PLC programming languages overview (IEC 61131-3) - Time & Space Complexity

Time Complexity: PLC programming languages overview (IEC 61131-3)

O(n)

Understanding Time Complexity

When working with PLC programming languages, it is important to understand how the time to run a program grows as the program or input size increases.

We want to know how the execution time changes when the program uses different language features or structures.

Scenario Under Consideration

Analyze the time complexity of this simple PLC program using Structured Text.

PROGRAM Example
VAR
  i : INT;
  sum : INT := 0;
END_VAR

FOR i := 1 TO 100 DO
  sum := sum + i;
END_FOR
END_PROGRAM

This program sums numbers from 1 to 100 using a loop.

Identify Repeating Operations

Look for loops or repeated steps.

• Primary operation: The FOR loop that adds numbers.
• How many times: Exactly 100 times in this example.

How Execution Grows With Input

The number of additions grows as the loop count grows.

Input Size (n)	Approx. Operations
10	10 additions
100	100 additions
1000	1000 additions

As the input number increases, the operations increase in a straight line.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows directly with the size of the input loop count.

Common Mistake

[X] Wrong: "The program runs in constant time because the loop count is fixed at 100."

[OK] Correct: If the loop count changes with input size, the time grows with it, so it is not constant time.

Interview Connect

Understanding how loops and program structure affect execution time helps you write efficient PLC code and explain your reasoning clearly in discussions.

Self-Check

What if the loop was nested inside another loop of the same size? How would the time complexity change?

Practice

1. Which of the following is NOT one of the five main PLC programming languages defined by IEC 61131-3?

easy

A. Function Block Diagram

B. Python

C. Structured Text

D. Ladder Diagram

Solution

1. Step 1: Recall IEC 61131-3 languages

   IEC 61131-3 defines Ladder Diagram, Function Block Diagram, Structured Text, Instruction List, and Sequential Function Chart.

2. Step 2: Identify the language not listed

   Python is a general-purpose programming language, not part of IEC 61131-3.

3. Final Answer:

   Python -> Option B

4. Quick Check:

   IEC 61131-3 excludes Python [OK]

Hint: Remember IEC 61131-3 languages exclude general programming languages [OK]

Common Mistakes:

• Confusing general programming languages with PLC languages
• Assuming all text-based languages are included
• Mixing up graphical and text-based languages

2. Which syntax correctly represents a simple assignment in Structured Text (ST)?

easy

A. Output := Input + 5;

B. Output = Input + 5

C. Output == Input + 5;

D. Output <- Input + 5;

Solution

1. Step 1: Understand Structured Text assignment

   In ST, the assignment operator is ':=' and statements end with a semicolon.

2. Step 2: Check each option

   Output := Input + 5; uses ':=' and ends with ';' correctly. Others use wrong operators or miss semicolons.

3. Final Answer:

   Output := Input + 5; -> Option A

4. Quick Check:

   ST assignment uses ':=' and ';' [OK]

Hint: Look for ':=' operator and semicolon in Structured Text [OK]

Common Mistakes:

• Using '=' instead of ':=' for assignment
• Omitting semicolon at statement end
• Confusing equality '==' with assignment

3. Given the following Ladder Diagram logic:
--[ ]--( )--
where the input contact is normally open and connected to output coil, what is the output when the input is OFF?

medium

A. Output is undefined

B. Output is ON

C. Output toggles ON and OFF

D. Output is OFF

Solution

1. Step 1: Understand Ladder Diagram symbols

   The normally open contact [ ] closes when input is ON, allowing current to flow to the coil ( ).

2. Step 2: Analyze input OFF condition

   When input is OFF, the contact remains open, so no current flows to the coil, output stays OFF.

3. Final Answer:

   Output is OFF -> Option D

4. Quick Check:

   Open contact means no output current [OK]

Hint: Open contact means output coil is OFF [OK]

Common Mistakes:

• Assuming output is ON when input is OFF
• Confusing normally open with normally closed contacts
• Thinking output toggles without input change

4. Identify the error in this Structured Text snippet:

IF Input = TRUE THEN
  Output := TRUE
END_IF

medium

A. Missing semicolon after Output assignment

B. Incorrect IF syntax, should be IF (Input = TRUE) THEN

C. Assignment operator should be '=' not ':='

D. END_IF should be lowercase

Solution

1. Step 1: Check Structured Text syntax rules

   Each statement must end with a semicolon ';' in ST.

2. Step 2: Inspect the code snippet

   The line 'Output := TRUE' lacks a semicolon at the end, causing a syntax error.

3. Final Answer:

   Missing semicolon after Output assignment -> Option A

4. Quick Check:

   Statements end with ';' in ST [OK]

Hint: Always end ST statements with semicolon [OK]

Common Mistakes:

• Forgetting semicolons after assignments
• Misusing '=' for assignment
• Assuming IF needs parentheses

5. You need to implement a control logic that turns ON an output only if two inputs are ON simultaneously, using Function Block Diagram (FBD). Which block combination correctly represents this?

hard

A. Two OR blocks connected in parallel

B. Two AND blocks connected in series

C. One AND block with both inputs connected to it

D. One NOT block with inputs connected

Solution

1. Step 1: Understand AND logic in FBD

   AND block outputs TRUE only if all inputs are TRUE simultaneously.

2. Step 2: Choose correct block setup

   One AND block with both inputs connected directly checks if both inputs are ON together.

3. Final Answer:

   One AND block with both inputs connected to it -> Option C

4. Quick Check:

   AND block with multiple inputs = simultaneous ON check [OK]

Hint: Use single AND block with all inputs for simultaneous ON [OK]

Common Mistakes:

• Using OR blocks instead of AND
• Connecting AND blocks in series unnecessarily
• Using NOT block incorrectly for AND logic