As part of industrial automation, PLC programming (Programmable Logic Controller programming) plays a central role in controlling and optimizing machines. PLCs (Programmable Logic Controllers) are now at the heart of many production systems, from complex installations to energy management systems. Their ability to execute instructions reliably and in real time makes them an essential pillar of modern industry.
To ensure a consistent and interoperable approach, PLC programming relies on an international standard, IEC 61131-3, which defines several languages tailored to the needs of engineers and technicians. Unlike traditional software development, PLC programming offers a variety of approaches, ranging from intuitive graphical representations to more advanced textual languages.
In this article, we will explore the five main languages defined by this standard, understand their specific features, advantages, and use cases, to help you better grasp the fundamentals of PLC programming.
What is PLC programming?
PLC programming refers to the set of methods used to program Programmable Logic Controllers (PLCs), which are devices designed to control and automate machines or industrial processes. In practice, a PLC receives information from sensors (temperature, pressure, position, etc.), processes it according to defined logic, and then sends commands to actuators (motors, valves, relays). This real-time operation ensures reliable, repeatable, and safe performance in often demanding environments.
One of the key characteristics of PLC programming is the coexistence of several programming languages. This diversity stems from the wide range of automation needs: some projects require a simple visual representation to facilitate maintenance, while others demand complex calculations or advanced logic. To address these different use cases, the IEC 61131-3 standard defines several complementary languages, each suited to a specific type of task.
Developments closely tied to the physical world
Unlike traditional software development, PLC programming is closely connected to the physical world. It must account for constraints such as real-time operation, system robustness, and operational safety. Where a software developer might tolerate certain delays or minor errors, a PLC program must operate deterministically and without interruption.
Finally, many manufacturers offer their own hardware and software solutions for PLC programming, such as Crouzet. These players develop controllers and programming environments tailored to the specific needs of industry, enabling companies to design high-performance and scalable automation systems.
Overview of PLC Languages
The programming of industrial controllers is based on a standardized framework defined by the IEC 61131-3 standard. This international standard was established to harmonize practices and enable engineers to work with common languages, regardless of PLC manufacturers. Today, it is the essential reference in PLC programming.
IEC 61131-3: An essential framework for PLC programming
The IEC 61131-3 standard is the international reference for programming industrial controllers. Published by the International Electrotechnical Commission (IEC), it was designed to standardize languages, data structures, and best practices in PLC programming, ensuring better interoperability between systems and manufacturers.
Before the introduction of this standard, each manufacturer offered its own programming methods, making projects difficult to maintain and transfer from one system to another. IEC 61131-3 unified these approaches by defining a consistent set of languages and shared concepts, thereby facilitating collaboration between engineers, technicians, and system integrators.
Beyond the five languages it formalizes, this standard also covers essential aspects such as program structuring (functions, function blocks, programs), variable management, and code modularity. It thus promotes the development of more robust, reusable, and scalable solutions.
Today, most PLC programming platforms are based on this standard, making it a fundamental pillar for anyone looking to work in the field of industrial automation.
Languages adapted to the needs
IEC 61131-3 defines five main languages, each designed to meet specific automation needs. These include Ladder Diagram (LD), widely used for its representation inspired by electrical schematics, as well as Function Block Diagram (FBD), which is based on a modular and visual approach. Structured Text (ST) provides a powerful textual syntax, ideal for complex processing. Instruction List (IL), like assembly language, is now declining in use, while Sequential Function Chart (SFC) enables clear and structured modeling of sequential processes.
These five languages are not opposed to one another—they are complementary and often used together within the same project. In the following sections, we will examine their characteristics, advantages, and use cases in detail.
Five languages for PLC programming
Before diving into the details of each language, it is important to understand that PLC programming does not rely on a single approach. On the contrary, it is based on several complementary languages, each designed to meet specific needs in industrial automation. These five languages offer different approaches—graphical or textual—allowing engineers and technicians to choose the most suitable tool for their project. Let’s now explore these five essential languages and their characteristics.
Ladder Diagram (LD) – The most widely used language
Ladder Diagram (LD) is undoubtedly the most emblematic language in PLC programming. It originates from traditional electrical schematics used by industrial electricians, which explains its lasting popularity. Its graphical representation is based on a “rung” logic (like the steps of a ladder), where contacts (inputs) and coils (outputs) are arranged to replicate electrical circuits.
In a Ladder program, contacts represent conditions (for example, a pressed button or an activated sensor), while coils trigger actions (such as starting a motor). This visual approach allows the program to be read almost like a wiring diagram, making it particularly accessible to maintenance technicians and operators.
Among its main advantages, LD stands out for its ease of understanding and its widespread adoption in industry. It is ideal for simple control logic, such as start/stop systems, interlocking, or basic sequences. Many manufacturers, including Crouzet, offer environments that facilitate its use.
However, Ladder Diagram shows its limitations when logic becomes complex. Advanced algorithms, loops, or data processing are more difficult to implement and maintain in this format. Despite this, it remains an essential standard for many industrial applications.
Function Block Diagram (FBD) – Modular visual programming
Function Block Diagram (FBD) is another graphical language defined by the IEC 61131-3 standard. It is based on a modular approach, where functional blocks are connected to one another to form a logical system. Each block represents a specific function (timing, addition, control, etc.), with clearly defined inputs and outputs.
FBD is particularly well suited to continuous systems or complex industrial processes that require calculations and multiple interactions between variables. Its visual representation makes it easier to understand the system, especially for engineers working on dynamic processes.
Among its advantages is the reusability of blocks, which saves time and improves code maintainability. Readability is also a strong point, especially for well-structured systems. However, when the number of blocks becomes too large, the diagram can quickly become difficult to read and organize.
FBD is often used in fields such as water treatment, energy management, or continuous industrial processes, where modularity and clarity are essential.
Structured Text (ST) – The powerful language
Structured Text (ST) is the textual language of PLC programming and is also defined by the IEC 61131-3 standard. Its syntax is like that of the Pascal language, making it familiar to developers with a background in traditional programming.
Unlike graphical languages such as Ladder or FBD, ST allows instructions to be written as structured code. It is particularly well suited for complex calculations, advanced algorithms, and processing large amounts of data. It also offers great flexibility, with structures such as loops, nested conditions, and custom functions.
Among its main advantages are its power and its ability to handle complex logic in a concise and efficient way. It is often used in applications requiring mathematical computations, signal processing, or advanced communications. Solutions offered by companies such as Crouzet make it possible to fully leverage this language in modern industrial environments.
On the other hand, Structured Text can be less intuitive for beginners or non-IT profiles. It requires a certain level of rigor and a good understanding of programming concepts, which can represent a barrier to entry for some users.
Instruction List (IL) – A declining language
Instruction List (IL) is a textual language historically used in PLC programming, with a syntax like assembly language. It is based on a sequence of simple instructions executed sequentially, providing precise control over the program’s operation.
In the past, this language was appreciated for its fast execution and low resource consumption, making it suitable for PLCs with limited capabilities. However, it has several major drawbacks, including poor readability and increased complexity when it comes to maintenance.
With technological advancements and the growing power of PLCs, Instruction List has gradually fallen out of use. In fact, the IEC 61131-3 standard has officially deprecated it in its recent versions. Today, it is largely replaced by Structured Text, which offers better readability and greater flexibility.
Sequential Function Chart (SFC) – For sequential processes
Sequential Function Chart (SFC) is a graphical language designed to describe sequential processes in a clear and structured way. It is based on the logic of steps and transitions, allowing the behavior of a system over time to be modeled.
SFC is closely related to GRAFCET, a method widely used in Europe to represent industrial automation systems. Each step corresponds to a system state, while transitions define the conditions required to move to the next step. This approach is particularly useful for processes involving multiple successive phases.
Among its main advantages, SFC offers excellent readability for complex systems by making logical sequences easier to understand. It also enables efficient structuring of programs that combine multiple languages, for example by integrating Ladder or Structured Text within certain steps.
SFC is commonly used in production lines, multi-stage automated systems, and industrial protocols requiring precise coordination between different operations.
Which PLC language should you choose?
Choosing the right language in PLC programming depends on several key factors, and there is no one-size-fits-all solution. Each automation project has its own technical, human, and industrial constraints, which directly influence the choice of language(s) to use.
- Project complexity is the first criterion to consider. For simple control systems (start/stop, input/output management), a language like Ladder Diagram is often sufficient. However, for more advanced applications involving complex calculations or algorithms, Structured Text is generally more appropriate. Continuous or modular processes often benefit from using Function Block Diagram.
- User profile also plays a decisive role. Technicians with a background in electrical engineering or maintenance are usually more comfortable with graphical languages like Ladder or FBD, while engineers or developers with programming experience tend to prefer Structured Text for its flexibility and power.
- Target industry is another important factor. Some sectors, such as manufacturing or the food industry, favor simple and robust approaches to facilitate maintenance. Others, like energy or complex automated systems, require languages capable of handling more advanced logic and data processing.
- It is also important to note that, in practice, the languages defined by the IEC 61131-3 standard are often used in combination within the same project. For example, a program may use SFC to structure an overall process, Ladder to manage simple inputs/outputs, and Structured Text for specific calculations. This hybrid approach allows you to leverage the strengths of each language.
Finally, to make the right choice, it is recommended to rely on suitable hardware and software solutions provided by specialized manufacturers such as Crouzet. These platforms generally support multiple languages, making it easier to develop, maintain, and scale automation systems.
In summary, the choice of a PLC language should be guided by the project’s requirements, the users’ skills, and the industrial context, while remaining open to a combined approach for greater efficiency.
PLC programming is built on a rich set of approaches that address the wide variety of needs in industrial automation. Through the five languages defined by the IEC 61131-3 standard—Ladder Diagram, Function Block Diagram, Structured Text, Instruction List, and Sequential Function Chart—engineers and technicians have access to a range of complementary tools for designing high-performance, reliable, and scalable systems.
Each of these languages has its own strengths: Ladder for its simplicity and readability, FBD for its modularity, Structured Text for its power, SFC for sequence management, and Instruction List—although now in decline—for its historical role. The challenge, therefore, is not to choose a single language, but to understand how to combine them effectively based on project constraints.
As industry evolves toward increasingly connected and intelligent environments, PLC programming continues to transform, particularly with the rise of Industry 4.0 and the integration of digital systems. Mastering these languages is therefore an essential foundation for designing the automation solutions of tomorrow and making the most of available technologies.