Function block instances in PLC Programming - Time & Space Complexity

Time Complexity: Function block instances

O(n)

Understanding Time Complexity

When using function block instances in PLC programming, it's important to know how the program's running time changes as you add more instances.

We want to see how the number of function block instances affects the total execution time.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

FUNCTION_BLOCK Counter
  VAR
    count : INT := 0;
  END_VAR
  count := count + 1;
END_FUNCTION_BLOCK

PROGRAM Main
  VAR
    fb1 : Counter;
    fb2 : Counter;
    fb3 : Counter;
  END_VAR
  fb1();
  fb2();
  fb3();
END_PROGRAM

This code defines a simple counter function block and creates three instances that each increment their own count.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Each function block instance runs its internal code once per cycle.
• How many times: The number of times equals the number of function block instances.

How Execution Grows With Input

As you add more function block instances, the total execution time grows in direct proportion.

Input Size (n)	Approx. Operations
3	3 executions of the block code
10	10 executions of the block code
100	100 executions of the block code

Pattern observation: Doubling the number of instances doubles the total work done.

Final Time Complexity

Time Complexity: O(n)

This means the total execution time grows linearly with the number of function block instances.

Common Mistake

[X] Wrong: "Adding more function block instances does not affect execution time much because each block is small."

[OK] Correct: Even small blocks add up when repeated many times, so total execution time increases with the number of instances.

Interview Connect

Understanding how function block instances affect execution time helps you design efficient PLC programs and explain your reasoning clearly in discussions.

Self-Check

"What if the function block contained a loop that runs based on an input parameter? How would that change the time complexity?"

Practice

1. What is the main purpose of using function block instances in PLC programming?

easy

A. To reuse code with separate memory for each instance

B. To write code that runs only once

C. To combine multiple programs into one file

D. To create global variables accessible everywhere

Solution

1. Step 1: Understand function block instances

   Function block instances allow you to reuse the same code but keep data separate for each instance.

2. Step 2: Compare options

   Only To reuse code with separate memory for each instance correctly describes this purpose. Other options describe unrelated concepts.

3. Final Answer:

   To reuse code with separate memory for each instance -> Option A

4. Quick Check:

   Function block instances = separate memory reuse [OK]

Hint: Think: same code, different data for each instance [OK]

Common Mistakes:

• Confusing instances with global variables
• Thinking instances run only once
• Mixing instances with program files

2. Which of the following is the correct syntax to declare two separate instances of a function block named TimerFB in IEC 61131-3 style?

easy

A. Timer1 -> TimerFB; Timer2 -> TimerFB;

B. Timer1 : TimerFB; Timer2 : TimerFB;

C. Timer1 = TimerFB(); Timer2 = TimerFB();

D. Timer1 : TimerFB() Timer2 : TimerFB()

Solution

1. Step 1: Recall IEC 61131-3 instance declaration

   Instances are declared with name : FunctionBlockName; syntax.

2. Step 2: Check options

   Timer1 : TimerFB; Timer2 : TimerFB; uses correct syntax with semicolons. Others use invalid or incorrect syntax.

3. Final Answer:

   Timer1 : TimerFB; Timer2 : TimerFB; -> Option B

4. Quick Check:

   Instance declaration uses colon and semicolon [OK]

Hint: Use colon and semicolon to declare instances [OK]

Common Mistakes:

• Using equal sign instead of colon
• Omitting semicolons
• Adding parentheses after function block name

3. Given the following code snippet in a PLC program:

Timer1 : TimerFB;
Timer2 : TimerFB;

Timer1(IN := TRUE);
Timer2(IN := FALSE);

// After one cycle, what is the state of Timer1.Q and Timer2.Q?

medium

A. Timer1.Q = TRUE, Timer2.Q = FALSE

B. Timer1.Q = FALSE, Timer2.Q = TRUE

C. Timer1.Q = TRUE, Timer2.Q = TRUE

D. Timer1.Q = FALSE, Timer2.Q = FALSE

Solution

1. Step 1: Understand instance independence

   Timer1 and Timer2 are separate instances with their own inputs and outputs.

2. Step 2: Analyze input signals

   Timer1.IN is TRUE, so Timer1.Q will become TRUE after timing. Timer2.IN is FALSE, so Timer2.Q remains FALSE.

3. Final Answer:

   Timer1.Q = TRUE, Timer2.Q = FALSE -> Option A

4. Quick Check:

   Separate instances keep outputs independent [OK]

Hint: Each instance acts independently with its own inputs [OK]

Common Mistakes:

• Assuming outputs share state between instances
• Confusing input signals for both instances
• Ignoring instance separation

4. Identify the error in this function block instance declaration:

Timer1 : TimerFB
Timer2 : TimerFB;

medium

A. Incorrect function block name

B. Extra semicolon after Timer2 declaration

C. Using colon instead of equal sign

D. Missing semicolon after Timer1 declaration

Solution

1. Step 1: Check syntax for instance declarations

   Each instance declaration must end with a semicolon.

2. Step 2: Locate the missing semicolon

   Timer1 declaration lacks a semicolon at the end, causing syntax error.

3. Final Answer:

   Missing semicolon after Timer1 declaration -> Option D

4. Quick Check:

   Each declaration ends with semicolon [OK]

Hint: Always end instance declarations with semicolon [OK]

Common Mistakes:

• Omitting semicolon after each instance
• Confusing colon with equal sign
• Assuming semicolon is optional

5. You want to control three independent motors using the same function block MotorControlFB. Which approach correctly uses function block instances to achieve this?

hard

A. Create three copies of MotorControlFB code with different names

B. Declare one instance MotorControlFB and call it three times with different inputs

C. Declare three instances: Motor1, Motor2, Motor3 of MotorControlFB and call each separately

D. Use global variables inside MotorControlFB to handle all three motors

Solution

1. Step 1: Understand instance independence for multiple devices

   Each motor needs its own instance to keep control data separate.

2. Step 2: Evaluate options

   Declare three instances: Motor1, Motor2, Motor3 of MotorControlFB and call each separately declares three separate instances, allowing independent control. Declare one instance MotorControlFB and call it three times with different inputs reuses one instance incorrectly. Use global variables inside MotorControlFB to handle all three motors uses globals, which mixes data. Create three copies of MotorControlFB code with different names duplicates code, which is inefficient.

3. Final Answer:

   Declare three instances: Motor1, Motor2, Motor3 of MotorControlFB and call each separately -> Option C

4. Quick Check:

   Multiple devices = multiple instances [OK]

Hint: One instance per device keeps data separate [OK]

Common Mistakes:

• Trying to reuse one instance for multiple devices
• Using global variables inside function block
• Copy-pasting code instead of using instances