The IEC 61131-3 standard defines five programming languages for PLCs. Each language has its strengths, and professional automation engineers are expected to be proficient in at least two or three of them.
1. Ladder Diagram (LD)
Ladder Diagram is the most widely used PLC programming language in the world. It was developed to resemble electrical relay logic schematics, making it intuitive for electricians and maintenance technicians.
Best for: Simple logic, motor control, interlocking, and discrete I/O applications.
Key features:
- Visual representation of relay circuits
- Easy to troubleshoot online
- Supported by every PLC manufacturer
- Ideal for maintenance personnel who need to read and modify programmes
2. Function Block Diagram (FBD)
Function Block Diagram uses graphical blocks to represent functions and operations. Data flows between blocks through connection lines, making it excellent for process control and analogue signal processing.
Best for: PID loops, analogue processing, and complex data flow applications.
Key features:
- Graphical representation of data flow
- Reusable function blocks
- Natural fit for process industries
- Easy to visualise signal processing chains
3. Structured Text (ST)
Structured Text is a high-level text-based language similar to Pascal or Python. It is the most powerful of the five languages and is increasingly preferred for complex algorithms, data handling, and mathematical computations.
Best for: Complex calculations, data manipulation, recipe management, and string handling.
Key features:
- High-level programming constructs (IF/ELSE, FOR, WHILE, CASE)
- Compact code for complex logic
- Preferred by software-minded engineers
- Essential for Industry 4.0 applications
4. Instruction List (IL)
Instruction List is a low-level text-based language similar to assembly language. While it is being phased out in favour of Structured Text, it is still found in legacy systems and some Asian-market PLCs.
Best for: Legacy system maintenance and memory-constrained applications.
Note: IL has been deprecated in the latest IEC 61131-3 edition, but understanding it helps when working with older installations.
5. Sequential Function Chart (SFC)
Sequential Function Chart is a graphical language used to programme sequential processes. It organises a programme into steps and transitions, making it ideal for batch processes and machine sequences.
Best for: Batch control, sequential machine operations, and state-based logic.
Key features:
- Clear visual representation of process steps
- Built-in parallel and alternative branching
- Excellent for documenting process sequences
- Often used alongside other languages for step actions
Which Language Should You Learn First?
Start with Ladder Diagram because it is universal and you will encounter it on every project. Then learn Structured Text as it is becoming the industry standard for complex applications and is essential for Industry 4.0 roles.
At EDWartens, our curriculum covers all five languages with practical exercises on both Siemens and Allen-Bradley platforms, ensuring you graduate with the versatility employers demand.