Programmable Logic Controllers (PLCs) are specialized computers used in industrial automation.

In order for PLCs to run must have some instructions or programming languages inside their CPU.  

The most famous languages like Ladder Logic (LD), Function Block Diagram (FBD), and Structured Text (ST) are under standard of International Electrotechnical Commission IEC-61131-3.

This organization is in charge of defining standard of several programming languages for PLCs

In this article we are going to see the foundation of each one, their advantages and disadvantages, and finally we will conclude by how to choose one. 

 Ladder Logic (LD)

Ladder Logic is a graphical programming language that is the oldest and most widespread of the IEC 61131-3 standards. It was designed to resemble the electrical diagrams of relay-based control systems. 

Structure of Ladder Logic

The structure of Ladder Logic has two vertical bars representing the power line connected by horizontal “rungs” that contain the logic as shown in the figure below. 

From the figure, the rungs are read from left to right and top to bottom. This simulates the physical flow of power through contacts (X) and coils (Y) to control output devices. 

Structure of Ladder Logic

Advantages of Ladder Logic

Simple for beginners

For fresh engineers, technicians and electricians, Ladder Logic is highly comfortable. 

Easy Debugging

Modern PLC software can animate Ladder Logic diagrams, highlighting active elements as the program runs. This provides real-time feedback, allowing you to quickly trace the logic flow.

Ideal for Discrete Logic

Ladder Diagram is highly effective for straightforward on/off control.

Broad Familiarity

With its long-standing use in industry, Ladder Diagram is the most widely adopted PLC language. 

Disadvantages of Ladder Logic

Data Handling Limitations

Ladder Logic is not naturally designed for working with complex data types such as arrays or strings. 

Low Portability

Compared to Structured Text, Ladder Logic is harder to transfer between different PLC platforms.

Differences in vendor-specific instructions and graphical layouts often mean programs must be rewritten from scratch.

Complexity Management Issues

Ladder Logic diagrams can become crowded with numerous rungs and intricate interconnections.

This visual complexity makes large programs difficult to read, troubleshoot, and maintain.

Weakness in Calculations

LD is inefficient for advanced arithmetic, algorithms, or heavy data manipulation. Implementing such tasks typically produces bulky code that is harder to understand and less efficient than text-based approaches.

Function Block Diagram (FDB)

Function Block Diagram is a graphical language that represents the program as a network of interconnected blocks.

Inside the block there may be other languages embedded such as Ladder Logic or any of the other PLC languages. 

Structure of Function Block Diagram

The figure below illustrates the structure of the Function Block Diagram. Notice that the block named function could be performing any specific task, such as a timer, a counter, PWM block, a PID controller, or a custom-defined function. 

Data and signals flow from the output of one block to the input of another, creating a clear visual representation of the program data flow.

Structure of Function Block Diagram

Advantages of Function Block Diagram

Reusable Modular Design

One of Function Block Diagram main advantages is its modular structure. Developers can build custom function blocks for specific tasks and reuse them across different programs or projects.

Clear Troubleshooting

FBD environments often provide animated data flow, making it easy to trace signals as they move between blocks.

This visual feedback helps technicians quickly identify where a value is being created, modified, or interrupted.

Language Flexibility

Many PLC platforms allow Function Block Diagrams to work seamlessly with other programming languages.

For example, an FBD routine can be called from Ladder Logic, enabling developers to apply the most appropriate language for each task within a project.

Process Control Strengths

FBD is particularly effective for continuous control applications, such as tuning PID loops for variables like temperature, flow, or pressure.

The graphical, block-based structure makes it easy to visualize how data moves and changes through the system.

Clear Representation of Complex Systems

Unlike Ladder Logic, which can become difficult to follow in large programs, FBD organizes operations into compact, functional blocks.

This provides a cleaner, more understandable view of complex logic, simplifying both analysis and maintenance.

Disadvantages of Function Block Diagram

Harder to learn

While more intuitive than Structured Text, FBD can be more challenging for beginners and maintenance staff to grasp compared to the straightforward relay logic of Ladder Logic. 

Potentially complex layout

For very large and complex systems, the diagram can still become a maze of interconnecting lines and blocks.

While still generally cleaner than complex LD, poor organization can hinder readability.

Overhead for simple tasks

For basic discrete logic, FBD can feel like overkill. Simple on/off logic is often faster and easier to implement directly in Ladder Logic.

Structured Text (ST)

Structured Text is a high-level, text-based programming language that uses a syntax similar to Pascal or C.

It is the most powerful and flexible of the IEC-61131-3 languages. So, offers advanced features like loops, conditional statements, and complex data structures. 

Structured Text is ideal for programmers with a traditional software background, as it closely mirrors the programming languages, they are familiar with.

Structure of ST

The following figures shows the structure of ST language. Notice the resemblance with the other high level programming languages like Pascal and/or C. 

Structure of ST language

Advantages of Structured Text

Efficiency for complex tasks

ST is excellent at handling complex mathematical calculations, data manipulation, and advanced algorithms. It can perform these tasks in a compact, efficient manner.

Modularity and portability

ST code is highly modular, supporting functions and function blocks that can be easily reused.

Because it is text-based, it is also the most portable language between different PLC manufacturers that adhere to the IEC standard.

Compact code

The text-based format of ST makes the code much more compact than the graphical representations of LD and FBD. This can reduce the program size and memory usage.

Advanced control structures

ST provides advanced programming constructs like FOR, WHILE and REPEAT loops, as well as CASE statements, which are very difficult or impossible to implement cleanly in Ladder Logic.

Data handling

ST is a natural fit for working with strings, arrays, and complex data types, making it ideal for tasks like data logging, report generation, and communication protocols.

Disadvantages of Structured Text

Hard to Learn

The biggest drawback of ST is its lack of visual representation, making it less intuitive for maintenance technicians without a programming background.

Troubleshooting a problem requires a deeper understanding of the code rather than simply looking at a visual flow.

Debugging challenges

While modern IDEs offer watch windows to monitor variable states, debugging ST is generally more abstract than the visual animation provided by graphical languages. 

Higher entry barrier

ST requires a higher level of programming knowledge to use effectively, which can increase training costs and limit the pool of available personnel.

Poor readability for simple logic

While excellent for complex tasks, Structured Text can be less readable and less immediately clear than Ladder Logic for simple, discrete logic sequences.

A straightforward interlocking circuit is much more intuitive when represented graphically.

What Language to Use?

The choice between Ladder Logic, Function Block Diagram, and Structured Text is not a matter of one being inherently superior, but rather of selecting the right tool for the specific application and environment.

For simple, discrete logic and high-speed troubleshooting

Ladder Logic is the clear winner. Its visual nature aligns with the skills of electrical and maintenance personnel, minimizing downtime when problems arise.

For complex, continuous processes and modularity

Function Block Diagram is the better choice. It provides a clean, modular structure for complex algorithms like PID control and makes data flow easy to follow.

For complex math, data handling, and large projects

Structured Text is the most powerful and efficient. It offers the flexibility and advanced control structures needed for sophisticated, algorithm-intensive applications.

In reality, most modern industrial projects use a combination of these languages within the same PLC program.

A common approach is to use Ladder Logic for simple I/O and discrete control, while using Function Blocks for analog control and Structured Text for complex calculations or data manipulation. 

This blended strategy leverages the strengths of each language, creating a robust, efficient, and maintainable program that is accessible to a wider range of technical personnel.

Conclusion

This article reviewed three PLC programming languages, Ladder Logic, Function Block Diagram, and Structured Text. It also studied the foundation of each one, their advantages and disadvantages. 

Finally, it showed an analysis of which language to choose between the three. So, any language of the three can be chosen depending what function, projects, or what is you are trying to achieve in your application.

FAQ: Ladder Logic vs Function Block diagram vs Structured Text

What are these languages—and are they officially recognized?

 Ladder logic (LD), Function Block Diagram (FBD) and Structured Text (ST) are standard PLC programming languages. Yes, they are recognized under IEC-61131-3 standard.

What is the origin and core purpose of each?

The LD was designed to get rid of relay-control systems due to larger numbers of relay in one system. FBD for reusable modular and ST to get high level languages advantage.

What are the strengths of each?

LD is simple good for those who start to learn about PLC. FBD is modular, so good for large scale project. While ST is better for complex data manipulation

What are the challenges or limitations of each language?

LD is not well for data manipulation, FBD may have complex layout when it comes to big program and ST as it’s high-level language, hard to learn and debug.

Which language is best for which scenarios?

LD simple to learn and for simple calculation, FBD for its modularity and ST for data manipulation and complex projects.

Is it common to use multiple languages in one project?

Yes, for example a PID controller block in many PLC as been implemented using all these languages.

Which language should beginners learn first?

Ladder Logic is usually the best starting point due to its intuitive visual nature and strong prevalence across PLC systems. Once you are comfortable, you can expand into FBD the ST.

A PLC, which stands for Programmable Logic Controller, is an industrial computer that acts as the “brain” that processes input to obtain the desired/controlled output, such as induction motor speed and industrial robot arm position, among others. 

The PLC works similarly to any computer, but it is just designed to work in an industrial environment.

So, it can survive in tough conditions such as excessive heat, dust, vibration, and electrical noise. 

In this article I share a brief history of a PLC, how a PLC is composed, how many types of PLCs there are, how many languages PLCs speak, and finally, the ways to connect a PLC to the load in a correct manner.

The Journey of a PLC

Relay logic is a method of electrical control that uses electromechanical relays to perform logical operations and control circuits.

Before PLCs, industrial automation relied on relay logic systems, large panels full of relays, timers, and wiring.

These were bulky, hard to troubleshoot, and expensive to modify whenever a production line changed.

This was back then in the 1960s; the technology changed, and today we have the PLCs that we are talking about in this article. So, in 1968 the engineers came up with the idea of substituting these bulky relays.

The Main Parts of a PLC

A PLC generally contains four main sections: A power supply, an input part, a Processor (CPU/brain) part, and an output part.

Power Supply

The power supply is the one that gives energy to the CPU. It has a DC-DC converter that takes the normal line voltage to 24VDC to provide DC power to the rest of the connected devices. 

Input Section

This is where the PLC receives signals from the outside world, just like your desktop PC when you click a mouse or when you type something using your keyboard (mouse and keyboard are the inputs).

 But when it comes to a PLC input can be sensors, switches, buttons, or any device that sends information (for example, a temperature sensor or a start button).

Processor (CPU/Brain)

As aforementioned, the CPU is the brain of the PLC. It reads the inputs, processes them according to the program written by the user, and decides/controls what the outputs should do.

Programming Device

Usually it’s a normal PC; after programming using special software, the instructions are loaded to the PLC CPU memory. Then the PLC will be ready to execute these instructions to get the desired output signal.

Output Section

Devices such as motors, lamps, valves, or relays are located at the output section of a PLC.

They are also known by the name of actuators. The CPU tells them when to turn ON or OFF or when to act. 

Note that the devices connected to the input section, as well as those that are connected to the output section, are in general called the field devices.

Types of PLC

PLCs are mainly categorized into two types based on their hardware design: Compact (or Fixed) PLCs and Modular PLCs. 

Although as the technology keeps growing, other classifications have emerged such as Nano/Micro/Mini PLC, safety PLCs and Soft PLCs.

Compact (Fixed) PLC

• All components (CPU, power supply, and I/O modules) are housed in a single unit.
• Pros: Simple and low cost.
• Cons: Limited expansion options.
• Use: Small, dedicated automation tasks.

Modular PLC

• Components are separate: CPU, I/O modules, and power supply are installed in racks.
• Pros: Scalable and flexible.
• Cons: More expensive and larger.
• Use: Medium- to large-scale automation.

Other Classifications

• Nano/Micro/Mini PLCs: These are small in size and have low processing power of the PLC, typically used in very small-scale operations.
• Safety PLCs: Designed with enhanced safety features to control hazardous processes and protect personnel.
• Soft PLCs: A general-purpose computer that runs PLC software and functions as a PLC, rather than using a dedicated hardware controller.

Sinking and Sourcing in PLCs

In PLCs, the terms “sinking” and “sourcing” are very important when it comes to physical connection of the PLCs with the input/output (I/O) devices.

They describe how current flows between the PLC I/O terminals and the field devices (sensors, switches, actuators). 

So, during sourcing, current goes from the PLC to the field device, and during sinking, current flows from the field device to the PLC.

Please refer to the diagram shown below for more understanding of these important terminologies. 

The difference between sink and source

Introduction to PLC Programming Languages

For the PLC brain (CPU) to do its job, it needs to be programmed with some instructions (Programming language). A PLC brain understands/talks the following languages.

Ladder Diagram (LD)

This is a graphical language equivalent to traditional electrical relay logic diagrams. It uses contacts as inputs, such as switches, sensors, and coils, and outputs, such as motors and lamps, to represent logical operations.

Function Block Diagram (FBD)

This is also a graphical language that uses predefined function blocks to represent logical functions.

The blocks can be fully functioning components such as timers, counters, and PID controllers, among other components. 

Structured Text (ST)

This language has the ability to use the standard low-level programming instructions like IF-THEN-ELSE, FOR loops, and WHILE loops.

Instruction List (IL)

This also has low-level capability, but it’s a text-based, assembly-like language that uses instructions like LD for load and AND for logical AND.

Sequential Function Chart (SFC)

A graphical language used to organize and structure a PLC program into a sequence of steps, transitions, and actions. 

Main PLC Brands

There are brands like Delta and Noark that are good choices if you have a small project.

But the main brands of PLCs include Siemens, Rockwell Automation (Allen-Bradley), Schneider Electric, Mitsubishi Electric, Omron, and ABB. 

These companies are known for offering innovative, reliable, and extensive ranges of products for industrial automation, though specific brand choice depends on application needs, budget, and industry focus. 

Key Takeaways: What is the PLC?

This article addressed a brief history of a PLC, how a PLC is composed, and types of PLCs.

Furthermore, it discussed how many languages PLCs speak and finally the ways to connect a PLC to the load in a correct manner.

As a result, it can be proved that PLC is indeed the brain of industrial automation. Furthermore, understanding concepts like sinking and sourcing also helps to ensure proper and safe connections between the PLC and field devices, as well as the field personnel/operator. 

If you’re just starting out, remember: learning PLCs is a step-by-step journey, but mastering them opens the door to countless opportunities in automation and control.

Want to keep learning more about the PLCs? Please do not hesitate to read our next article for more beginner-friendly guides.

FAQ: What is a PLC?

What is a PLC and how does it work?

A PLC, which stands for Programmable Logic Controller, is an industrial computer that acts as the “brain” that processes input to obtain the desired/controlled output, such as induction motor speed and industrial robot arm position, among others.

Why were PLCs invented?

Initially, industrial automation relied on banks of hard-wired relays—complex, bulky, and difficult to modify.

PLCs were developed to replace these systems, offering flexibility through programmable logic. 

Who invented the first PLC?

The first PLC (Modicon-084) was created by Richard Morley in 1968, and his team at Bedford Associates. It was specifically made for General Motors.

What are the main components of a PLC?

A PLC generally contains four main sections: an input part, a Processor (CPU/brain) part, and the output part.It may also contain a power source, or you can just add it yourself.

What programming languages are used in PLCs?

PLCs are programmed using IEC 61131-3 standard languages, including:Ladder Diagram (LD): which is a graphical format resembling a relay circuit. Function Block Diagram (FBD): uses reusable logic blocks.

Structured Text (ST): textual, high-level logic with IF/THEN/LOOP structures. Instruction List (IL): low-level, assembly-like text. Sequential Function Chart (SFC): flowchart-like steps and transitions.

Where can I use a PLC?

PLCs are mostly used in industries and control systems, including: Manufacturing lines, automotive assembly, packaging, and industrial robotics. Also, in building systems like elevators, traffic lights, and automatic doors.

How do I choose the right PLC?

This depends on your application requirements, such as the required number of inputs/outputs, Types of inputs/outputs (the latter could be either digital or analog), and the processing capability of a CPU, among other factors. 

What are the steps to learn PLC programming?

Start with beginner-friendly projects like switching the diode ON and OFF then proceed with traffic lights or sump pumps.

Using vendor learning platforms such as Siemens S7-1200 or Rockwell CompactLogix is often recommended for ease of programming.

How does a PLC read sensors?

A PLC reads sensors through its input modules. The sensors send signals to the input modules, which convert them into digital data for the CPU (brain) to manipulate them before sending them to the output modules.

Which of the PLC programming languages is easier to learn for PLC?

Ladder Diagram (LD) is generally considered the easiest programming language to learn for PLCs, especially for beginners with a background in electrical circuits or relay logic.

So, it is a graphical language equivalent to traditional electrical relay logic diagrams.