Function block instances in PLC Programming - Deep Dive

Overview - Function block instances

What is it?

Function block instances are like reusable mini-programs in PLC programming that you can create once and use many times. Each instance is a separate copy of the function block with its own data and behavior. This lets you control multiple devices or processes independently using the same logic. Think of it as having multiple identical machines, each working on its own task.

Why it matters

Without function block instances, you would have to write the same control logic over and over for each device or process, which is time-consuming and error-prone. Instances let you save time, reduce mistakes, and keep your program organized. They make complex automation systems manageable and scalable, so you can add or change parts without rewriting everything.

Where it fits

Before learning function block instances, you should understand basic PLC programming concepts like variables, functions, and function blocks themselves. After mastering instances, you can explore advanced topics like data encapsulation, state machines, and modular program design to build robust automation systems.

Mental Model

Core Idea

A function block instance is a unique copy of a reusable control unit that keeps its own data and runs independently within a PLC program.

Think of it like...

Imagine a cookie cutter (function block) that shapes dough into cookies. Each cookie (instance) looks the same but is separate and can be decorated differently or eaten at different times.

┌─────────────────────┐
│ Function Block Code  │
│  (Reusable Logic)    │
└─────────┬───────────┘
          │
  ┌───────┴────────┐  ┌───────┴────────┐  ┌───────┴────────┐
  │ Instance #1     │  │ Instance #2     │  │ Instance #3     │
  │ Own Data & State│  │ Own Data & State│  │ Own Data & State│
  └─────────────────┘  └─────────────────┘  └─────────────────┘

Build-Up - 7 Steps

1

Foundation

What is a Function Block

Concept: Introduce the basic idea of a function block as a reusable piece of code with inputs, outputs, and internal memory.

A function block is like a small machine inside your PLC program. It has inputs where you give it signals, outputs where it sends results, and internal memory to remember things between runs. You write the logic once, and then you can use it many times.

Result

You understand that a function block is a reusable logic unit that can be used multiple times in a program.

Understanding function blocks as reusable machines helps you see why instances are needed to run multiple copies independently.

2

Foundation

Difference Between Function and Function Block

Concept: Explain how functions differ from function blocks by having no internal memory and no instances.

Functions take inputs and return outputs but do not remember anything between calls. Function blocks, however, keep their own data inside. This means function blocks can keep track of states, timers, or counters, which functions cannot.

Result

You can distinguish when to use a function versus a function block based on whether you need memory or state.

Knowing this difference clarifies why function blocks need instances to keep separate memories.

3

Intermediate

Creating Multiple Instances

🤔Before reading on: do you think multiple instances share the same data or have separate data? Commit to your answer.

Concept: Show how to declare and use multiple instances of the same function block with independent data.

In your PLC program, you declare instances by giving each a unique name. For example: MyTimer1 : TON; MyTimer2 : TON; Each instance MyTimer1 and MyTimer2 runs the same timer logic but counts independently. You call them separately in your code to control different parts of your machine.

Result

Each instance runs independently, so starting MyTimer1 does not affect MyTimer2.

Understanding that each instance has its own data prevents bugs where timers or counters interfere with each other.

4

Intermediate

Instance Data Storage Explained

🤔Before reading on: do you think instance data is stored globally or locally inside each instance? Commit to your answer.

Concept: Explain how each instance stores its own internal variables separately in memory.

When you create an instance, the PLC allocates memory for all its internal variables. This memory is private to that instance. So if you have two instances of a counter, each keeps its own count value. This separation is why instances behave independently.

Result

You see that instance data is isolated, so changes in one instance do not affect others.

Knowing how memory is allocated for instances helps you design programs that avoid unintended data sharing.

5

Intermediate

Calling and Using Instances in Code

Concept: Show how to call methods or execute logic on specific instances and read their outputs.

You use the instance name to call the function block logic. For example: MyTimer1(IN:=Start1, PT:=T#5s); MyTimer2(IN:=Start2, PT:=T#10s); Then you can read outputs like MyTimer1.Q or MyTimer2.Q to check if the timer finished. This lets you control different parts of your system with the same logic but different timing.

Result

You can control multiple timers independently by calling their instances with different inputs.

Understanding instance calls as separate objects lets you build modular and flexible control programs.

6

Advanced

Instance Lifecycle and Initialization

🤔Before reading on: do you think instance data resets automatically each scan or persists? Commit to your answer.

Concept: Explain how instance data persists between scans and how to initialize or reset it properly.

Instance data is stored in memory and persists between PLC scans. This means timers, counters, or states keep their values until you reset them. To initialize, you can write code that sets variables to default values on startup or when needed. Without proper initialization, instances may hold old or random data.

Result

You learn to manage instance data lifecycle to avoid unexpected behavior.

Knowing data persistence helps prevent bugs caused by uninitialized or stale data in instances.

7

Expert

Memory and Performance Considerations

🤔Before reading on: do you think creating many instances has no impact on PLC memory or scan time? Commit to your answer.

Concept: Discuss how each instance consumes memory and CPU time, affecting system resources and performance.

Each instance uses memory for its internal variables and CPU time to execute its logic every scan. Creating too many instances can exhaust memory or slow down the PLC scan cycle. Experts balance the number of instances with available resources and optimize code to keep performance smooth.

Result

You understand the tradeoff between modular design and system resource limits.

Recognizing resource costs of instances helps you design efficient and reliable automation programs.

Under the Hood

Function block instances work by allocating separate memory areas for each instance's internal variables and states. When the PLC runs, it executes the function block code once per instance, using that instance's memory. This isolation ensures that data and behavior do not mix between instances. The PLC runtime manages these memory blocks and schedules execution in each scan cycle.

Why designed this way?

This design allows reuse of logic without rewriting code for each device or process. It balances modularity with independence, letting programmers build scalable systems. Alternatives like global variables would cause data conflicts and make programs fragile. The instance model was chosen to keep code clean, maintainable, and safe.

┌─────────────┐
│ PLC Runtime │
└──────┬──────┘
       │ Executes each scan
       ▼
┌───────────────┐   ┌───────────────┐   ┌───────────────┐
│ Instance #1   │   │ Instance #2   │   │ Instance #3   │
│ Memory Block  │   │ Memory Block  │   │ Memory Block  │
│ + Logic Code  │   │ + Logic Code  │   │ + Logic Code  │
└───────────────┘   └───────────────┘   └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do multiple instances share the same internal data? Commit to yes or no.

Common Belief: All instances of a function block share the same internal data because they use the same code.

Reality: Each instance has its own separate internal data and memory, so they operate independently.

Why it matters: Believing instances share data can cause programmers to expect changes in one instance to affect others, leading to confusing bugs.

Quick: Does creating many instances have no effect on PLC performance? Commit to yes or no.

Common Belief: You can create unlimited instances without worrying about memory or scan time.

Reality: Each instance consumes memory and CPU time, so too many instances can slow down the PLC or cause memory issues.

Why it matters: Ignoring resource limits can cause system crashes or slow response times in critical automation.

Quick: Does instance data reset automatically every PLC scan? Commit to yes or no.

Common Belief: Instance data resets to default values every scan, so no need to initialize it.

Reality: Instance data persists between scans and must be explicitly initialized if needed.

Why it matters: Assuming automatic reset can cause unpredictable behavior from stale or garbage data.

Quick: Can you use a function block instance without declaring it first? Commit to yes or no.

Common Belief: You can call a function block instance directly without declaring it.

Reality: You must declare each instance with a unique name before using it in code.

Why it matters: Not declaring instances leads to compilation errors and confusion about how to manage multiple copies.

Expert Zone

1

Some PLC environments allow dynamic creation of instances at runtime, but most require static declaration, affecting flexibility.

2

Stacked or nested function block instances can lead to complex data flows that require careful management to avoid hidden dependencies.

3

Optimizing instance memory layout can improve scan performance, especially in large systems with many instances.

When NOT to use

Avoid using many function block instances when simple functions or direct code can achieve the goal with less overhead. For very simple or stateless operations, use functions instead. Also, if your PLC has very limited memory or CPU, consider consolidating logic to reduce instance count.

Production Patterns

In real-world automation, function block instances are used to control multiple identical devices like motors, valves, or sensors. Each device gets its own instance to keep control logic clean and independent. Experts also use instance arrays to manage many devices efficiently and implement state machines inside instances for complex behaviors.

Connections

Object-Oriented Programming (OOP)

Function block instances are similar to objects created from classes in OOP.

Understanding instances as objects helps bridge PLC programming with software engineering concepts, improving modular design skills.

Memory Management in Embedded Systems

Instance data allocation parallels how embedded systems manage memory for multiple tasks.

Knowing embedded memory principles clarifies why instance data isolation is critical for reliable automation.

Manufacturing Assembly Lines

Each function block instance controls a separate machine or station on an assembly line.

Seeing instances as independent stations helps grasp why separate data and control are necessary for smooth production.

Common Pitfalls

#1 Using the same instance name for multiple devices, causing data overlap.

Wrong approach:

MotorControl : FB_Motor;
MotorControl(IN:=Start1);
MotorControl(IN:=Start2);

Correct approach:

Motor1 : FB_Motor;
Motor2 : FB_Motor;
Motor1(IN:=Start1);
Motor2(IN:=Start2);

Root cause:

Confusing the function block code with its instances leads to reusing the same instance name instead of creating separate ones.

#2 Assuming instance data resets every scan and not initializing variables.

Wrong approach:

IF FirstScan THEN
// no initialization
END_IF;

Correct approach:

IF FirstScan THEN
InstanceVar := 0; // explicit initialization
END_IF;

Root cause:

Misunderstanding data persistence causes unpredictable behavior from leftover values.

#3 Creating too many instances without considering PLC memory limits.

Wrong approach:

Declare hundreds of instances without checking resources.

Correct approach:

Plan instance count carefully and optimize code to reduce unnecessary instances.

Root cause:

Lack of awareness about resource consumption leads to performance degradation.

Key Takeaways

Function block instances let you reuse control logic while keeping data separate for each device or process.

Each instance has its own memory, so they operate independently without interfering with each other.

You must declare each instance with a unique name before using it in your PLC program.

Instance data persists between PLC scans and requires explicit initialization to avoid errors.

Creating many instances affects memory and CPU usage, so balance modularity with system resources.

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