One of the most common questions aspiring PLC programmers ask is whether to learn Ladder Logic or Structured Text first. Both are defined in the IEC 61131-3 standard, and both are widely used in industry. The answer depends on your background, your industry, and the complexity of your projects.
Ladder Logic (LAD)
Ladder Logic is the oldest and most widely used PLC programming language. It was designed to mirror electrical relay diagrams, making it intuitive for electricians and maintenance technicians.
Strengths of Ladder Logic:
- Visual and intuitive: Easy to read for anyone with an electrical background
- Excellent for discrete logic: Start-stop circuits, interlocking, sequencing, and basic Boolean operations
- Industry standard: Required knowledge for virtually every PLC engineering role
- Easy troubleshooting: Online monitoring shows power flow through contacts and coils in real time
- Maintenance friendly: Technicians on the factory floor can understand and modify basic logic
Limitations:
- Becomes unwieldy for complex mathematical calculations
- Difficult to implement string handling and data manipulation
- Large programmes can become visually cluttered and hard to navigate
Structured Text (ST)
Structured Text is a high-level programming language similar to Pascal or Python. It is increasingly popular in modern automation projects, particularly for complex process control and data handling.
Strengths of Structured Text:
- Powerful for calculations: Mathematical operations, PID tuning parameters, and recipe management are far easier in ST
- Compact code: Complex logic can be expressed in fewer lines compared to LAD
- Familiar syntax: Engineers with software development experience adapt quickly
- Better for loops and arrays: Handling large data sets and repetitive operations is more efficient
- Modern approach: Aligns with Industry 4.0 and IIoT requirements
Limitations:
- Less intuitive for electricians and maintenance personnel
- Online troubleshooting is less visual than ladder logic
- Not universally supported in older PLC platforms
The Professional Approach
In practice, most experienced PLC engineers use a combination of both languages within the same project:
- Ladder Logic for I/O handling, interlocking, alarm management, and motor control
- Structured Text for calculations, data processing, communication handling, and complex sequencing
Our Recommendation
If you are starting from scratch, begin with Ladder Logic to build a solid foundation in PLC concepts. Then progress to Structured Text to expand your capabilities. At EDWartens UK, our PLC training courses cover both languages in depth, with hands-on projects that demonstrate when and how to use each language effectively.