PLC LANGUAGES
PLC Programming Languages: Which One to Learn First in 2026
Ladder, structured text, FBD, SFC and the retired Instruction List: where each one turns up at work, what each vendor calls it, and a sensible learning order.
The PLC programming languages are those defined in IEC 61131-3: Ladder Diagram, Structured Text, Function Block Diagram and Sequential Function Chart, plus the older Instruction List you will still meet in legacy code. Learn ladder first, because it is what you will most often read on a machine, then structured text. Learn to read Instruction List, not write it.
In this lesson by plcgoods, the same Factory I/O project is built in ladder, function block diagram and structured text in CODESYS, which is the fastest way to see how the three differ. Follow along, then check your understanding against the notes below.
The PLC programming languages, one paragraph each
Ladder Diagram (LD) draws logic as rungs between two power rails, with contacts for conditions and coils for outputs. It came from relay panels, which is why electricians and maintenance technicians read it easily. It is at its best for interlocks, permissives, seal-in circuits and simple timers.
Structured Text (ST) is a Pascal-like text language with IF, CASE, FOR, WHILE and REPEAT. It is at its best for arithmetic, arrays, recipes, string handling and state machines, where ladder becomes a wall of rungs.
Function Block Diagram (FBD) wires blocks together like a signal-flow drawing. Process and DCS engineers use it for analogue scaling, alarms and PID loops, because you can follow a signal from input to output across the page.
Sequential Function Chart (SFC) describes a process as steps and transitions: fill, heat, hold, drain. Each step's actions are written in one of the other languages. It suits batch plants and machines with a clear sequence.
Instruction List (IL) is a low-level, assembly-like list of instructions. The third edition of IEC 61131-3, published in 2013, marked it as deprecated, and the fourth edition, IEC 61131-3:2025, describes only structured text, ladder, FBD and SFC. You will still meet IL, or Siemens' similar STL, in older programs.
What each vendor calls them
The standard gives the languages their names, but every vendor's software uses its own. Knowing the mapping saves confusion in job adverts and manuals. PLCopen, the vendor-independent association built around IEC 61131-3, is a good starting point if you want to read about the standard itself.
- Siemens TIA Portal: LAD (ladder), FBD, SCL (structured text), GRAPH (SFC). STL, Siemens' instruction list, is available on the S7-300, S7-400 and S7-1500, not on the S7-1200.
- Rockwell Studio 5000: Ladder Diagram, Function Block Diagram, Structured Text and Sequential Function Chart, all inside one Logix project.
- CODESYS and the brands built on it: the four current languages plus IL, which the CODESYS Development System marks as obsolete but lets you enable, and CFC (Continuous Function Chart), a free-placement version of FBD. The development system itself is free.
- Mitsubishi GX Works3: ladder, structured text, FBD/LD and SFC.
The ideas carry over between brands far better than the menus do. A timer is a timer: once you understand an on-delay timer in one tool, the others are a matter of finding it.
The same job in ladder and in structured text
Take a simple job: count bottles passing a sensor, stop the conveyor at 24, and reset when an operator presses a button.
In ladder, one rung feeds the sensor into a CTU up-counter with a preset of 24. A second rung uses the counter's done bit to open a contact in the conveyor's run circuit. A third puts the reset button on the counter's reset input. Three short rungs, and anyone standing at the machine can see which one is stopping the belt.
In structured text, you detect the sensor's rising edge with an R_TRIG block, add one to a counter variable when it fires, and write an IF statement that clears the run command once the count reaches 24. Reset is another IF. It is about the same length, but it scales far better: changing to a box size read from a recipe, or counting three lanes in an array, is a few lines of ST and a lot of copied rungs in ladder.
That is the pattern with most tasks. Ladder wins for visibility; structured text wins once there is data, maths or repetition. To start writing it, follow our structured text tutorial for beginners, and for the counter itself see PLC counters: CTU, CTD and CTUD.
Which language does your target job use?
- Maintenance and service technician: ladder, read online while the machine runs. Being able to follow a rung and force nothing you do not understand is the core skill.
- Machine builder or OEM programmer: ladder for the safety and I/O layers, structured text for motion, recipes and reusable function blocks.
- Process, water and oil and gas: FBD and SFC, often inside a DCS rather than a PLC, alongside ladder for packaged units.
- Systems integrator: all of them, often in the same project, and the discipline to keep each language where it is strongest.
A learning order that works
Start with ladder on a simulator and write the classic exercises: start/stop with a seal-in (our start/stop ladder logic walkthrough explains it rung by rung), a traffic light, a conveyor with a counter. Then rewrite the same programs in structured text. Rewriting something you already understand is the quickest way into a new language, because you only have to learn the syntax, not the problem.
After that, use structured text on its own for something ladder handles badly, such as averaging ten readings in an array. Add FBD when you reach analogue signals and PID, and SFC when you have a machine with obvious steps. Leave Instruction List for last and learn to read it, since you may inherit it but should not write new code in it.
For how long each stage takes, see how long it takes to learn PLC programming. For a vendor-neutral place to practise every language, including CFC, see CODESYS for free.
Common mistakes when switching languages
- Writing ladder in structured text. A hundred IF statements that each set one bit is ladder with extra typing. Use CASE for state machines and arrays for repeated equipment.
- Forgetting the scan. Every language runs inside the same scan cycle. A FOR loop that runs ten thousand times will stretch the cycle time and can trip the watchdog.
- Mixing languages without a rule. Decide per project which language does what, for example ladder for I/O and interlocks, ST inside function blocks. Your colleagues will thank you.
- Losing the edge. In ST, a condition that is true for many scans fires every scan. Use R_TRIG or F_TRIG where you mean "once, when it changes".
- Trusting generated code blindly. AI assistants now write structured text in several vendor tools; our look at AI for PLC programming covers what they do well and why you still have to read every line.
Practise the languages free
Siemens TIA Portal takes you from zero to ladder programs on a simulated S7-1200, and Siemens SCL Structured Text in TIA Portal moves the same controller into text. CODESYS and IEC 61131-3 Programming covers every standard language on a free soft PLC, and Allen-Bradley Studio 5000 shows the Rockwell side. All of our free PLC courses are free to learn in full: video lessons by independent creators, written notes, a practice task per module and one final assessment. If you pass and want a certificate, it is a small one-off fee, US$8.99 for a beginner course. It is an EDWartens Certificate of Completion with a verification code, not a Siemens or Rockwell certification.
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Questions
Which PLC programming language should I learn first?
Ladder logic. It is the language you will most often be asked to read on a machine, and its rungs make scan-based thinking visible. Move to structured text once start/stop, timers and counters feel routine.
How many PLC programming languages are there?
Four in the current edition: Ladder Diagram, Function Block Diagram, Structured Text and Sequential Function Chart. Older editions also had Instruction List; the 2013 edition deprecated it, and the fourth edition, published in May 2025, no longer lists it.
Is structured text replacing ladder logic?
No. Structured text is growing for maths, data handling and reusable function blocks, but ladder stays common for discrete machine logic because maintenance staff can read it online while the machine runs. Most real projects mix both.
Is Siemens SCL the same as structured text?
SCL is Siemens' implementation of IEC 61131-3 structured text, with some Siemens-specific extensions. If you can write ST in CODESYS or Studio 5000, SCL will feel familiar within a day.
Can I learn PLC languages without buying a PLC?
Yes. Vendor trial software with a simulator, such as TIA Portal with PLCSIM, or the free CODESYS development system with its soft PLC, lets you write and run all of these languages on a laptop.
Sources
- IEC: IEC 61131-3:2025, Programmable controllers, Part 3: Programming languages
- IEC: IEC 61131-3:2013 (third edition)
- PLCopen: IEC 61131-3
- CODESYS Store: CODESYS Development System
Written by the EDWartens engineering team for general education. Product names are trademarks of their owners; mentioning them does not imply endorsement. Prices and terms of other providers were checked on the date shown and can change.
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