Siemens PLC programming language - Solutions - Huaqiang Electronic Network

Different industries use various programming languages for PLCs, but within a specific application, only a few are commonly used. In this article, we will focus on the programming languages available in Siemens PLCs to explore their similarities and differences.

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

This is a graphical language used on top of other PLC programming languages to create sequential control programs, such as those used in robotic systems. The process is divided into multiple steps, each with specific actions and conditions that trigger transitions between steps. It's particularly effective for manufacturing processes where a clear sequence of operations is required.

In Siemens STEP7, this is implemented through S7 Graph.

2. Ladder Diagram (LAD - Ladder Diagram)

The most widely used PLC programming language, it resembles relay logic circuits, making it easy for electrical engineers familiar with traditional relay systems to understand. It's ideal for digital logic control applications.

A ladder diagram consists of contacts, coils, and instruction blocks. Contacts represent input conditions, coils represent output results, and instruction blocks are used for functions like timers, counters, or arithmetic operations. The signal flow typically moves from left to right, starting from the main line.

However, it's not well-suited for large-scale or complex programs due to its visual limitations.

3. Statement List (STL - Statement List)

This is a text-based programming language similar to assembly language. It uses lines of code and is more suitable for experienced programmers who need to implement complex functions that may be difficult to achieve with ladder diagrams.

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

Function block diagrams use graphical symbols similar to Boolean algebra to represent control logic. They are especially useful for programmers with a background in digital electronics. Logic operations are shown using blocks like AND or OR gates, with inputs on the left and outputs on the right. A small circle at the input or output indicates an inversion, and signals flow from left to right through connecting wires.

5. Structured Text (ST - Structured Text)

Structured Text is a high-level programming language designed for the IEC61131-3 standard. It allows for more complex mathematical operations and can produce compact, efficient code compared to ladder diagrams. It is similar in structure to C or Pascal, making it ideal for users who are accustomed to high-level programming languages.

In Siemens PLCs, this is implemented through S7SCL, which offers a powerful and flexible approach to automation programming.