1. Introduction Since its introduction in the 1960s, the Programmable Logic Controller (PLC) has maintained an annual sales growth rate of over 25% for more than 30 years, and has been developing towards higher functionality, higher speed, larger capacity, and networking, with increasingly sophisticated programming languages. To adapt to this development, the International Electrotechnical Commission (IEC) has revised the PLC standard multiple times. The IEC 1131 standard for programmable logic controllers mainly specifies the definition, vocabulary, PLC hardware structure, operating environment, electrical requirements, mechanical requirements, necessary information to be provided by the manufacturer, testing and verification methods, programming languages, user guides, and information communication aspects. However, it does not specify the electrical graphic symbols for PLCs. The IEC 617 standard, "Graphical Symbols for Electrical Diagrams," also does not provide specific regulations on the electrical graphic symbols for PLCs, and the corresponding Chinese national standard, GB/T 4728, "Graphical Symbols for Electrical Diagrams," does not mention them either. With the increasing number of application systems for programmable logic controllers (PLCs), this article discusses how to draw electrical diagrams for PLCs using existing national standards . II. Types of Electrical Diagrams for PLC Control Systems The main types of electrical diagrams for PLC control systems include schematic diagrams, sequence diagrams, timing diagrams, program diagrams, circuit diagrams, and wiring diagrams. Schematic diagrams represent relatively simple diagrams showing the main relationships and connections between various items in a system, subsystem, device, component, equipment, and software. Sequence diagrams represent the working order or state of each unit in the system, with the working or state of each unit arranged in one direction and the process steps or times drawn at right angles on the diagram. Sequence diagrams are most widely used in programmable step sequence control, such as in automated processing lines. Timing diagrams are sequential diagrams drawn to scale with a timeline. Program diagrams are simplified diagrams that detail program units, modules, and their interconnections. Circuit diagrams are simplified diagrams representing the actual circuits of a system, subsystem, device, component, equipment, and software, using graphic symbols arranged by function to represent each component and its connection relationships. Wiring diagrams are simplified diagrams showing or listing the connection relationships of a device or equipment. 3. Drawing of common electrical diagrams for programmable controller control systems 1. Drawing of programmable controllers According to the International Electrotechnical Commission IEC1131 standard for programmable controllers, the programming language of programmable controllers can be either text-based or graphical. Text-based languages ​​include list-based and structured text-based languages, while graphical languages ​​include ladder diagrams and flowcharts. For text-based programming languages, each PLC manufacturer has specific regulations, which will not be discussed here. However, graphical languages ​​have their own characteristics, which should be noted when drawing them. (1) Drawing of ladder diagrams Ladder diagrams are a unique programming language for programmable controllers. They still use relay symbols from relay logic control, such as normally open contacts, normally closed contacts, and coils. The arrangement method is also similar to that of relay control circuit diagrams. Although different manufacturers use different instruction mnemonics, the graphic symbols of relays are the same. For example, normally open contacts are represented by the symbol 1, normally closed contacts by the symbol {, and coils by the symbol 0. "Relay in programmable controller" is a figurative term used in programming. It is represented by the state of the corresponding bit in the memory and is not an actual electrical component. There is no such graphic symbol in the national standard. For example, if a ladder diagram is drawn using PROTEL98, there is no such symbol in the PROTEL component library. A separate library must be created. In addition, for complex arithmetic operations, timing and counting, PID regulators and other functions, block diagrams are generally used, with different symbols marked and input/output parameter ports provided. Figure 1 is a ladder diagram of a Mitsubishi F1 series containing timing and counting functions. (2) Flowchart drawing PLC control flowcharts generally use logic diagrams. Logic diagrams, also known as logic function diagrams, are function diagrams that mainly use binary logic element symbols. Programming with logic diagrams makes the logical relationships very clear. It is mainly used in large and medium-sized PLCs. For example, Siemens S5 series PLCs use this programming language. Although logic diagrams represent the logical relationships between quantities in the program and are a software implementation method, there are no actual binary logic elements. However, when designing logic diagrams, we choose the national standard GB/T4728 12 1996. The logic symbols in "Graphic Symbols and Binary Symbols for Electrical Diagrams" are shown in the attached table. When drawing logic diagrams using PROTEL98, if the circuit diagram component library uses national standard symbols, the symbols in the library can be directly called. If they do not conform to the national standard, a new library must be created according to the national standard. Furthermore, when drawing logic diagrams, they must conform to the national standard GB/T6988-1997 "Preparation of Documents for Electrical Engineering". Figure 2 shows the logic diagram of a Siemens S5 series PLC. 2. Circuit Diagram Drawing: A programmable logic controller (PLC) control system circuit diagram typically includes input/output sections and the PLC itself. Input sections usually refer to various buttons or switches and various switching information on the controlled object, such as limit switches and photoelectric tube signals. Output sections include various electromagnetic coils, contactors for connecting motors, indicator lights, and other actuators. National standards specify graphic symbols for these electrical components, and these symbols must be used when drawing circuit diagrams. However, national standards do not specify graphic symbols for PLCs. Therefore, when drawing circuit diagrams, we must design the graphic symbols for PLCs according to the design rules for graphic symbols. The design principles for electrical graphic symbols are: ① The content of the graphic symbol should be functional and specific information. ② The shape of the graphic symbol should be simple, easy to understand, easy to repeat, easy to associate with its meaning, and easy to remember. ③ The meaning of the graphic symbol should be normally identifiable through the preceding and following content; if not, additional information should be provided. The line width to module ratio of the graphic symbol is 1:10. For different line widths… The line width ratio should be 1:2. The terminals of the lines should be placed at the net points. ⑤ The minimum spacing between parallel lines should be at least twice the width of the widest line. If text is written between parallel lines, the distance between lines should be at least 2m. ⑥ Text description: The font should be standard B-type upright. Text should be written horizontally and vertically. When text is part of a graphic symbol, it should be placed at the top or center. Input/output text should be placed in the input/output position. The distance between text and symbol should be at least twice the width of the thickest line. ⑦ There are no special requirements for the size of the graphic symbol. Based on the above design rules and the distribution of PLC input/output terminals, the PLC symbol should be designed as a rectangle, with input terminals at the top and output terminals at the bottom, and the text numbers of the input/output terminals should be marked in the corresponding positions. Considering that the input/output numbers of PLCs from different manufacturers are different, the above design symbols cannot be used as universal symbols for PLCs. Graphic symbols for PLCs from different manufacturers should be designed independently according to the above principles. Figure 3 shows the circuit diagram of the Mitsubishi F1-40MR programmable controller control system. 3. Wiring Diagram Drawing: The national standard GB/T6988.3-1997, "Electrical Technical Documentation: Wiring Diagrams and Tables," stipulates that components in wiring diagrams should be represented by simple outlines such as squares, rectangles, or circles, or by simplified graphics. Graphical symbols specified in GB4728 may also be used. Terminals should be clearly indicated, but terminal symbols are not required. Therefore, when drawing wiring diagrams for PLC control systems, the representation of PLC components directly adopts the graphic symbols from the circuit diagram. Other drawing rules are the same as for general electrical control systems. It is also worth noting that when the programmable controller uses a large number of modules or units, their actual placement must be considered. Units can be placed horizontally or vertically. When units are placed horizontally, there should be no wire ducts or distribution cables passing between the CPU unit and the I/O unit.