Use ELSE to catch all unexpected values in CASE [OK]

Common Mistakes:

• Not using ELSE for unexpected values
• Assigning error to a specific case instead of ELSE
• Missing cases for valid inputs