Abstract: With the rapid increase of electronic devices in automobiles, in-vehicle electronic networks have emerged. This paper details the characteristics of CAN bus and LIN bus technologies and focuses on the application of CAN/LIN technology in automotive electronic networks. Keywords: CAN; LIN; in-vehicle network; fieldbus 1 Introduction Fieldbus networks can significantly reduce the number of control lines and improve reliability. Therefore, the adoption of fieldbus networks has become an inevitable trend in the development of automotive electronic control system technology. The Automotive Electronics Association (SAE) classifies in-vehicle communication networks into the following four types: ① Type A. Suitable for low-speed environments (<10 kbit/s), used for applications that improve convenience, such as body electronics, including the control of headlights, windows and doors, and the adjustment of electric rearview mirrors; ② Type B. Suitable for medium-speed environments (10-125 kbit/s), used for general information transmission, such as in-vehicle climate control, including air conditioning and heating units; ③ Type C. Suitable for high-speed environments (125 kbit/s-1 Mbit/s), used for real-time control, such as powertrain synthesis; ④ Type D. Suitable for environments with speeds greater than 1 Mbit/s, it is used for multimedia applications such as the Internet, digital television, and real-time hardware functions such as X-by-wire applications. Communication between electronic control units can be achieved by selecting different communication networks based on the requirements of communication bandwidth and speed. Since the electronic control units in the body control section transmit relatively little data and have low real-time requirements, a Class A low-speed communication network is sufficient. The LIN (Cal Interconnect Network) protocol is a multiplexed serial communication protocol for low-end distributed applications in vehicles, conforming to the SAE Class A standard for automotive network communication. Its initial design goal was to serve as a lower-level network for CAN (Controller Area Network), and combined with CAN, it can form a layered network structure in vehicle applications. 2 CAN Bus Technology 2.1 CAN Standard Modern automotive application environments place extremely stringent requirements on the interconnection between various electronic units in automobiles. Therefore, Robert Bosch began developing the "Controller Area Network (CAN)" standard in 1982. The number of electronic systems in automobiles is increasing (such as anti-lock braking, airbags, air conditioning, central locking, engine management, and remote keyless access control). They were initially interconnected in a point-to-point manner, with each connection using independent copper wires, making the connectors very bulky. Therefore, it was necessary to develop a common "network" standard for automotive electronic systems to reduce the number of wires and connection points, and to improve vehicle reliability, facilitate error detection, simplify maintenance processes, and reduce overall weight (improving performance and reducing fuel consumption). The CAN standard, as a bus standard, is characterized by its high speed, low cost, ease of implementation, and ease of use, and can be used in any situation where electronic devices need to be interconnected. 2.2 CAN Principles and Implementation Basic CAN characteristics include the relative logical states between buses and the driver input and receiver output. Bus access is event-driven and occurs randomly; if two nodes attempt to occupy the bus simultaneously, access is implemented through non-destructive bit-by-bit arbitration. The allocation of message priorities in identifiers is a major feature of CAN, making it extremely suitable for real-time control environments. There are four different types or frames of messages that can be transmitted on the CAN bus: ① Data frames are the most common message type, consisting of an arbitration field, a data field, a CRC field, and an acknowledgment field; ② Remote frames are used to request data transmission from another node. Remote frames are similar to data frames, but with two significant differences: firstly, this type of message is explicitly marked as a remote frame by the implicit RTR bit in the arbitration field; secondly, it does not involve data; ③ Error frames are special messages that do not follow the CAN message formatting rules. When a node detects an error in a message, it transmits an error frame, causing all other nodes in the network to also send error frames. The original transmitter then automatically retransmits the message. The CAN controller has a sophisticated error counter system to ensure that nodes do not repeatedly transmit error frames, preventing the bus from being continuously occupied; ④ Overload frames are similar in format to error frames, but are transmitted by excessively busy nodes, primarily used to provide more inter-message delay. Balanced differential signal transmission reduces noise coupling and allows for higher signal transmission rates over twisted-pair cables. Balance means that the current flowing through each signal line is equal but in opposite directions, creating a field cancellation effect, which is key to reducing noise emissions. Using balanced differential receivers and twisted-pair cables improves the common-mode rejection and noise immunity of the CAN bus. The CAN standard defines a communication network connecting all nodes on the bus, enabling them to communicate with each other. A central control node may or may not exist, and nodes can be added at any time, even while the network is operational (hot-plugging). 3 LIN Bus Technology 3.1 LIN Protocol The LIN (Local Interconnect Network) protocol is a multiplexed serial communication protocol for low-end distributed applications in vehicles. The LIN standard includes transmission protocol specifications, transmission media specifications, development tool interfaces, and interfaces for software programming. The LIN specification comprises three main parts: ① The LIN protocol specification describes the LIN physical layer and data link layer; ② The LIN configuration language description defines the format of LIN configuration files, which are used to configure the entire network and serve as a common interface between OEMs and different network node vendors, while also acting as an input for development and analysis tools; ③ The LIN API section describes the interface between the network and applications, enabling seamless connectivity between development and design tools, improving development speed, and enhancing network reliability. LIN bus features include: ① Low-cost single-wire 12V data transmission, with drive and receive characteristics conforming to the improved ISO 9141 single-wire standard; ② Transmission rates up to 20 kbit/s; ③ A single-master/multiple-slave architecture, eliminating the need for bus arbitration, with the master node controlling bus access; ④ A hardware interface based on the universal UART/SCI, implemented using low-cost semiconductor processes, with almost all microcontrollers having the necessary LIN hardware; ⑤ Slave nodes can achieve self-synchronization without crystal or ceramic oscillators, reducing slave node hardware costs; ⑥ Guaranteeing worst-case signal transmission latency, avoiding bus access conflicts. 3.2 LIN Principles and Operating Mechanism The LIN protocol was designed to allow for the addition of new slave nodes without requiring changes to the existing software and hardware. The LIN protocol provides internal error detection for node malfunctions and transmission failures. Each data transmission is initialized by the single master node in the system; slave nodes only respond when the master node accesses the data, thus avoiding arbitration in network transmissions. Slave nodes use an internal resynchronization mode to lock onto the master node during data transmission and reception. Slave nodes require only inexpensive oscillators for timing, simplifying circuitry and reducing costs. In a LIN network, each node has a slave task module, and the master node also contains a master task module. All communication in a LIN network is initiated by the master task. The master task sends a frame header to initiate communication in the network. The frame header contains three parts: a synchronization gap, a synchronization field, and an information identifier. The slave task uses the information identifier to determine whether to respond to the master task. If a response is required, it begins sending a response message. The response message consists of 2, 4, or 8 data bytes and 1 checksum byte. A LIN data frame consists of a frame header and a response message group. Another frame in the LIN bus is the sleep frame, issued by the master task, which puts the bus and nodes into a low-power state. The biggest advantage of the LIN protocol is that it doesn't require a dedicated hardware controller; it simply adds a set of C language API functions to the SCI interface. When developing LIN applications, users don't directly read or write to the microcontroller's internal registers; they only need to call the corresponding API to perform the same operations. Currently, there are two types of API functions: the LIN API and the Motorola API. The LIN API has 20 functions that can perform functions including system initialization, data reading and writing, node online/offline operation, and interrupt masking. The Motorola API is entirely based on the information frame mode, operating on a frame of information through LIN-Get.Msg and LIN-PutMsg. The information identifier for each node is configured during compilation via header files, requiring no other auxiliary tools. 4. Application of CAN/LIN Technology in Vehicles Due to the complex environment inside a car, characterized by extremely high electrical noise, mechanical stress, temperature, and flammability, any error in network connectivity can have serious consequences, even endangering lives. Therefore, the CAN standard provides the following guarantees for data communication: reliable data communication, employing CRC checksum and a unique data signal representation method, with error identification and automatic retransmission functions; real-time data communication, high data transmission rate, and high-priority data having priority in occupying the bus; flexible data communication, multi-master bus structure, direct communication between bus nodes, data block-oriented communication method, and communication media that can be twisted pair, coaxial cable, or fiber optic, easily connected through standard connectors. LIN is a protocol for low-end communication, mainly used in applications with low communication rates. LIN was initially proposed for automotive applications, primarily for integrating body electronic control units, connecting intelligent actuators and sensors to the main body network, etc. Achieving communication between nodes through a single wire greatly reduces the complexity of automotive body wiring, which is the potential of LIN applications. LIN's initial design goal was to serve as a lower-layer network for CAN, combining with CAN to form a layered network structure for the vehicle. The main body network of the entire vehicle is structured by CAN, while LIN is used for local communication. The structural diagram of the in-vehicle CAN/LIN bus application is shown in Figure 1. [align=center]Figure 1. In-vehicle bus application structure diagram[/align] In the design of automotive electronic control units, safety should be the primary consideration. Therefore, for critical control components such as the engine, automatic transmission, braking system, and airbags, distributed control should be adopted. For units unrelated to safety, such as door and window, steering wheel, seats, air conditioning, lighting, humidity sensors, and alternator control units, a centralized control method combining CAN and LIN can be used for cost reasons. For these cost-sensitive units, LIN allows for the widespread application of mechanical components such as smart sensors, brakes, or photosensitive devices. These devices can be easily connected to the automotive network and are conveniently maintained and serviced. In LIN-implemented systems, analog signals are typically replaced with digital signals, which optimizes bus performance. 5. Conclusion CAN bus is a serial data communication bus developed for automobiles. Currently, some automakers worldwide are actively using CAN bus in new vehicles. Moreover, due to its inherent advantages, its application scope is not limited to the automotive industry but is also developing in fields such as mechanical manufacturing and robotics. The application of LIN bus in vehicle body control systems reduces the cost of low-end communication networks while increasing design flexibility and improving system reliability, which is beneficial for the development of distributed vehicle body control systems. Therefore, LIN bus has good application prospects in the automotive field. In addition, as an open protocol, LIN can also be used in industrial and home appliance fields to complete communication tasks between devices with low speed requirements and short connection distances.