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PLC Programmingscripting~30 mins

Reusable code patterns in PLC Programming - Mini Project: Build & Apply

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Reusable Code Patterns in PLC Programming
📖 Scenario: You work in a factory where machines need to be controlled by a PLC (Programmable Logic Controller). You want to create a simple reusable code pattern to turn a motor on or off based on a start and stop button.
🎯 Goal: Build a reusable code pattern (function block) in PLC programming that controls a motor using start and stop buttons.
📋 What You'll Learn
Create a function block called MotorControl with inputs Start and Stop and an output MotorOn
Add a variable MotorOn inside the function block to hold the motor state
Use a simple logic to turn MotorOn true when Start is pressed and false when Stop is pressed
Instantiate the MotorControl function block in the main program
Print or display the motor state after pressing start and stop buttons
💡 Why This Matters
🌍 Real World
Factories use PLCs to control machines safely and reliably. Reusable code patterns like function blocks help programmers write clear and maintainable control logic.
💼 Career
PLC programming skills with reusable code patterns are essential for automation engineers and technicians working in manufacturing and industrial automation.
Progress0 / 4 steps
1
Create the MotorControl function block
Write a function block called MotorControl with inputs Start and Stop of type BOOL and an output MotorOn of type BOOL.
PLC Programming
Hint

Define the function block with inputs and outputs exactly as described.

2
Add motor control logic inside MotorControl
Inside the MotorControl function block, write code to set MotorOn to TRUE when Start is TRUE, and set MotorOn to FALSE when Stop is TRUE.
PLC Programming
Hint

Use an IF statement to check Start and Stop inputs and set MotorOn accordingly.

3
Instantiate MotorControl in main program
In the main program, declare a variable motor1 of type MotorControl. Then set motor1.Start to TRUE and motor1.Stop to FALSE to simulate pressing the start button.
PLC Programming
Hint

Declare motor1 as MotorControl and assign Start and Stop values.

4
Display the motor state
Add code in the main program to call the motor1 function block logic and then print the value of motor1.MotorOn to show if the motor is ON or OFF.
PLC Programming
Hint

Call the function block instance as motor1(); and then access motor1.MotorOn.

Practice

(1/5)
1. What is the main benefit of using FUNCTION_BLOCK in PLC programming?
easy
A. It creates a graphical interface for the PLC.
B. It makes the program run faster by using less memory.
C. It automatically fixes errors in the code.
D. It allows you to reuse code and simplify program management.

Solution

  1. Step 1: Understand the purpose of FUNCTION_BLOCK

    FUNCTION_BLOCK is designed to package code that can be reused multiple times in a PLC program.

  2. Step 2: Identify the main benefit

    Reusing code saves time and makes the program easier to manage and maintain.

  3. Final Answer:

    It allows you to reuse code and simplify program management. -> Option D

  4. Quick Check:

    Reusable code = Simplify management [OK]
Hint: FUNCTION_BLOCK means reusable code blocks [OK]
Common Mistakes:
  • Thinking FUNCTION_BLOCK speeds up execution
  • Believing it fixes errors automatically
  • Confusing it with user interface creation
2. Which of the following is the correct way to declare a FUNCTION_BLOCK named Counter in PLC programming?
easy
A. FUNCTION Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION
B. FUNCTION_BLOCK Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK
C. FUNCTION_BLOCK Counter() VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK
D. FUNCTION_BLOCK Counter VAR_OUTPUT Count: INT; END_VAR END_FUNCTION_BLOCK

Solution

  1. Step 1: Identify correct FUNCTION_BLOCK syntax

    FUNCTION_BLOCK declaration starts with 'FUNCTION_BLOCK Name' and ends with 'END_FUNCTION_BLOCK'. Inputs are declared inside VAR_INPUT.

  2. Step 2: Check each option

    FUNCTION_BLOCK Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK correctly declares FUNCTION_BLOCK Counter with VAR_INPUT and ends properly. FUNCTION Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION uses FUNCTION instead of FUNCTION_BLOCK. FUNCTION_BLOCK Counter() VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK adds parentheses which are not used. FUNCTION_BLOCK Counter VAR_OUTPUT Count: INT; END_VAR END_FUNCTION_BLOCK uses VAR_OUTPUT instead of VAR_INPUT for input variable.

  3. Final Answer:

    FUNCTION_BLOCK Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK -> Option B

  4. Quick Check:

    Correct FUNCTION_BLOCK syntax = FUNCTION_BLOCK Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK [OK]
Hint: FUNCTION_BLOCK uses VAR_INPUT for inputs, no parentheses [OK]
Common Mistakes:
  • Using FUNCTION instead of FUNCTION_BLOCK
  • Adding parentheses after FUNCTION_BLOCK name
  • Declaring inputs under VAR_OUTPUT
3. Given this FUNCTION_BLOCK code snippet, what will be the output value of Result after execution?
FUNCTION_BLOCK AddTwo
VAR_INPUT
A: INT;
B: INT;
END_VAR
VAR_OUTPUT
Result: INT;
END_VAR
Result := A + B;
END_FUNCTION_BLOCK

// Usage
AddTwoInstance: AddTwo;
AddTwoInstance.A := 3;
AddTwoInstance.B := 5;
// Execute block
// What is AddTwoInstance.Result?
medium
A. 8
B. 35
C. 0
D. Error: Result not assigned

Solution

  1. Step 1: Understand the FUNCTION_BLOCK logic

    The block adds inputs A and B and stores the sum in Result.

  2. Step 2: Substitute input values and calculate

    A = 3, B = 5, so Result = 3 + 5 = 8.

  3. Final Answer:

    8 -> Option A

  4. Quick Check:

    3 + 5 = 8 [OK]
Hint: Add inputs to get output Result [OK]
Common Mistakes:
  • Concatenating numbers as strings
  • Assuming Result is zero by default
  • Thinking Result is not assigned automatically
4. Identify the error in this FUNCTION_BLOCK code snippet:
FUNCTION_BLOCK Multiply
VAR_INPUT
X: INT;
Y: INT;
END_VAR
VAR_OUTPUT
Product: INT;
END_VAR
Product = X * Y;
END_FUNCTION_BLOCK
medium
A. Incorrect use of '=' instead of ':=' for assignment.
B. VAR_OUTPUT should be declared before VAR_INPUT.
C. Missing semicolon at the end of the assignment line.
D. FUNCTION_BLOCK name must be lowercase.

Solution

  1. Step 1: Check assignment syntax in FUNCTION_BLOCK

    In PLC programming, assignment uses ':=' not '='.

  2. Step 2: Verify other syntax elements

    Semicolon is present. VAR_INPUT before VAR_OUTPUT is correct. FUNCTION_BLOCK names are case-sensitive but uppercase is allowed.

  3. Final Answer:

    Incorrect use of '=' instead of ':=' for assignment. -> Option A

  4. Quick Check:

    Use ':=' for assignment [OK]
Hint: Use ':=' for assignments in PLC code [OK]
Common Mistakes:
  • Using '=' instead of ':='
  • Thinking semicolon is optional
  • Believing VAR_OUTPUT must come first
5. You want to create a reusable FUNCTION_BLOCK that counts how many times an input boolean Trigger changes from FALSE to TRUE. Which approach correctly implements this behavior?
hard
A. Reset the counter to zero every time Trigger is TRUE.
B. Increment the counter every time the FUNCTION_BLOCK runs regardless of Trigger state.
C. Use a VAR to store previous Trigger state and increment a counter only when Trigger changes from FALSE to TRUE.
D. Use VAR_INPUT to store the counter and increment it inside the block.

Solution

  1. Step 1: Understand the counting condition

    The counter should increase only when Trigger changes from FALSE to TRUE (rising edge).

  2. Step 2: Implement edge detection logic

    Store previous Trigger state in a VAR variable. Compare current and previous states. Increment counter only if previous was FALSE and current is TRUE.

  3. Final Answer:

    Use a VAR to store previous Trigger state and increment a counter only when Trigger changes from FALSE to TRUE. -> Option C

  4. Quick Check:

    Edge detection needs previous state storage [OK]
Hint: Store previous input to detect rising edge [OK]
Common Mistakes:
  • Incrementing counter every cycle
  • Resetting counter incorrectly
  • Using VAR_INPUT for internal state
Recommended

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10 questions~5 mineasy hard