The international standard IEC 61131-3 defines the methods for writing logic in industrial controllers. It organizes the world of automation by dividing languages into graphical and textual formats. The choice of a specific solution directly affects development time, ease of subsequent troubleshooting, and code performance.
Below, we analyze the key PLC programming languages—from the classic ladder to advanced structured text—to help you better understand and select the optimal tool for your application requirements.
PLC Programming languages according to IEC 61131-3: overview and applications
The IEC 61131-3 standard is a fundamental document for anyone providing PLC programming services. It defines a set of standard programming languages commonly used in the industry. Each of these languages has unique features and purposes, allowing for flexible adaptation of tools to specific tasks.
Ladder Diagram (LD)
The Ladder Diagram, known as LD, originates directly from electrical relay logic diagrams. Its graphical nature makes it incredibly intuitive for electricians and automation technicians. The basic elements of the language are:
- Contacts (normally open and normally closed), which symbolize input signals or logical states.
- Coils, representing outputs or actions to be performed.
The power rails connecting these elements create logical signal flow paths. LD is ideal for simple control logic, such as switching states, implementing start/stop signals, or basic safety interlocks. Due to its readability and direct link to the electrical world, the ladder language is one of the oldest and still most frequently used languages for programming PLC controllers. It allows for the rapid creation of clear diagrams for typical tasks.
Structured Text (ST)
Structured Text, or ST, is a high-level textual language that differs significantly from ladder logic. Its syntax is similar to programming languages like Pascal or Python, making it a more flexible and powerful tool. Using conditional statements (IF-THEN-ELSE), loops (FOR, WHILE), and advanced data structures, a programmer can implement complex algorithms and mathematical calculations.
ST is invaluable when creating complex control algorithms, mathematical calculations, or implementing advanced controllers like PID regulators. It allows for efficient management of variables and data structures, which is crucial in more demanding applications. The syntax of ST enables the concise writing of complex logic.
Function Block Diagram (FBD)
The Function Block Diagram, or FBD, is another graphical PLC programming language. It is based on function blocks that represent specific operations or functions, such as logic gates, timers, counters, or control algorithms. An FBD program is built by connecting these blocks based on data flow, creating a graphical diagram of signal flow and logic. This visual character makes programs easy to read and understand.
FBD is particularly useful for creating modular and reusable program components. The ability to define custom function blocks increases its flexibility. It allows for the creation of transparent and easy-to-maintain code structures, which is valued in large-scale projects. Each block performs a specific logical or mathematical function.
Sequential Function Chart (SFC)
SFC is a powerful tool for describing and programming processes characterized by a sequential flow. Its structure is based on steps (states) representing individual stages of the process and transitions, which define the conditions for moving between steps. This enables easy modeling of complex sequences of operations, loops, and even parallel actions within a single program.
SFC is invaluable in managing complex production processes where the order of operations is critical. It allows for a clear representation of control logic, which facilitates diagnostics and program modification. It is an ideal solution for systems requiring precise control over the sequence of actions.
Structured Control Language (SCL)
SCL is an implementation of the Structured Text (ST) standard, dedicated primarily to programming Siemens S7 PLC controllers. It is a high-level textual language that allows for much more efficient programming of complex operations than graphical methods. SCL is based on Pascal syntax, making it clear and logical for those with experience in classic IT programming.
Like ST, the key elements of SCL are mathematical operators, conditional statements, and loops. It is the tool of choice for creating custom Function Blocks (FB) and Functions (FC) that must perform bulk data operations, manage recipes, or communicate with databases. SCL allows for concise code that is easier to edit and copy between projects, significantly speeding up the automation engineer’s work.
Instruction List (IL)
IL, or Instruction List, is a low-level language resembling Assembly. It consists of short command codes and operand addresses that directly manipulate controller registers. Due to its nature, IL requires the programmer to have deep knowledge of the controller’s architecture and a detailed understanding of its processor’s operation.
It is mainly used where the priority is maximum code optimization, minimal resource consumption, or the need for direct access to memory and registers. It allows for very precise and fast execution of operations, but its readability and ease of debugging are limited compared to higher-level languages. This language is often used to optimize critical code fragments.
Comparison of PLC programming languages and choosing the right tool
Each of the discussed PLC languages has its strengths and weaknesses. Ladder Diagram (LD) is ideal for simple electrical logic. Structured Text (ST) excels in complex calculations and algorithms. Function Block Diagram (FBD) facilitates modular graphical structures, while Sequential Function Chart (SFC) perfectly models sequential processes. Instruction List (IL) offers maximum performance and low-level control.
A PLC programmer often chooses a language or a combination of languages depending on the project’s specifics, the complexity of the algorithm, and the team’s experience.
| Language | Type | Main Application |
| LD (Ladder Diagram) | Graphical | Simple sequences, production machine logic |
| FBD (Function Block Diagram) | Graphical | Industrial processes, PID regulation, visualization |
| SFC (Sequential Function Chart) | Graphical | Complex multi-stage processes, step modeling |
| ST (Structured Text) | Textual | Complex algorithms, calculations, data analysis |
| SCL (Structured Control Language) | Textual | Complex math, data processing, loops, arrays, recipes |
| IL (Instruction List) | Textual | Max code optimization, low-level control |
Tools and development environments for PLC languages
Creating effective PLC programs requires the right tools. The most popular Integrated Development Environments (IDEs) include Siemens TIA Portal, Codesys, and Rockwell Automation Studio 5000. They offer extensive code editors for various languages, advanced debuggers for real-time program execution tracking, and simulators to test logic without physical hardware.
This advanced software consists of complex platforms supporting the entire project lifecycle—from code creation and testing to deployment and system diagnostics.
Advanced concepts in PLC programming
Beyond basic languages, PLC programming includes advanced concepts like subprograms, modular code building, and communication between controllers or integration with HMI/SCADA systems. Utilizing ready-made libraries provided by manufacturers or the community significantly speeds up the process, allowing engineers to focus on unique application logic.
Best practices for PLC programmers
- Readability: Use comments, logical naming for variables, and a uniform structure.
- Testing: Thoroughly test in both simulators and on actual hardware.
- Documentation: Maintain detailed project documentation.
- Safety: Prioritize safe code to prevent failures or hazards.
The future of PLC programming
The world of automation is moving toward cloud integration, AI data analysis, and object-oriented approaches. While high-level languages are gaining popularity, traditional languages like LD remain preferred for fast on-site diagnostics and machine maintenance.
Summary: the key to success in automation
Understanding various PLC programming languages and using them effectively is the foundation of success in modern automation. From simple ladder tasks to complex ST algorithms, every PLC opens new perspectives.
