Abstract: In-vehicle networks are an inevitable trend in the development of modern automotive electronics technology. This paper systematically analyzes the necessity and application of in-vehicle networks to better understand the next generation of automotive electronic control systems. Keywords: In-vehicle network, vehicle body system, powertrain system, safety. I. Introduction With the rapid development of the automotive industry, modern cars use a large number of electronic control devices. Many mid-to-high-end cars employ a dozen or even twenty electronic control units (ECUs). Each ECU needs to communicate with multiple related sensors and actuators, and information exchange is also required between control units. If each piece of information is transmitted through its own independent data line, it will lead to an increase in the number of pins in the ECU, an increase in the wiring harness and connectors of the entire electronic control system, and an increase in the failure rate, among other problems. To simplify the wiring, improve the communication speed between ECUs, and reduce the failure frequency, a new type of data network, the CAN data bus, has emerged. The CAN bus has strong real-time performance, long transmission distance, and strong anti-electromagnetic interference capabilities. In applications such as automotive engine control components, sensors, and anti-skid systems in the field of automation electronics, the bit rate of CAN can reach up to 1 Mbps. At the same time, it can be used inexpensively in the electrical systems of transportation vehicles. II. Introduction to CAN Bus CAN, short for "Controller Area Network," is a serial communication bus defined by ISO. It is primarily used to exchange information between various electronic control units (ECUs) in a vehicle, forming an in-vehicle network system. The CAN data bus is also known as the CAN-BUS. It offers advantages such as information sharing, reduced wiring, significantly lighter wiring harnesses, minimized control unit and pin counts, and improved reliability and maintainability. CAN is designed as a microcontroller communication system in the automotive environment, exchanging information between various ECUs to form an automotive electronic control network. It uses a microcontroller as the direct control unit for direct control of sensors and actuators. Each microcontroller is a node on the control network. Regardless of the number of ECUs or the data capacity of a vehicle, each ECU only needs two wires connected to a node; these two wires are called the data bus. In a CAN data bus, data transmission is analogous to a conference call. Each user acts as a control unit, "speaking" data into the network. Other users "listen" to the data, utilizing it only when interested, ignoring it when not. Theoretically, an unlimited number of nodes can be connected to a single CAN bus network; however, in practice, the number is limited by the electrical characteristics or latency of the network hardware. The prerequisite for communication using computer networks is that each electronic control unit (ECU) must use and interpret the same "electronic language," called a "protocol." Various transmission protocols are common in automotive computer networks. To facilitate data exchange with numerous control and testing instruments, a standard communication protocol must be established. With the application and promotion of CAN in various fields, Philips Semiconductors developed and released the CAN technical specification (Version 2.0) in September 1991. This technology includes two parts: A and B. Version 2.0A defines the standard CAN message format, while Version 2.0B provides both standard and extended formats. In November 1993, ISO issued the international standard ISO 11898 for road transport vehicles—data exchange—high-speed communication local area networks, paving the way for the standardization and normalization of control area networks. The SAE J1939 standard, proposed in 2000, became the universal standard for control area networks in trucks and buses. [b]III. Composition and Structure of the CAN-BUS Data Bus[/b] The CAN-BUS system mainly includes the following components: CAN controller, CAN transceiver, CAN-BUS data transmission line, and CAN-BUS terminating resistor. : 1. CAN Controller and CAN Transceiver Each control unit on the CAN-BUS has a CAN controller and a CAN transceiver. The CAN controller is mainly used to receive information from the microprocessor, process this information, and transmit it to the CAN transceiver. Simultaneously, the CAN controller also receives data from the CAN transceiver, processes this data, and transmits it to the microprocessor of the control unit. The CAN transceiver receives data from the CAN controller and sends it to the CAN data transmission bus. Simultaneously, the CAN transceiver also receives data from the CAN data bus and transmits it to the CAN controller. 2. Data Bus Termination Resistor: The CAN-BUS data bus is connected at both ends by a termination resistor. This resistor prevents data from echoing back after reaching the line terminal, thus preventing interference with the original data and ensuring correct data transmission. The termination resistor is installed inside the control unit. 3. Data Transmission Bus: Most vehicle models use two bidirectional data lines, divided into a high-order (CAN-H) and a low-order (CAN-L) data line. To prevent external electromagnetic interference and outward radiation, the two data lines are twisted together, requiring a twist at least once every 2.5cm. The potentials on the two lines are opposite, and the sum of their voltages is constant. IV. Application Classification of In-Vehicle Networks In-vehicle networks can be broadly classified into four systems based on their applications: body system, powertrain system, safety system, and information system. 1. Within the powertrain system, the powertrain modules are relatively concentrated and can be fixed in one location, connected to modules located in the engine compartment via a network. A high-speed network is required when connecting the car's main functions—running, stopping, and turning—via a network. The powertrain CAN data bus typically connects three computers: the engine, ABS/EDL, and automatic transmission computers (the powertrain CAN data bus can also connect to airbag, four-wheel drive, and instrument cluster computers). The bus can transmit 10 sets of data simultaneously: 5 sets from the engine computer, 3 sets from the ABS/EDL computer, and 2 sets from the automatic transmission computer. The data bus transmits data at a rate of 500 Kbit/s, with each data set taking approximately 0.25 ms to transmit, and each electronic control unit (ECU) sending data every 7–20 ms. The priority order is ABS/EDL ECU → engine ECU → automatic transmission ECU. In the powertrain system, data transmission should be as fast as possible for timely data utilization; therefore, a high-performance transmitter is needed. A high-speed transmitter accelerates data transmission between ignition systems, allowing received data to be immediately applied to the next ignition pulse. CAN data bus connection points are typically located in the wiring harness outside the control unit. In special cases, the connection point may also be located inside the engine control unit. 2. Compared to the powertrain system, the body system components are located throughout the vehicle. Therefore, the wiring harness is longer and more susceptible to interference. To prevent interference, communication speed should be minimized. In the body system, due to the increased number of human-machine interface modules and nodes, communication speed control is not an issue, but the cost is relatively higher. To address this, cheaper solutions are being explored, and currently, direct-drive buses and auxiliary buses are commonly used. The comfort CAN data bus connection typically connects seven control units, including the central control unit, one controlled unit each at the front and rear of the vehicle, and control units for the four doors. Comfort CAN data transmission has seven main functions: central locking, power windows, lighting switches, air conditioning, instrument cluster, rearview mirror heating, and self-diagnostic functions. The transmission lines of the control units converge at a single point in a star configuration. The advantage of this is that if one control unit fails, the other control units can still send their data. This system reduces the number of wires passing through the doors, simplifying the wiring. If a short circuit to ground, a short circuit to positive terminal, or a short circuit between lines occurs somewhere in the circuit, the CAN system will immediately switch to emergency mode or single-wire mode. The data bus transmits data at a rate of 62.5 Kbit/s, with each data transmission taking approximately 1 ms, and each electronic control unit (ECU) sending data every 20 ms. The priority order is: central control unit → driver's side door control unit → front passenger side door control unit → left rear door control unit → right rear door control unit. Because data in the comfort system can be transmitted at a lower rate, the transmitter performance is lower than that of the powertrain system transmitter. The entire vehicle body system circuit mainly consists of three parts: the main control unit circuit, the controlled unit circuit, and the door control unit circuit. After receiving a switch signal, the main control unit first analyzes and processes it, then sends control commands to each controlled terminal via the CAN bus. Each controlled terminal responds and takes corresponding actions. The front and rear control terminals only receive commands from the main control unit, execute them according to the main control unit's requirements, and feed back the execution results to the main control unit. The gate control unit not only receives instructions from the main control unit via CAN, but also receives switch signals from the car door. Based on the instructions and switch signals, the gate control unit will perform corresponding actions and then send the execution results to the main control unit. (1) Safety system This refers to the system that activates the airbag based on information from multiple sensors. Since the safety system involves human life safety, and there are many airbags and collision sensors in the car, the safety system must have the characteristics of fast communication speed and high communication reliability. (2) Information system Information systems are widely used in cars, such as the application of car telephones and audio systems. The requirements for the communication bus of the information system are: large capacity and very high communication speed. The communication media generally use optical fiber or copper wire because these two media have very fast transmission speeds, which can meet the high-speed requirements of the information system. [b]V. Key Technologies for the Application of CAN Bus Technology in Automobiles[/b] To build an in-vehicle network using CAN bus, the key technical issues that need to be addressed are: (1) Technical issues such as the rate, capacity, priority level, and node capacity of bus information transmission (2) Reliable data transmission under high electromagnetic interference environment (3) Determining the delay size when the maximum transmission is achieved (4) Fault tolerance technology of the network (5) Monitoring and fault diagnosis functions of the network (6) Time characteristics of real-time control network (7) Wiring in installation and maintenance (8) Addition of network nodes and software and hardware updates (scalability) VI. Conclusion As a reliable automotive computer network bus, CAN bus has begun to be applied in advanced automobiles, enabling various automotive computer control units to share all information and resources through CAN bus, so as to simplify wiring, reduce the number of sensors, avoid duplication of control functions, improve system reliability and maintainability, reduce costs, and better match and coordinate various control systems. With the development of automotive electronics technology, the CAN bus communication protocol, which has high flexibility, simple scalability, excellent anti-interference and error correction capabilities, will be more widely used in automotive electronic control systems. References [1] Wang Zhen. [1] Application of CAN bus in automobiles [N]. China Automotive News. 2004. [2] Wu Kuanming. CAN bus principle and application system design. Nanjing University of Aeronautics and Astronautics Press. 1996. [3] Zhou Zhen. Body control module based on CAN bus. 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