Structured Text

Structured Text uses familiar programming constructs that will be recognizable to anyone with software development experience:

Variables and Data Types:
- BOOL: Boolean values (TRUE/FALSE)
- INT, DINT, REAL: Integer and floating-point numbers
- STRING: Text data for messages and recipes
- ARRAY: Collections of same-type elements
- STRUCT: User-defined composite data types

Operators:
- Arithmetic: +, -, *, /, MOD (modulo)
- Comparison: =, <>, <, >, <=, >=
- Logical: AND, OR, XOR, NOT
- Assignment: := (not = like in some languages)

Basic Syntax Example:
```
IF Sensor_1 AND NOT Fault_Active THEN
Motor_Speed := Set_Point * 0.85;
Motor_Run := TRUE;
ELSE
Motor_Run := FALSE;
Motor_Speed := 0.0;
END_IF;
```

Understanding proper syntax, including statement terminators (semicolons) and case sensitivity rules (varies by platform), is essential for writing error-free code.

Structured Text provides robust control structures for implementing complex logic:

Conditional Statements:
- IF-THEN-ELSE: Basic conditional execution
- CASE: Multi-branch selection based on a variable's value

Loop Structures:
- FOR: Execute a block a specific number of times
- WHILE: Repeat while a condition remains true
- REPEAT-UNTIL: Execute at least once, then check condition

Example - Recipe Selection with CASE:
```
CASE Recipe_Number OF
1: Mix_Time := 300; Temperature := 150.0;
2: Mix_Time := 450; Temperature := 175.0;
3: Mix_Time := 600; Temperature := 200.0;
ELSE
Mix_Time := 0; Temperature := 0.0;
Alarm_Invalid_Recipe := TRUE;
END_CASE;
```

Example - Averaging with FOR Loop:
```
Sum := 0.0;
FOR i := 1 TO 10 DO
Sum := Sum + Sensor_Readings[i];
END_FOR;
Average := Sum / 10.0;
```

These structures enable implementation of algorithms that would require dozens of ladder rungs, making code more readable and maintainable.

Structured Text excels in applications requiring sophisticated data handling:

PID Control Implementation:
Writing custom PID (Proportional-Integral-Derivative) control algorithms allows fine-tuning beyond standard function blocks. This is valuable in temperature control, flow regulation, and motion systems.

Recipe and Batch Management:
Structured Text efficiently handles multi-parameter recipes, storing configurations in arrays or structures and applying them dynamically during production changeovers.

Data Logging and Statistics:
Calculate running averages, standard deviations, and process capability metrics in real-time. Store historical data for quality analysis and regulatory compliance.

Communication Protocols:
Parse and construct messages for serial communication, Modbus, or custom protocols. Handle string manipulation for barcode data, RFID tags, and database integration.

Motion Control:
Implement complex motion profiles, cam tables, and coordinated multi-axis movements. Calculate trajectories and interpolate positions for smooth operation.

Integration with Other Languages:
Most modern PLCs allow mixing languages within a project. Use Structured Text for calculations within a ladder logic program, or call ladder subroutines from Structured Text main programs. This hybrid approach leverages each language's strengths.

Structured Text proficiency is increasingly valued across industries:

Process Industries:
Chemical, pharmaceutical, and food processing rely on Structured Text for batch control, recipe management, and regulatory compliance (FDA 21 CFR Part 11). These industries often pay premium salaries for programmers with strong ST skills.

Packaging and Converting:
High-speed packaging lines use Structured Text for registration control, cut-length calculations, and quality statistics. The mathematical precision required makes ST the natural choice.

Water and Wastewater:
Treatment plants use ST for flow calculations, chemical dosing algorithms, and SCADA integration. Environmental compliance requires accurate data logging and reporting.

Career Advancement:
Adding Structured Text to your skillset can increase earning potential by 15-25% compared to ladder-logic-only programmers. Senior controls engineers and automation architects are expected to be proficient in multiple IEC 61131-3 languages.

Certifications:
Several organizations offer certifications that validate Structured Text competency:
- Rockwell Automation certifications
- Siemens SITRAIN programs
- ISA (International Society of Automation) credentials

These certifications demonstrate expertise to employers and can accelerate career progression.