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Function block definition in PLC Programming - Time & Space Complexity

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Time Complexity: Function block definition
O(n)
Understanding Time Complexity

When we define a function block in PLC programming, it is important to understand how the time it takes to run grows as the inputs or internal operations increase.

We want to know how the execution time changes when the function block is used with different input sizes or repeated operations.

Scenario Under Consideration

Analyze the time complexity of the following function block definition.

FUNCTION_BLOCK FB_SumArray
VAR_INPUT
  n : INT;
  arr : ARRAY[1..100] OF INT; (* Fixed size array for PLC compatibility *)
END_VAR
VAR_OUTPUT
  total : INT;
END_VAR
VAR
  i : INT;
END_VAR

METHOD Calculate : VOID
total := 0;
FOR i := 1 TO n DO
  total := total + arr[i];
END_FOR
END_METHOD
END_FUNCTION_BLOCK

This function block sums all elements of an input array and stores the result in an output variable.

Identify Repeating Operations
  • Primary operation: The FOR loop that adds each element of the array to the total.
  • How many times: The loop runs once for each element in the array, so n times.
How Execution Grows With Input

As the size of the input array grows, the number of additions grows at the same rate.

Input Size (n)Approx. Operations
1010 additions
100100 additions
10001000 additions

Pattern observation: The number of operations grows directly with the input size.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the function block grows in a straight line as the input array gets bigger.

Common Mistake

[X] Wrong: "The function block runs in constant time because it just sums values once."

[OK] Correct: Even though it looks simple, the function must add each element, so the time depends on how many elements there are.

Interview Connect

Understanding how loops inside function blocks affect execution time helps you explain how your code scales and performs in real automation tasks.

Self-Check

"What if the function block called itself recursively for each element? How would the time complexity change?"

Practice

(1/5)
1. What is the main purpose of a Function Block in PLC programming?
easy
A. To group inputs, outputs, and code for repeated tasks
B. To store data permanently in the PLC
C. To create a graphical user interface
D. To connect the PLC to external devices

Solution

  1. Step 1: Understand the role of function blocks

    Function blocks group inputs, outputs, and code to perform tasks repeatedly in PLC programs.

  2. Step 2: Compare options with the definition

    Only To group inputs, outputs, and code for repeated tasks describes grouping inputs, outputs, and code for reuse, which matches the function block purpose.

  3. Final Answer:

    To group inputs, outputs, and code for repeated tasks -> Option A

  4. Quick Check:

    Function block purpose = Group inputs, outputs, and code [OK]
Hint: Function blocks bundle inputs, outputs, and code for reuse [OK]
Common Mistakes:
  • Confusing function blocks with data storage
  • Thinking function blocks create user interfaces
  • Assuming function blocks connect hardware directly
2. Which of the following is the correct way to start defining a function block named MotorControl in IEC 61131-3 structured text?
easy
A. PROGRAM MotorControl
B. FUNCTION MotorControl
C. VAR MotorControl
D. FUNCTION_BLOCK MotorControl

Solution

  1. Step 1: Recall syntax for function block definition

    In IEC 61131-3, function blocks start with the keyword FUNCTION_BLOCK followed by the name.

  2. Step 2: Check each option

    FUNCTION_BLOCK MotorControl uses FUNCTION_BLOCK MotorControl, which is correct. Options A, B, and D use other keywords for different constructs.

  3. Final Answer:

    FUNCTION_BLOCK MotorControl -> Option D

  4. Quick Check:

    Function block syntax = FUNCTION_BLOCK Name [OK]
Hint: Function blocks start with FUNCTION_BLOCK keyword [OK]
Common Mistakes:
  • Using FUNCTION instead of FUNCTION_BLOCK
  • Confusing PROGRAM with function block
  • Using VAR keyword to define function blocks
3. Given this function block code snippet, what will be the output of FB1.Output after calling FB1.Run() twice?
FUNCTION_BLOCK FB1
VAR_INPUT
  In: BOOL;
END_VAR
VAR_OUTPUT
  Output: INT;
END_VAR
VAR
  Counter: INT := 0;
END_VAR

METHOD PUBLIC Run
IF In THEN
  Counter := Counter + 1;
END_IF
Output := Counter;
END_METHOD

END_FUNCTION_BLOCK

// Usage:
FB1.In := TRUE;
FB1.Run();
FB1.Run();
medium
A. 2
B. 1
C. 0
D. TRUE

Solution

  1. Step 1: Understand the counter increment logic

    Each call to Run checks if In is TRUE, then increments Counter by 1.

  2. Step 2: Trace two calls with In = TRUE

    First call increments Counter from 0 to 1, second call increments from 1 to 2. Output is set to Counter.

  3. Final Answer:

    2 -> Option A

  4. Quick Check:

    Counter increments twice with In=TRUE = 2 [OK]
Hint: Count increments each Run call if input TRUE [OK]
Common Mistakes:
  • Assuming Output resets each call
  • Confusing BOOL and INT output types
  • Forgetting Counter is retained between calls
4. Identify the error in this function block definition:
FUNCTION_BLOCK FB_Error
VAR_INPUT
  Start: BOOL;
END_VAR
VAR_OUTPUT
  Status: BOOL;
END_VAR
VAR
  Count: INT;
END_VAR

METHOD Run
IF Start THEN
  Count := Count + 1
  Status := TRUE;
END_IF
END_METHOD
END_FUNCTION_BLOCK
medium
A. Function block name cannot contain underscore
B. Missing semicolon after Count := Count + 1
C. Output variable Status should be input
D. Incorrect keyword METHOD used

Solution

  1. Step 1: Check syntax inside the method

    The line Count := Count + 1 is missing a semicolon at the end, which is required in structured text.

  2. Step 2: Verify other parts

    METHOD keyword is valid, output variable can be named Status, and underscores are allowed in names.

  3. Final Answer:

    Missing semicolon after Count := Count + 1 -> Option B

  4. Quick Check:

    Semicolon missing after assignment line [OK]
Hint: Check each line ends with semicolon in structured text [OK]
Common Mistakes:
  • Ignoring missing semicolons
  • Thinking METHOD keyword is invalid
  • Assuming variable naming rules are stricter
5. You want to create a function block TimerFB that counts seconds when Enable input is TRUE and resets when Reset input is TRUE. Which internal variable type should you use to store the count so it keeps its value between calls?
hard
A. VAR_INPUT
B. VAR_OUTPUT
C. VAR (internal variable)
D. VAR_TEMP

Solution

  1. Step 1: Understand variable types in function blocks

    VAR_INPUT and VAR_OUTPUT are for inputs and outputs, not for storing internal state. VAR_TEMP is temporary and does not keep value between calls.

  2. Step 2: Identify variable type that retains value

    VAR variables inside function blocks keep their value between calls, acting like memory.

  3. Final Answer:

    VAR (internal variable) -> Option C

  4. Quick Check:

    Use VAR for internal memory in function blocks [OK]
Hint: Use VAR for internal memory that persists between calls [OK]
Common Mistakes:
  • Using VAR_TEMP which resets each call
  • Confusing inputs/outputs with internal storage
  • Assuming VAR_INPUT can store internal state
Recommended

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