Key Takeaway
Siemens PLCs mainly support several standard IEC 61131-3 programming languages, including LAD (Ladder Logic Diagram), FBD (Function Block Diagram), and STL (Statement List). Additionally, SCL (Structured Control Language, similar to Pascal) and Graph (for sequence programming) are available. LAD and FBD are graphical languages popular for ease of visualization, while STL is a text-based language ideal for precise, low-level programming. Advanced Siemens PLCs like the S7-1500 also support object-oriented extensions in SCL. Choosing a language depends on project requirements, complexity, and engineer preference. TIA Portal and older Step 7 platforms allow users to switch between languages for different program parts, providing flexibility and maximizing efficiency across various automation applications.
Overview of IEC 61131-3 Languages
Siemens PLCs are built on a powerful international standard known as IEC 61131-3. This is the gold standard for PLC programming, accepted across industries worldwide.
Now, what does that mean for you? It means Siemens gives you the flexibility to choose from five standardized languages:
Ladder Logic (LAD)
Function Block Diagram (FBD)
Structured Text (ST)
Instruction List (IL) (now obsolete)
*Sequential Function Chart (SFC)
Among these, Siemens mainly focuses on LAD, FBD, and ST inside their TIA Portal software. These languages aren’t just code—they’re tools you can use based on your logic preference, project type, and team workflow.
And here’s the best part: you can mix and match them within the same project. That means you can build your main process in Ladder Logic and write advanced mathematical logic in Structured Text—no restrictions!
Ladder Logic (LAD) in Siemens PLCs
If you’re from an electrical background, Ladder Logic is going to feel like second nature. It’s visually intuitive and looks a lot like electrical relay logic diagrams.
Siemens’ LAD environment is clean, easy to use, and very effective when working with basic to moderately complex logic. You’ll see inputs, outputs, timers, counters—all laid out like a ladder. It’s like reading a circuit diagram but with a twist of programming.
Many technicians and field engineers prefer it because you don’t need to be a software developer to use it effectively. Just understand the logic, and you’re good to go.
So, whether you’re starting your first PLC program or debugging a running machine, LAD gives you clarity at every rung.
Structured Text (ST) and When to Use It
Now, let’s talk about Structured Text. This is where you go full programmer mode. ST is a high-level textual language, similar to Pascal or C.
It’s ideal for:
Complex mathematical operations
Loops (FOR, WHILE)
Conditional branching (IF-THEN-ELSE)
Array and data structure manipulation
If you’re building a batching system, performing PID control, or managing recipe-based logic, ST can make your life easier. You write fewer lines and get more done, faster.
In Siemens’ TIA Portal, you can switch a function block to ST anytime. This flexibility is what makes Siemens powerful. And once you get the hang of it, ST can be your go-to tool for building clean, scalable, and efficient automation code.
Function Block Diagram (FBD) for Modular Design
Function Block Diagram is a fantastic middle-ground. It’s graphical, like Ladder Logic, but more modular and clean—especially when your logic gets bulky.
You connect predefined blocks like timers, logic gates, mathematical operators, and even custom blocks. It’s perfect when you want to build reusable modules.
Let’s say you’re automating a conveyor system with multiple zones. You can build one zone as a function block, test it, and then reuse that same logic across all other zones with minor tweaks.
FBD is also great for teamwork. Different team members can work on different blocks in parallel and integrate later. Siemens makes this easier with user-defined function blocks (UDFBs), which allow teams to maintain large projects in a clean, scalable way.
Choosing the Right Language for Your Project
Now here comes the million-dollar question: Which one should you use?
There’s no fixed rule—but there is a good strategy.
Use LAD if you’re just starting, or your team has a strong electrical background.
Go with FBD for modular projects where logic needs to be reused and visual clarity matters.
Choose ST when dealing with algorithms, advanced control, or when you want compact and powerful code.
The real beauty of Siemens PLCs and the TIA Portal is that they let you use all three. You’re not stuck with one. As your skills grow, so can your code sophistication.
And always remember—your PLC program is not just for the computer. It’s for the people on the shop floor, the maintenance team, and the next engineer who comes after you. So choose the language that offers clarity, reliability, and future-proofing.
Conclusion
Siemens PLCs support multiple programming languages based on the IEC 61131-3 standard, including LAD, FBD, and ST. Each language brings its own strength, catering to different types of logic and user preferences.
For beginners and electricians, Ladder Logic offers an easy-to-understand approach. For modular and visual thinkers, Function Block Diagram shines. And for those diving into complex, data-heavy automation, Structured Text is the powerhouse.
By choosing the right language—or combining them—you can build smarter, faster, and more reliable automation solutions. That’s what makes Siemens PLCs a favorite among engineers worldwide.