CASE statement in PLC Programming - Mini Project: Build & Apply

Using CASE Statement in PLC Programming

📖 Scenario: You are programming a simple PLC controller that controls a machine mode based on an input number. The machine can be in one of four modes: Idle, Start, Stop, or Error.The input is a number from 1 to 4, and you want to use a CASE statement to set the machine mode accordingly.

🎯 Goal: Build a PLC program using a CASE statement that reads an input number and sets the machine mode string based on the input.

📋 What You'll Learn

Create a variable input_mode with a value between 1 and 4

Create a variable machine_mode to hold the mode string

Use a CASE statement to assign machine_mode based on input_mode

Print the machine_mode as output

💡 Why This Matters

🌍 Real World

PLC programs often use CASE statements to control machines based on input signals or sensor values.

💼 Career

Understanding CASE statements is essential for PLC programmers to write clear and maintainable control logic.

1

Set up the input variable

Create an integer variable called input_mode and set it to 3.

PLC Programming

# Create an integer variable called input_mode and set it to 3
# Your code here

Hint

Use the assignment operator := to set input_mode.

2

Create the output variable

Create a string variable called machine_mode to hold the machine mode text.

PLC Programming

input_mode : INT := 3;
# Create a string variable called machine_mode
# Your code here

Hint

Declare machine_mode as a string variable.

3

Use CASE statement to assign machine_mode

Write a CASE statement using input_mode to assign machine_mode these values:
- 1: "Idle"
- 2: "Start"
- 3: "Stop"
- 4: "Error"
Use ELSE to assign "Unknown".

PLC Programming

input_mode : INT := 3;
machine_mode : STRING;

# Write a CASE statement to assign machine_mode based on input_mode
# Your code here

Hint

Use CASE input_mode OF and END_CASE; to create the CASE block.

4

Output the machine_mode

Write a statement to print the value of machine_mode.

PLC Programming

input_mode : INT := 3;
machine_mode : STRING;

CASE input_mode OF
    1: machine_mode := 'Idle';
    2: machine_mode := 'Start';
    3: machine_mode := 'Stop';
    4: machine_mode := 'Error';
ELSE
    machine_mode := 'Unknown';
END_CASE;

# Print the machine_mode
# Your code here

Hint

Use the PRINT() function to display the string.

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