Siemens PLC programming language - Solutions - Huaqiang Electronic Network

Different industries use various programming languages for PLCs, but for a specific application, the number of programming languages used is usually limited. In this article, we will take Siemens PLC as an example to explore the similarities and differences among common PLC programming languages.

1. Sequential Function Chart (SFC - Sequential Function Chart)

This is a graphical language that builds on other programming languages, specifically designed for sequential control programs such as those in robotics or automated production lines. The process is divided into several steps, each with defined actions and transition conditions that determine when one step moves to the next. It's particularly useful in manufacturing and batch processing environments.

In Siemens STEP7, this is implemented through S7 Graph, which allows users to design complex sequences in a visual and intuitive way.

2. Ladder Diagram (LAD - Ladder Diagram)

The most commonly used PLC programming language, the ladder diagram closely resembles relay logic diagrams, making it easy for electricians familiar with traditional relay systems to understand and use. It is especially effective for digital logic control applications.

A ladder diagram consists of contacts, coils, and instruction blocks. Contacts represent input conditions, coils represent output actions, and instruction blocks are used for timers, counters, and mathematical operations. The signal flow typically moves from left to right, with the main power rail on the left side.

However, it may not be the best choice for large-scale or complex programs due to its visual nature and limitations in handling advanced logic structures.

3. Statement List (STL - Statement List)

This is a text-based programming language similar to assembly language, offering a more direct and compact way to write PLC programs. It’s often used by experienced developers who prefer a more code-centric approach. STL allows for greater flexibility and can implement functions that are difficult or impossible to achieve with ladder diagrams.

4. Function Block Diagram (FBD - Function Block Diagram)

Function block diagrams use graphical symbols similar to Boolean logic to represent control processes. They are ideal for engineers with a background in digital electronics. Each block represents a function, with inputs on the left and outputs on the right. A small circle indicates negation, and signals flow from left to right through connecting lines.

5. Structured Text (ST - Structured Text)

Structured Text is a high-level programming language designed for IEC61131-3 standard PLCs. It supports complex mathematical operations and offers a more concise and efficient way to write programs compared to ladder diagrams. Its syntax is similar to C and Pascal, making it accessible to programmers who are familiar with high-level languages.

In Siemens, this is implemented through S7 SCL, which provides a powerful and flexible environment for developing complex control systems.