Heya! Welcome to Crypto To You. Today on this occasion I am going to share PLC Programming Languages Explained: Ladder Logic, FBD, SCL, and ST.
"Ladder Diagram (LD) is probably the most widely used controller programming language". But it is far from the only one. In fact, the IEC 61131-3 standard (third edition, 2013) defines five first-class programming languages for PLCs—Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC).
Yet many automation engineers—especially those just starting their careers—stick to a single language. This is a mistake. "No. For 90% of tasks, LD + ST is sufficient. FBD is useful for analog signals. SFC is only for batch processes". The key is knowing which language to use for which task.
This comprehensive guide explains the five IEC 61131-3 PLC programming languages. You'll learn the strengths, weaknesses, and best applications for each language—and how mastering multiple languages will make you a more versatile and valuable automation engineer.
📌 THE IEC 61131-3 STANDARD
What Is IEC 61131-3?
IEC 61131-3 is the international standard for PLC programming languages. Before this standard existed, "each PLC manufacturer would have their own dialect for their machines". This meant that a programmer who knew Siemens PLCs had to start from scratch to learn Allen Bradley or Mitsubishi.
The standard defines three graphical languages and two text-based languages:
| Category | Language | Abbreviation |
|---|---|---|
| Graphical | Ladder Diagram | LD (or LAD) |
| Graphical | Function Block Diagram | FBD |
| Graphical | Sequential Function Chart | SFC |
| Text-based | Structured Text | ST (SCL in Siemens) |
| Text-based | Instruction List | IL (deprecated) |
The standard has been widely adopted, making it easier for programmers to move between different PLC brands.
Why Learn Multiple Languages?
"One PLC Language Is Not Enough". Different tasks require different tools. "The job of the controls engineer is to pick the right one per task, encapsulate the hard parts inside reusable blocks, and document the result".
📌 LADDER DIAGRAM (LD / LAD)
What Is Ladder Logic?
"Ladder Diagram is the default graphical language" in most PLC programming environments. It "represents boolean logic as a vertical power rail with horizontal rungs of contacts driving a single output coil".
"Ladder Diagram (LD or simply Ladder) is probably the most widely used controller programming language". In practice, "70% of programs are written in LD".
Key Features
Ladder Logic looks like an electrical schematic: "contacts on the left, coils on the right, logic in the middle". This is why "an engineer who reads electrical drawings can read Ladder without explanation".
Strengths:
"Intuitive for electricians" — "Lowest training cost for technicians familiar with relay logic"
"Excellent for discrete I/O, motor starters, safety circuits, and alarm handling"
Weaknesses:
Best Applications
"Alarm handling, interlock logic, simple start/stop motor control"
"State machines, PID controls and systems with an abundance of Boolean algebra"
When NOT to Use Ladder
"Math-heavy code" — scaling 4-20 mA signals, recipe management, and statistical filtering
"Array and loop handling" — Ladder has no native
FORconstruct"Complex state machines" — "LD forces the engineer to encode the state in bit flags"
"When the logic passes these thresholds, the answer is not 'better ladder' — it is to step sideways into the right tool for that job".
📌 FUNCTION BLOCK DIAGRAM (FBD)
What Is FBD?
"Function Block Diagram or FBD is another popular graphical PLC programming language that consists of various kinds of blocks that have inputs and outputs". "The output of one block can feed into another block's input".
"FBD extends LAD by replacing contacts and coils with function boxes (AND, OR, ADD, MUL, SR, RS, TON, CTU, MOVE, etc.) connected by signal flow lines".
Key Features
Strengths:
"Compact expression of arithmetic, comparison, and PID math"
"Direct mapping to closed-loop control loops (PID, ramp, integrator)"
Weaknesses:
"Customer acceptance varies; some end users insist on LAD because maintenance staff do not interpret FBD cleanly"
Best Applications
"PID control blocks, flow-totalizer math, scaling/linearization"
"Process interlocks with mixed arithmetic and boolean logic"
📌 STRUCTURED TEXT (ST / SCL)
What Is Structured Text?
"Structured Text (ST) is a high level PLC programming language" that "resembles Pascal or C". In Siemens TIA Portal, "ST is called SCL (Structured Control Language). Functionally identical, just a different name".
"Structured text, that's going to look more like your high level language, whether it's C or Python".
Key Features
Strengths:
"Compact code" — "The SCL version is 8 effective lines versus 12 STL statements and 4 LAD networks"
"Variables, FOR/WHILE loops, IF/THEN/ELSE conditions, arrays, structures"
"Very good and powerful for doing things like looping and doing string manipulation"
Weaknesses:
Less intuitive for maintenance electricians
Best Applications
"PID control, recipe management, analog signal processing, communication protocols"
"Large amounts of math or data manipulation"
"Lookup tables and moving a lot of data at once repetitively"
ST vs. Ladder: A Practical Example
For the same logic, "SCL keeps the conditional and the increment in a single coherent block" while Ladder "scatters the logic". This makes ST much more maintainable for complex algorithms.
📌 SEQUENTIAL FUNCTION CHART (SFC)
What Is SFC?
"SFC describes a process as a sequence of steps and transitions". "A step is a set of actions; a transition is the condition for moving to the next step".
Key Features
Strengths:
"Ideal for batch processes, recipe control, and startup sequences"
Clear visualization of sequential processes
Weaknesses:
"Only for batch processes"
Not suitable for all applications
Best Applications
Sequential startup and shutdown sequences
Processes with well-defined steps and transitions
📌 INSTRUCTION LIST (IL) — THE LEGACY LANGUAGE
What Is Instruction List?
"Instruction list is assembly language for PLCs". "Popular in the 1990s when controller memory was measured in kilobytes".
"IL has practically died out — ST replaced it". The IEC 61131-3 third edition "removed IL as a normative language". "Now practically unused since ST does the same thing more readably".
The Verdict: If you're starting your PLC programming journey, skip IL. Focus on Ladder, FBD, and ST/SCL instead.
📌 LANGUAGE SELECTION QUICK GUIDE
| Language | Type | Complexity | Best For | When to Use |
|---|---|---|---|---|
| Ladder (LD) | Graphical | Low | Discrete logic, interlocks, safety | Most discrete control tasks |
| FBD | Graphical | Low-Medium | Analog processing, PID control | Process control, math-heavy tasks |
| ST/SCL | Text | Medium | Math, arrays, recipes, algorithms | Complex calculations, data processing |
| SFC | Graphical | Medium | Sequences, batch processes | Batch control, startup sequences |
| IL | Text | High | Legacy systems | Avoid for new projects |
Source: Compiled from IEC 61131-3 language guides
📌 WHY THIS COURSE: MASTER PLC PROGRAMMING LANGUAGES
Mastering multiple PLC programming languages makes you a more versatile and valuable automation engineer. The Complete Siemens PLC Programming Ladder Logic - All In One course provides comprehensive training on:
Ladder Logic fundamentals and advanced techniques
Function Block Diagram (FBD) for process control
Structured Control Language (SCL) for complex algorithms
Real-world applications and project-based learning
Additional Learning Resources
Siemens TIA PLC Programming: Beginner Course with SCL, Ladder — SCL programming fundamentals
PLC Programming - Learn the basics with CoDeSys — Multi-language PLC programming
Curso Lenguaje de programacion ladder, bfd, awl, scl, grafce — Spanish-language multi-language course
📌 WHO SHOULD TAKE THESE COURSES
Aspiring automation engineers learning PLC programming
Maintenance technicians upgrading their skills
Electrical engineers expanding into controls
Computer science graduates entering industrial automation
Students preparing for careers in automation
📌 FINAL THOUGHTS
"Ladder Diagram (LD) is probably the most widely used controller programming language". But it is not the only one—and it is not the best for every task.
The IEC 61131-3 standard defines five programming languages for PLCs. The smart automation engineer learns multiple languages and uses the right one for each task.
"No. For 90% of tasks, LD + ST is sufficient. FBD is useful for analog signals. SFC is only for batch processes". But for the remaining 10%—complex math, array handling, and advanced algorithms—knowing Structured Text (SCL) is essential.
Start building your multi-language PLC programming skills today with quality training and practice.
📌 AFFILIATE DISCLAIMER
Disclosure: Some of the links in this article are affiliate links. This means I may earn a commission if you click through and make a purchase, at no additional cost to you. I only recommend products and courses that I believe will provide value to my readers. All opinions expressed are my own