Reusable code patterns in PLC Programming - Deep Dive

Overview - Reusable code patterns

What is it?

Reusable code patterns in PLC programming are pre-designed blocks or templates of code that can be used multiple times across different parts of a program or in different projects. They help automate common tasks or control logic without rewriting the same code again. This saves time and reduces errors by using tested and reliable code pieces. Think of them as building blocks that you can combine to create complex machine controls.

Why it matters

Without reusable code patterns, every new machine or process would require writing all control logic from scratch, which is slow and error-prone. Reusable patterns make programming faster, more consistent, and easier to maintain. They also help teams share best practices and improve safety by using proven code. This leads to more reliable machines and less downtime in factories.

Where it fits

Before learning reusable code patterns, you should understand basic PLC programming concepts like ladder logic, function blocks, and simple control structures. After mastering reusable patterns, you can explore advanced topics like modular programming, object-oriented PLC programming, and system integration for large automation projects.

Mental Model

Core Idea

Reusable code patterns are like ready-made puzzle pieces that fit into many different control problems, saving time and ensuring reliability.

Think of it like...

Imagine cooking with a set of pre-made sauces and spice mixes instead of making each flavor from scratch every time. These mixes save effort and keep the taste consistent across dishes.

┌─────────────────────────────┐
│ Reusable Code Pattern Block │
├─────────────┬───────────────┤
│ Input       │ Output        │
├─────────────┼───────────────┤
│ SensorData  │ MotorControl  │
│ TimerValue  │ AlarmSignal   │
└─────────────┴───────────────┘
        ▲                 ▲
        │                 │
   Used in multiple    Used in multiple
   parts of program    projects or machines

Build-Up - 7 Steps

1

Foundation

Understanding basic PLC code blocks

Concept: Learn what a code block is and how it groups instructions in PLC programming.

In PLC programming, a code block is a set of instructions grouped together to perform a specific task. Common types include functions, function blocks, and programs. For example, a function block might control a motor start/stop sequence. These blocks can be called or used multiple times in a program.

Result

You can identify and create simple code blocks that perform specific control tasks.

Understanding code blocks is essential because reusable patterns are built by combining and reusing these blocks.

2

Foundation

Basics of modular programming in PLCs

Concept: Learn how to divide a program into smaller, manageable parts for reuse.

Modular programming means breaking a large control program into smaller modules or blocks. Each module handles one part of the machine or process. For example, one module might handle conveyor control, another handles sensors. This makes the program easier to understand and reuse.

Result

You can organize PLC code into modules that can be reused or updated independently.

Modularity is the foundation for creating reusable code patterns that simplify complex automation tasks.

3

Intermediate

Creating parameterized function blocks

🤔Before reading on: do you think function blocks can work with fixed inputs only, or can they accept different inputs each time? Commit to your answer.

Concept: Learn how to design function blocks that accept parameters to work flexibly in different situations.

Function blocks can be designed to accept inputs called parameters. These parameters allow the same block to behave differently depending on the input values. For example, a timer block can be reused with different time delays by passing different parameters.

Result

You can create flexible function blocks that adapt to different control needs without rewriting code.

Using parameters in function blocks unlocks the power of reuse by making code adaptable to many scenarios.

4

Intermediate

Using libraries for reusable code

🤔Before reading on: do you think libraries are just collections of code, or do they also help with sharing and updating code? Commit to your answer.

Concept: Learn how to use and create libraries to store reusable code blocks for easy sharing and maintenance.

Libraries are collections of pre-made function blocks, functions, or data types that can be imported into PLC projects. They help programmers reuse tested code easily and keep it updated in one place. For example, a motor control library might include blocks for start, stop, and fault handling.

Result

You can import and use libraries to speed up programming and ensure consistency across projects.

Libraries centralize reusable code, making it easier to maintain and share best practices across teams.

5

Intermediate

Implementing state machines as reusable patterns

🤔Before reading on: do you think state machines are only for complex systems, or can they simplify common control tasks? Commit to your answer.

Concept: Learn how to use state machines to model control logic that changes states, making it reusable and clear.

A state machine is a pattern where a system moves between defined states based on inputs or events. For example, a conveyor might have states like 'Stopped', 'Running', and 'Fault'. Implementing state machines as reusable blocks helps manage complex logic clearly and consistently.

Result

You can design reusable state machine blocks that handle control sequences reliably.

State machines provide a clear structure for control logic, making reuse safer and easier to understand.

6

Advanced

Parameterizing reusable patterns with data structures

🤔Before reading on: do you think simple variables are enough for reusable patterns, or do complex data structures improve flexibility? Commit to your answer.

Concept: Learn how to use structured data types to pass complex information into reusable code blocks.

Using data structures like arrays or user-defined types allows passing multiple related values into a function block at once. For example, a motor control block might receive a structure with speed, direction, and acceleration parameters. This makes reusable patterns more powerful and easier to manage.

Result

You can create reusable blocks that handle complex inputs cleanly and flexibly.

Structured data inputs reduce errors and improve clarity when reusing complex control logic.

7

Expert

Advanced reuse with generic and polymorphic blocks

🤔Before reading on: do you think PLC programming supports generic code blocks that work with any data type, or is it always fixed? Commit to your answer.

Concept: Explore how modern PLC languages support generic and polymorphic function blocks for maximum reuse.

Some modern PLC programming languages allow creating generic function blocks that can work with different data types without rewriting code. Polymorphism lets blocks behave differently based on input types. This advanced reuse reduces code duplication and increases flexibility in large projects.

Result

You can write highly flexible reusable blocks that adapt to many data types and situations.

Understanding generics and polymorphism in PLCs unlocks expert-level reuse and future-proofs your code.

Under the Hood

Reusable code patterns work by encapsulating control logic into discrete blocks that can be called multiple times with different inputs. Internally, the PLC runtime manages memory and execution context for each block instance, allowing independent operation. Parameters and data structures are passed by value or reference depending on the language, enabling flexible behavior. Libraries store compiled blocks for easy import and linking during program download.

Why designed this way?

PLC programming evolved from simple relay logic to complex automation requiring modularity and reuse. Early PLCs had limited memory and processing, so reusable blocks saved space and development time. Standardizing code blocks and libraries improved reliability and maintainability. Alternatives like monolithic code were error-prone and hard to update, so modular reusable patterns became the industry standard.

┌───────────────┐       ┌───────────────┐
│ PLC Program   │──────▶│ Function Block│
│ (Main Logic)  │       │ Instance 1    │
└───────────────┘       ├───────────────┤
                        │ Parameters   │
                        │ Local Memory │
                        └───────────────┘
                             ▲
                             │
                        ┌───────────────┐
                        │ Function Block│
                        │ Instance 2    │
                        └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do you think reusable code blocks always run faster than custom code? Commit to yes or no.

Common Belief: Reusable code blocks always improve performance because they are optimized.

Reality: Reusable blocks improve development speed and reliability but may not always be faster at runtime due to added abstraction.

Why it matters: Assuming reusable blocks are always faster can lead to ignoring performance bottlenecks in critical control paths.

Quick: Do you think you can reuse code blocks without understanding their internal logic? Commit to yes or no.

Common Belief: You can safely reuse code blocks without fully understanding how they work inside.

Reality: Reusing code without understanding can cause unexpected behavior or errors if the block assumptions don't match your application.

Why it matters: Blind reuse risks machine faults or safety issues if the block is misapplied.

Quick: Do you think all PLC languages support generic reusable blocks? Commit to yes or no.

Common Belief: All PLC programming languages support generic and polymorphic reusable blocks.

Reality: Only some modern PLC languages support generics; many older or simpler languages do not.

Why it matters: Expecting generics where unsupported can cause design confusion and limit reuse options.

Quick: Do you think reusable code patterns eliminate the need for testing? Commit to yes or no.

Common Belief: Once a reusable code block is tested, it never needs testing again.

Reality: Reusable blocks must be tested in each new context because inputs and environment can differ.

Why it matters: Skipping tests on reused code can lead to hidden bugs and unsafe machine operation.

Expert Zone

1

Reusable code blocks often require careful management of internal state to avoid unintended interactions when used multiple times.

2

Parameter defaults and overloading in function blocks can simplify reuse but may introduce subtle bugs if not documented clearly.

3

Using polymorphism in PLCs can complicate debugging because behavior changes based on data types, requiring advanced diagnostic tools.

When NOT to use

Avoid reusable code patterns when the control logic is extremely simple or unique to one machine, as the overhead of abstraction may add unnecessary complexity. In such cases, direct coding is clearer and faster. Also, for safety-critical code with strict certification requirements, custom verified code may be preferred over generic reusable blocks.

Production Patterns

In industry, reusable code patterns are used to build standard libraries for common equipment like motors, valves, and sensors. Teams create certified blocks for safety functions and diagnostics. Large projects use modular architectures where reusable blocks are combined into hierarchical systems, enabling parallel development and easier maintenance.

Connections

Software Design Patterns

Reusable code patterns in PLCs build on the same principles as software design patterns in general programming.

Understanding software design patterns helps PLC programmers create more flexible and maintainable reusable blocks.

Manufacturing Process Optimization

Reusable code patterns enable faster automation changes, which supports continuous improvement in manufacturing processes.

Knowing how reusable code accelerates automation helps connect programming skills to real-world factory efficiency gains.

Modular Furniture Assembly

Both involve building complex systems from standard, reusable parts that fit together easily.

Seeing reusable code as modular furniture helps appreciate the value of standardization and interchangeability in engineering.

Common Pitfalls

#1 Copy-pasting code instead of using reusable blocks

Wrong approach:

LD rung 1: Motor start logic
LD rung 2: Motor start logic (copied and pasted with minor changes)

Correct approach:

Create a function block 'MotorStart' and call it with different parameters where needed.

Root cause:

Lack of understanding of modularity and reuse leads to duplicated code that is hard to maintain.

#2 Using global variables inside reusable blocks

Wrong approach:

Function block uses global flags and timers directly inside its logic.

Correct approach:

Pass all needed data as parameters to the function block to keep it self-contained.

Root cause:

Misunderstanding of encapsulation causes blocks to depend on external state, reducing reuse safety.

#3 Ignoring parameter validation in reusable blocks

Wrong approach:

Function block assumes input parameters are always valid and does not check ranges.

Correct approach:

Add checks inside the block to validate parameters and handle errors gracefully.

Root cause:

Overconfidence in input correctness leads to runtime errors and unpredictable behavior.

Key Takeaways

Reusable code patterns save time and reduce errors by letting you use tested control logic multiple times.

Designing flexible function blocks with parameters and data structures is key to effective reuse.

Libraries and modular programming organize reusable code for easy sharing and maintenance.

Advanced reuse techniques like generics and polymorphism increase flexibility but require deeper understanding.

Always understand and test reusable blocks in your specific context to avoid hidden bugs and safety risks.

Practice

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