Structured text programming gives engineers a clear, text-based method for controlling machines. The language uses readable statements and familiar control structures. It lets developers write logic that runs on PLCs and controllers. This article explains core concepts, modular design, examples, and practical tips.
Key Takeaways
- Structured text programming is a high-level, readable IEC 61131-3 language ideal for PLCs and embedded controllers, enabling compact logic for sequencing, math, and safety tasks.
- Follow IEC 61131-3 conventions—use strong typing (BOOL, INT, REAL, STRING), VAR blocks, and consistent naming (DI_, DO_, AI_, AO_) to improve portability and reduce runtime errors.
- Modularize code with small, testable functions for pure logic and stateful function blocks for controllers or timers, instantiating blocks like objects to preserve state across cycles.
- Implement clear control flow (IF, CASE, FOR, WHILE, REPEAT) and common patterns—latches for interlocks, ENUM+CASE state machines, and function-block PID loops—for readable, debuggable logic.
- Adopt unit testing, simulation, version control, and performance safeguards (limit heavy math in main cycle, use slower background tasks, validate safety logic) to speed troubleshooting and ensure safe operation.
What Is Structured Text?
Structured text programming is a high-level language for industrial control. It uses statements similar to Pascal and BASIC. Engineers use it to express logic in a compact, readable form. The language targets controllers, PLCs, and embedded devices that follow industrial standards.
History And Standards (IEC 61131-3)
IEC 61131-3 defines the standard for programmable controller languages. The standard lists structured text programming as one of five languages. It specifies syntax, data types, and program organization. Vendors carry out the standard with minor extensions. The standard gives teams a common baseline for code portability.
Where It’s Used (PLCs, Industrial Automation, Embedded Controllers)
Manufacturers use structured text programming in process, discrete, and hybrid control. Systems engineers use it for sequencing, math, and algorithms. Process plants use it for recipe and batch logic. Machine builders use it for motion and safety-related tasks. Embedded controller projects use it when text is clearer than ladder diagrams.
Core Language Concepts
Structured text programming centers on types, expressions, and control flow. The language supports strong typing and clear scoping. The next subsections show common building blocks and examples.
Data Types And Variables
The language defines basic types: BOOL, INT, DINT, REAL, and STRING. The standard also defines arrays, structures, and user types. Developers declare variables in VAR blocks. The compiler checks types at build time. Clear types reduce runtime errors.
Expressions And Operators
Expressions use arithmetic, logical, and bitwise operators. Engineers write math using +, -, *, and /. They use AND, OR, NOT for boolean logic. They use comparisons like =, <>, >, and <=. The language evaluates expressions left to right when needed. Functions and function blocks return typed values.
Control Flow: IF, CASE, FOR, WHILE, REPEAT
The language offers IF and CASE for branching. It offers FOR, WHILE, and REPEAT for loops. Engineers use IF for simple branches. They use CASE for multi-way selection. They use FOR for counted loops. They use WHILE for condition-driven loops. They use REPEAT for post-test loops. Structured text programming keeps flow clear with these structures.
Modularization: Functions, Function Blocks, And Programs
Structured text programming supports modular code units. Functions, function blocks, and programs provide roles that match design needs. Developers pick the unit type based on state and reuse.
Defining And Using Functions
Functions return a value and do not keep state between calls. Developers define inputs and a single return variable. The compiler calls the function like f(x, y). Functions suit math, conversions, and pure logic. Teams keep functions small and testable.
Function Blocks Vs. Objects: State And Instantiation
Function blocks keep internal state. Engineers instantiate function blocks like objects. Each instance stores its data. Developers use function blocks for PID controllers, timers, and motor drives. Function blocks let the code manage resources and state across cycles. Structured text programming integrates blocks with global and local scopes.
Practical Examples And Common Patterns
This section gives short, practical patterns that engineers reuse. The examples use clear logic and common idioms. They show how structured text programming solves routine tasks.
Simple Control Logic Example (Start/Stop, Interlocks)
The program checks start and stop inputs. It sets an output when conditions pass. An interlock prevents start if a fault exists. The code uses IF for conditions and a latch pattern to hold the output. Engineers protect outputs with explicit fault checks.
State Machine Implementation Pattern
The pattern stores the current state in an ENUM variable. The main cycle uses CASE on the state value. Each CASE branch handles one state and sets the next state when ready. Developers add timers and guards to avoid rapid transitions. This approach makes logic clear and debuggable.
Basic PID/Control Loop Structure In Structured Text
Developers carry out PID as a function block with PV, SP, and output. The block stores the integral term and previous error. The controller computes error, updates integral, applies limits, and writes output. Engineers tune gains offline or with a simple autotune routine. Structured text programming supports deterministic updates for control loops.
Best Practices And Debugging Tips
This section lists practical rules that teams can apply. The rules aim to reduce bugs and speed troubleshooting. Engineers follow them on most projects.
Naming Conventions, Comments, And Documentation
Teams use clear names for variables and functions. They prefix I/O with a consistent tag like DI_, DO_, AI_, AO_. They group related variables in structures. They add short comments that explain intent, not restate code. They keep documentation up to date with code changes.
Testing, Simulation, And Version Control
Engineers write unit tests for functions and blocks. They run simulation where the controller supports it. They test safety logic in a safe lab environment. Teams use version control for source code and program configurations. They tag releases and keep change logs.
Performance Considerations And Safety Practices
Developers avoid heavy math in the main cycle when the controller has limited CPU. They move noncritical tasks to slower cycles or background tasks. They bound loop times and watch for watchdog resets. They design safety logic with hardware interlocks and fail-safe defaults. They validate safety code with formal tests and peer review.
