How to Start Learning SCL Programming in Siemens TIA Portal

Many PLC beginners start their programming journey with Ladder Logic (LAD) because it looks like traditional electrical control circuits.

And that's exactly where you should begin.

When a beginner sees contacts, coils, timers, counters, and interlocks in Ladder Logic, the relationship with conventional control panels is easy to understand.

Ladder Logic helps you visualize how a machine works.

It helps you understand:

• Inputs and outputs
• Start and stop commands
• Interlocks
• Permissive conditions
• Motor control
• Timers and counters
• Sequence control
• Alarm conditions

Once these fundamentals are clear, however, the next important step is to learn SCL – Structured Control Language in Siemens TIA Portal.

SCL does not replace Ladder Logic.

It complements it.

A good PLC programmer should understand both and, more importantly, know when to use each one.

 

Why Learn SCL?

As PLC programs become more complex, the amount of logic and data that needs to be processed also increases.

Consider a simple machine.

You may need to calculate:

• Production quantity
• Speed
• Temperature
• Pressure
• Flow
• Energy consumption
• Cycle time
• Production efficiency

You may also need to perform comparisons, mathematical calculations, data processing, and repetitive operations.

Ladder Logic can certainly perform many of these tasks.

But for calculation-heavy or data-processing applications, SCL can provide a much more compact and structured approach.

Instead of creating several graphical blocks, you can often express the same operation in a few lines of text.

This is one of the major advantages of SCL.

 

Start With a Very Simple Example

Suppose we have two INT variables:

Value_1

and

Value_2

We want to add them and store the result in another variable:

Result

In SCL, the basic concept can be represented as:

Result := Value_1 + Value_2;

This looks very simple.

But this single line introduces several important programming concepts.

And this is exactly where a beginner should start.

Don't begin SCL learning with complex machine sequences, arrays, loops, or advanced Function Blocks.

Start with simple operations.

Understand what each line means.

Then gradually increase the complexity.

 

Concept 1: Variables

The first important concept is variables.

A variable is used to store information that the PLC program needs to use.

For example:

Start_Command

Motor_Speed

Temperature

Production_Count

Pressure

Result

Different variables can have different data types.

For example:

BOOL can be used for TRUE/FALSE conditions.

INT can be used for integer values.

DINT can be used when a larger integer range is required.

REAL can be used for decimal or floating-point values.

WORD and DWORD can be used for bit-oriented or numerical data depending on the application.

Understanding variables is one of the foundations of SCL programming.

 

Concept 2: The Assignment Operator

One of the most important symbols in SCL is:

:=

This is called the assignment operator.

For example:

Result := Value_1 + Value_2;

It means:

Calculate Value_1 + Value_2 and assign the result to Result.

Another example:

Motor_Run := TRUE;

This means that TRUE is assigned to the variable Motor_Run.

Similarly:

Motor_Run := FALSE;

assigns FALSE to Motor_Run.

Once you understand the assignment operator, many SCL statements become easier to read.

 

Concept 3: Arithmetic Operations

SCL allows you to perform arithmetic operations directly in the program.

The basic operations include:

Addition (+)

Subtraction (-)

Multiplication (*)

Division (/)

For example:

Total := Quantity * Price;

or:

Average := Total / Count;

This is especially useful in industrial applications involving measurements and calculations.

For example, a PLC may receive a raw analog value and convert it into an engineering value.

You may need to calculate:

Temperature

Pressure

Flow

Speed

Level

Percentage

SCL can make these calculations easier to read and maintain.

 

Concept 4: Program Readability

One of the biggest advantages of SCL is readability.

Imagine a calculation that requires several mathematical operations.

In Ladder Logic, you may need several graphical blocks connected together.

In SCL, the same calculation may be expressed in a few lines.

For example:

Output := (Input_1 + Input_2) * Factor;

An experienced programmer can immediately understand the data flow.

This does not mean Ladder Logic is difficult or inferior.

LAD has a major advantage: visual understanding.

The point is that different programming languages provide different ways of representing the same control logic.

 

Concept 5: Understanding Data Flow

SCL also teaches an important programming concept:

Data flow.

Consider:

Sum := Value_1 + Value_2;

Average := Sum / Count;

First, the PLC calculates the sum.

Then that result is used to calculate the average.

This creates a clear flow:

Input → Calculation → Intermediate Result → Final Result

Understanding this type of data flow becomes increasingly important as PLC programs become larger.

 

Don't Jump Directly Into Advanced SCL

A common mistake beginners make is trying to learn everything at once.

They start with:

• Arrays
• Loops
• Structures
• Complex Function Blocks
• Advanced data types
• Large machine sequences

without first understanding basic Boolean logic and variables.

This often creates confusion.

A better approach is:

Start simple → Practice → Apply → Increase complexity

SCL is a programming language.

Like any language, you need to learn the basic vocabulary and grammar before writing complex programs.

 

Step 1: Convert Simple LAD Into SCL

One of the best methods for learning SCL is to take an existing Ladder Logic program and convert it into SCL.

Start with simple examples.

For example:

LAD: Start command → Motor ON

Then write the equivalent SCL logic.

Next:

LAD: Start + Stop + Interlock → Motor ON

Convert it into SCL.

Then move to:

LAD: Two sensors → Conveyor control

Convert it into SCL.

This approach is extremely effective because you already understand what the Ladder program is doing.

Now your objective is simply to express the same logic using SCL.

 

Step 2: Learn Boolean Logic

Before writing complex SCL programs, become comfortable with:

AND

OR

NOT

For example:

IF Safety_OK AND Start_Command THEN

    Motor_Run := TRUE;

END_IF;

Or:

IF Field_Start OR SCADA_Start THEN

    Start_Command := TRUE;

END_IF;

Parentheses are also important when combining multiple conditions.

The key is not to memorize syntax.

Understand the logic first.

 

Step 3: Learn IF...THEN...ELSE

Once Boolean logic is comfortable, move to decision-making statements.

For example:

IF Temperature > 80.0 THEN

    High_Temperature_Alarm := TRUE;

ELSE

    High_Temperature_Alarm := FALSE;

END_IF;

This is useful for:

• Alarms
• Interlocks
• Process conditions
• Equipment control
• Quality decisions
• Machine sequences

Again, start with small examples.

 

Step 4: Learn Timers, Counters and Comparisons

After IF statements, gradually introduce industrial functions.

Practice:

Timers

Counters

Greater than

Less than

Equal to

Not equal to

For example:

IF Pressure > Pressure_Limit THEN

    High_Pressure_Alarm := TRUE;

END_IF;

Then build a small machine application around it.

This is much more effective than learning individual instructions without understanding their purpose.

 

Step 5: Understand Data Types

As you progress, spend time understanding data types.

Start with:

• BOOL
• INT
• DINT
• REAL
• WORD
• DWORD

Then understand how different data types behave during calculations and assignments.

For example, if you are working with temperature values containing decimals, REAL may be appropriate.

If you are counting production parts, an integer-based data type may be more suitable.

Correct data-type selection is an important part of reliable PLC programming.

 

Step 6: Move to Advanced SCL

Once your fundamentals are strong, gradually move toward:

• CASE statements
• FOR loops
• WHILE loops
• Arrays
• Structures
• User-defined data types
• Functions
• Function Blocks
• Data Blocks
• Recipe management
• Data processing
• Sequence programming

At this stage, SCL becomes much more powerful.

But remember:

Advanced SCL is built on simple programming concepts.

 

SCL and Function Blocks

SCL becomes particularly powerful when combined with Function Blocks (FBs).

For example, you can develop a standard motor-control FB using SCL.

The FB could include:

• Start/stop logic
• Interlocks
• Permissives
• Trip handling
• Feedback monitoring
• Alarm generation
• Operating modes
• Status information

The same FB can then be reused for multiple motors with appropriate instance data.

This approach can make large automation programs more structured, reusable, and maintainable.

 

Should You Stop Using Ladder Logic?

Absolutely not.

Ladder Logic remains extremely valuable.

For many machine-control applications, LAD is excellent for:

• Motor control
• Interlocks
• Start/stop circuits
• Troubleshooting
• Maintenance
• Simple sequence logic

SCL becomes particularly useful when you have:

• Complex calculations
• Data processing
• Arrays
• Repetitive operations
• Complex conditions
• Structured algorithms
• Large data sets

Therefore, don't think:

LAD vs SCL

Think:

LAD + SCL

Use the right programming language for the right application.

 

A Practical SCL Learning Roadmap

For a beginner, I would recommend the following learning sequence:

Level 1 – PLC Fundamentals

Inputs → Logic → Outputs

↓

Level 2 – Ladder Logic

Contacts → Coils → Timers → Counters → Interlocks

↓

Level 3 – SCL Fundamentals

Variables → Assignment → Arithmetic

↓

Level 4 – Boolean Logic

AND → OR → NOT → Parentheses

↓

Level 5 – Decision Making

IF → THEN → ELSE → END_IF

↓

Level 6 – Data

BOOL → INT → DINT → REAL → WORD → DWORD

↓

Level 7 – Industrial Applications

Motor → Pump → Conveyor → Valve → Heating → Alarms

↓

Level 8 – Advanced SCL

CASE → Loops → Arrays → Structures → FBs

This gradual approach can make SCL much easier to learn.

 

Final Thought

SCL programming should not be treated as something completely different from Ladder Logic.

The control philosophy remains the same.

The programming representation changes.

Ladder Logic teaches you how the machine works visually.

SCL teaches you how to express that logic in a structured and scalable way.

A strong PLC programmer should be able to look at a problem and decide:

Should I use LAD?

Should I use SCL?

Should I use an FC?

Should I use an FB?

How should I structure the data?

That is the real programming skill.

Don't try to become an SCL expert in one day.

Start with one simple calculation.

Then one Boolean condition.

Then one IF statement.

Then one machine function.

Practice converting LAD into SCL.

Learn the logic first.

Learn the syntax second.

Apply it to real industrial problems third.

Because programming languages will continue to evolve, but the ability to understand logic, processes, machines, and problems will always remain the foundation of a good automation engineer.

LAD for visualization.

SCL for structure.

Logic for engineering.

 

How to Learn SCL Programming Language in Siemens TIA Portal

Many PLC beginners start their programming journey with Ladder Logic (LAD).

And that is absolutely the right place to start.

Ladder Logic is easy to understand because its structure looks similar to traditional electrical control circuits. Contacts, coils, normally open contacts, normally closed contacts, timers, counters, and interlocks provide a visual representation of how a machine works.

But once you understand the fundamentals of PLC logic, there is another important programming language you should start learning:

 

SCL – Structured Control Language.

SCL is a high-level, text-based programming language used in Siemens TIA Portal for programming SIMATIC PLCs.

For beginners, SCL can initially look more difficult than Ladder Logic.

But when you start using it step by step, you will realize that SCL can make many types of PLC programming cleaner, shorter, more structured, and easier to maintain.

The key is:

Do not try to learn everything at once.

Start with simple PLC logic and gradually increase the complexity.

Why Should PLC Engineers Learn SCL?

Ladder Logic is excellent for visualizing control logic.

For example, if we have:

START + STOP + Emergency Stop + Motor Feedback

we can easily understand the logic by looking at the ladder network.

However, some PLC programs involve:

• Mathematical calculations
• Data processing
• Comparisons
• Multiple conditions
• Repeated operations
• Arrays
• Structured data
• Recipe management
• Complex sequences
• Data manipulation

Writing these types of operations in Ladder Logic can sometimes result in large and complicated networks.

SCL can make the same logic much more compact and structured.

Therefore, learning SCL gives an automation engineer another powerful tool for solving PLC programming problems.

 

Start With Simple Ladder Logic

One of the best ways to learn SCL is not to start with SCL directly.

Start with a simple Ladder Logic program.

Understand what the ladder is doing.

Then convert the same logic into SCL.

This approach helps you understand the relationship between:

Electrical Logic → Ladder Logic → Boolean Logic → SCL

For example, imagine a lamp should turn ON when:

• Emergency Stop is healthy
• STOP push button is not pressed
• START push button from the field OR START command from SCADA is active

The logic can be represented conceptually as:

E-Stop Healthy AND STOP Not Pressed AND (Field START OR SCADA START)

This simple example introduces some of the most important concepts in SCL.

 

Understand AND, OR and NOT

Before learning complex SCL programs, become very comfortable with Boolean logic.

Three operators are particularly important:

AND

AND means all required conditions must be TRUE.

For example:

Motor can start when:

Safety OK AND Start Command

Both conditions must be TRUE.

OR

OR means any one of the conditions can be TRUE.

For example:

Field Start OR SCADA Start

Either command can initiate the start request.

NOT

NOT reverses the Boolean condition.

For example:

NOT Stop_PB

means the stop push button condition is not active.

 

These three concepts form a major foundation for SCL programming.

If Boolean logic is clear, learning SCL becomes much easier.

 

Step 1: Learn Basic SCL Syntax

After understanding Boolean logic, start with basic SCL statements.

For example:

IF Start_Command AND Safety_OK THEN

    Motor_Run := TRUE;

END_IF;

Don't worry about writing complicated programs initially.

Focus on understanding:

IF → condition → THEN → action → END_IF

Then move to:

IF...ELSE...END_IF

For example, the PLC can perform one action when a condition is true and another action when it is false.

This is the foundation for writing decision-based logic.

 

Step 2: Practice Boolean Conditions

Once you understand IF statements, start combining conditions.

For example:

IF Safety_OK AND

   (Field_Start OR SCADA_Start) AND

   NOT Motor_Trip THEN

    Motor_Run := TRUE;

END_IF;

Notice the importance of parentheses.

The parentheses clearly define which conditions belong together.

This becomes especially important when the logic contains multiple AND and OR conditions.

A good programmer should not only write logic that works.

The logic should also be easy for another engineer to understand.

 

Step 3: Learn Comparisons

Industrial automation involves a lot of comparison.

For example:

• Temperature > 80°C
• Pressure < 2 bar
• Speed = 1500 RPM
• Level >= 70%
• Counter value <> preset value

SCL makes these comparisons very readable.

For example:

IF Temperature > 80.0 THEN

    High_Temperature_Alarm := TRUE;

END_IF;

This type of programming becomes very useful in process control, machine monitoring, alarms, and data processing.

 

Step 4: Learn Timers and Counters

After basic Boolean logic and comparisons, move toward timers and counters.

Understand how timers are used for:

• Delays
• ON-delay operations
• Sequence timing
• Fault monitoring
• Equipment protection

Counters can be used for:

• Production counting
• Part counting
• Cycle counting
• Maintenance intervals

Do not simply memorize the syntax.

Understand why the timer or counter is required in the machine sequence.

That process understanding is more important than syntax.

 

Step 5: Learn Variables and Data Types

This is another important area for SCL beginners.

You should understand data types such as:

• BOOL
• INT
• DINT
• REAL
• WORD
• DWORD
• TIME
• DATE_AND_TIME
• STRING

For example:

A motor start command may be:

BOOL

A production quantity may be:

DINT

A pressure value may be:

REAL

Understanding data types helps prevent programming errors and makes your code more reliable.

 

Step 6: Practice With Real Machine Examples

This is where SCL learning becomes much more effective.

Instead of practicing only theoretical examples, take real industrial applications.

For example:

Motor Control

Create logic for:

Start → Stop → Trip → Reset → Feedback → Interlock

Conveyor Control

Create logic for:

Start → Sensor detection → Conveyor movement → Part detection → Stop

Pump Control

Create logic for:

Auto/Manual → Start command → Pressure condition → Feedback → Fault

Heating System

Create logic for:

Temperature measurement → Setpoint comparison → Heater ON/OFF → High-temperature alarm

Tank Filling

Create logic for:

Low-level detection → Pump start → High-level detection → Pump stop

These examples help connect programming syntax with actual industrial processes.

 

Step 7: Convert Existing Ladder Programs Into SCL

One of the best exercises for learning SCL is:

Take a Ladder Logic program and convert it into SCL.

For example:

Start with:

LAD → Motor Start/Stop

Then convert it into:

SCL → Motor Start/Stop

Next:

LAD → Conveyor Sequence

Convert it into:

SCL → Conveyor Sequence

Then:

LAD → Alarm Logic

Convert it into:

SCL → Alarm Logic

This exercise develops both logical thinking and programming skills.

You begin to recognize that the programming language may change, but the control logic remains the same.

 

Step 8: Move Toward Advanced SCL

Once you are comfortable with the basics, gradually move toward advanced topics.

Learn:

• CASE statements
• FOR loops
• WHILE loops
• Arrays
• Structures
• User-defined data types
• Functions
• Function Blocks
• Data Blocks
• Recipe handling
• Data manipulation
• Sequence programming

But don't rush.

There is no advantage in learning FOR loops before you understand Boolean logic and IF statements.

A strong foundation is more important than learning advanced syntax quickly.

 

SCL and FB: A Powerful Combination

SCL becomes particularly powerful when used inside Function Blocks (FBs).

For example, you can create a standard motor-control FB using SCL.

The FB can contain:

• Start/Stop logic
• Interlocks
• Trip handling
• Feedback monitoring
• Alarm generation
• Operating modes
• Timers
• Status information

The same FB can then be reused for multiple motors with appropriate instance data.

This is one of the ways structured PLC programming becomes valuable in larger industrial automation projects.

 

LAD or SCL – Which One Should You Learn?

The answer is:

Learn both.

It should not be a competition between Ladder Logic and SCL.

Each has its strengths.

LAD is excellent for:

• Visual troubleshooting
• Electrical control logic
• Interlocks
• Simple machine logic
• Beginner learning
• Maintenance-friendly programming

SCL is excellent for:

• Calculations
• Data processing
• Complex conditions
• Arrays
• Loops
• Structured programming
• Repetitive operations
• Advanced algorithms

A good automation engineer should be comfortable moving between programming languages based on the application.

 

A Simple SCL Learning Roadmap

If I were training a beginner, I would suggest this sequence:

Level 1 – PLC Fundamentals

Understand:

Inputs → Logic → Outputs

↓

Level 2 – Ladder Logic

Learn:

Contacts → Coils → Timers → Counters → Interlocks

↓

Level 3 – Boolean Logic

Learn:

AND → OR → NOT → Parentheses

↓

Level 4 – Basic SCL

Learn:

IF → ELSE → END_IF

↓

Level 5 – Data

Learn:

Variables → Data Types → Comparisons

↓

Level 6 – Industrial Logic

Practice:

Motor → Pump → Conveyor → Valve → Alarm

↓

Level 7 – Advanced SCL

Learn:

CASE → Loops → Arrays → Structures → Functions → FBs

This gradual approach is much easier than trying to learn SCL syntax from complex programs.

 

Final Thought

Learning SCL is not about replacing Ladder Logic.

It is about expanding your PLC programming capability.

Ladder Logic helps you visualize the machine.

SCL helps you structure the logic.

PLC fundamentals help you understand the control system.

And most importantly:

Logic is the real skill—not the programming language.

A good automation engineer should be able to look at a machine problem and decide:

Should I use LAD?

Should I use SCL?

Should I use an FC?

Should I use an FB?

How should the data be structured?

How can the program be made reusable and maintainable?

That is the real progression from PLC programmer to automation engineer.

So, if you already understand basic Ladder Logic, don't stop there.

Start converting simple LAD programs into SCL.

Practice a little every day.

Start with:

AND → OR → NOT → IF → ELSE → Comparisons → Timers → Data Types → Machine Logic → FB → Advanced SCL

You don't need to learn everything in one day.

Learn the logic first. Learn the syntax second. Apply it to real machines third.

Because when the fundamentals are strong, learning a new programming language becomes much easier.

LAD for visualization.

SCL for structure.

Logic for engineering.

 

OB vs FC vs FB in TIA Portal: Why Do We Really Need an FB?

OB vs FC vs FB in TIA Portal: Why Do We Really Need an FB?

One common question I hear during TIA Portal and Siemens PLC training is:

“Sir, if we can do programming in OB and FC, then why do we need an FB? Is FB really necessary?”

My answer is always:

Yes, FB has a very important purpose.

For a beginner, OB, FC, and FB can look very similar. All three are program blocks, and we can write logic inside them.

But they are designed for different programming requirements.

Once a student understands the difference between these three blocks, PLC programming becomes much easier—and more importantly, the student starts thinking like an automation engineer rather than simply writing ladder logic.

Let's understand it in a simple way.

 

 

OB – The Organisation Block

OB stands for Organisation Block.

You can think of the OB as the entry point or organiser of the PLC program.

The PLC executes different OBs based on the type of OB and the event that triggers them.

For example, the OB1 is commonly used as the main cyclic program block.

Inside OB1, we can call:

• FCs
• FBs
• Other program structures

A simple structure could look like:

OB1 → FC1 → Logic

or:

OB1 → FB1 → Instance DB

So, OB does not necessarily contain all the detailed machine logic.

Instead, it helps organize how the program is executed.

For a simple machine, we might have:

OB1

→Motor Control
→ Conveyor Control
→ Pump Control
→ Valve Control
→ Alarm Logic

But as the machine becomes larger, putting everything directly into OB1 can make the program difficult to understand and maintain.

This is where FCs and FBs become extremely useful.

 

FC – Function Block Without Retained Instance Memory

FC stands for Function.

FCs are very useful when we want to create reusable logic that does not require its own instance memory.

For example, an FC can be used for:

• Mathematical calculations
• Scaling
• Comparisons
• Signal processing
• Common interlocks
• Data conversion
• Simple calculations
• Reusable logic

Suppose we have an analog input from a pressure sensor.

We receive a raw value from the PLC and need to convert it into engineering units such as:

0–27648 → 0–10 bar

Instead of putting all the scaling calculations into OB1, we can create an FC for the calculation.

The OB can call the FC and provide the required inputs.

The FC performs the calculation and returns the result.

This makes the program cleaner and easier to maintain.

But there is an important point:

An FC does not have its own instance DB.

Therefore, it does not have the same type of dedicated instance memory that an FB has.

This makes FC very suitable for calculations and logic where dedicated persistent state is not required.

 

FB – Function Block with Memory

Now comes the most important question:

Why do we need FB?

The major advantage of an FB (Function Block) is that it is designed to work together with an Instance DB (Instance Data Block).

The Instance DB stores the data associated with a particular FB instance.

This allows the FB to maintain its own data and state between calls.

This becomes extremely useful when we are developing reusable equipment or machine-control logic.

Let's take a practical example.

 

Example: Controlling 10 Motors

Suppose a manufacturing machine has:

10 motors.

Every motor has:

• Start command
• Stop command
• Motor feedback
• Trip feedback
• Permissive conditions
• Interlocks
• Running status
• Fault status
• Reset command

We could create separate logic for every motor.

But that would create a lot of repeated programming.

Instead, we can create one standard motor-control FB.

For example:

FB_Motor

Inside this FB we can develop the complete motor logic.

Then we can create multiple instances:

Motor 1 → FB_Motor + Instance DB
Motor 2 → FB_Motor + Instance DB
Motor 3 → FB_Motor + Instance DB

And so on.

Each motor uses the same standard logic, but each instance has its own data.

This is one of the most powerful concepts in structured PLC programming.

 

Why Is Instance DB Important?

Think of the FB as a standardized machine component and the Instance DB as the memory/data associated with that particular component.

For example:

FB_Motor

contains the standard motor-control logic.

The Instance DB contains data for:

Motor 1

Another Instance DB contains data for:

Motor 2

Another Instance DB contains data for:

Motor 3

The logic is standardized, while the instance data is separated.

This provides significant advantages in large automation projects.

 

A Simple Real-Life Analogy

Imagine you have a motor-control template.

The template says:

“When Start is pressed and all permissive conditions are healthy, start the motor. If a trip occurs, stop the motor and generate a fault.”

You don't want to rewrite this entire logic ten times.

Instead, you create one standard FB:

FB_Motor

Then you use it for:

Motor 1
Motor 2
Motor 3
Motor 4
...
Motor 10

Each motor gets its own instance data.

This is similar to creating one standard design and using it for multiple machines.

 

Where Are FBs Commonly Used in Industry?

In real industrial automation projects, FBs are commonly useful for equipment and systems that have their own state, parameters, commands, feedback, or operating modes.

Examples include:

Motor Control

Start, stop, trip, feedback, permissives, interlocks and reset.

Pump Control

Auto/manual operation, start/stop, feedback, pressure conditions, alarms and protection.

Valve Control

Open/close commands, feedback, timeout monitoring and fault detection.

Conveyor Control

Start/stop, sequence, sensors, interlocks, speed control and fault handling.

Compressor Control

Operating modes, pressure conditions, alarms, permissives and protection.

Machine Sequence

Step control, sequence status, timers, transitions and fault conditions.

The exact architecture depends on the machine and programming standard, but the principle remains the same:

Create reusable logic and manage the associated data in a structured way.

 

Why Not Put Everything in OB1?

A beginner may think:

“If OB1 can do everything, why should I create FCs and FBs?”

Technically, a lot of logic can be written directly in OB1.

But imagine a large manufacturing line with:

• 50 motors
• 20 valves
• 10 conveyors
• Multiple pumps
• Several sensors
• Safety interlocks
• Alarms
• Sequence control
• HMI communication

If everything is written inside OB1, the program can become difficult to understand, troubleshoot, modify and maintain.

Good PLC programming is not only about making the machine run.

It is also about creating a program that another engineer can understand months or years later.

That's why structured programming matters.

 

The Real Industrial Advantage: Standardization

One of the biggest benefits of FB programming is standardization.

Suppose an organization develops a standard motor FB.

The same FB can potentially be used across multiple projects according to the company's programming standards.

This means engineers don't need to develop motor logic from scratch every time.

They can reuse the tested structure.

This can improve:

• Development time
• Troubleshooting
• Maintenance
• Standardization
• Program readability
• Project consistency

And when a standard function needs improvement, the organization can manage that improvement systematically according to its software architecture and project requirements.

 

The Simplest Way to Remember OB, FC and FB

For beginners, I usually explain it in three simple lines:

OB

OB organizes and executes the program.

FC

FC performs logic without its own instance DB.

FB

FB performs logic with associated instance data stored in an Instance DB.

Or even more simply:

OB → Program execution

FC → Reusable logic/calculation

FB → Reusable logic + instance data

This simple understanding gives students a strong foundation for advanced Siemens PLC programming.

 

Is FB Always Necessary?

There is one important clarification.

FB is not required for every piece of PLC logic.

You should not use FB simply because it exists.

If you need a simple calculation, scaling operation, comparison, or reusable logic without dedicated instance data, an FC may be a better choice.

If you have a reusable equipment function that needs associated data and state, an FB is often a better architecture.

The objective is not:

“Use FB everywhere.”

The objective is:

“Choose the correct block based on the programming requirement.”

That is the mindset an automation engineer should develop.

 

Final Thought

When students first start working with TIA Portal, OB, FC and FB can appear to be just three different programming blocks.

But they represent something much more important:

Different approaches to structuring an industrial control program.

As machines become more complex, structured programming becomes increasingly important.

The goal of PLC programming is not only to make the machine work.

It is to make the program:

Readable.
Reusable.
Maintainable.
Scalable.
Standardized.
Easy to troubleshoot.