Abstract: With new energy vehicles being included in the seven strategic emerging industries that the country is accelerating the cultivation and development of, the intelligentization, digital networking, and energy conservation of automobiles have become the major directions of automotive development. The automotive bus is the foundation for realizing digital networking. This article will discuss the four major automotive buses currently in use: CAN, LIN, Flexray, and MOST.
Today, society has entered the information age. People hope that cars are not just a means of transportation, but also an extension of their life and work, so that they can make phone calls, go online, entertain themselves, and work as if they were in their own office or home.
The increasing functionality has led to a dramatic increase in the number of electronic devices in automobiles, resulting in the emergence of various automotive buses . The most familiar automotive bus is CAN, while LIN and Flexray may be less familiar. Therefore, we will introduce these four automotive buses in the following sections.
Automotive bus is the communication network that interconnects the underlying automotive devices or instruments in an in-vehicle network. Currently, there are four main types of automotive buses: CAN bus, LIN bus, FlexRay bus, and MOST bus.
Use a table to illustrate the differences between various buses.
I. The Birth of the Automotive Bus
The development of automotive bus technology is inseparable from the advancement of automotive electronics. The degree of automotive electronics integration is also considered an important indicator of the level of modern automobiles. Traditional automotive electronics mostly use point-to-point single communication methods with little interaction between components, inevitably leading to a massive wiring system. Statistics show that in a high-end car using traditional wiring methods, the wire length can reach 2000 meters, and the number of electrical nodes can reach 1500, and this number roughly doubles every 10 years.
This further exacerbates the conflict between bulky wiring harnesses and the limited available space in a car. Traditional wiring methods are unsuitable for modern automobiles, both in terms of material costs and efficiency. Furthermore, to meet the real-time requirements of various electronic systems, common automotive data (such as engine speed, wheel speed, and throttle pedal position) must be shared, while each control unit has different real-time requirements. Therefore, traditional electrical networks are no longer adequate for the development of modern automotive electronic systems, leading to the emergence of new automotive bus technologies.
II. CAN Bus
CAN bus, also known as automotive bus, stands for "Controller Area Network," a serial communication network that effectively supports distributed and real-time control. It connects individual control units in a certain form (mostly star topology) to form a complete system.
The CAN bus was originally developed by the German company Bosch to solve the data exchange problem between numerous electronic control modules (ECUs) in modern automobiles. It is now widely used in automotive electronic systems, becoming the main industry standard in the European automotive manufacturing industry and representing the mainstream development trend of automotive electronic control networks.
Many world-renowned car manufacturers, such as Volkswagen, Benz, BMW, Porsche, and Rolls-Royce, have adopted CAN bus to realize data communication in the internal control system of automobiles.
III. LIN Bus
LIN is a new, low-cost, open serial communication protocol jointly developed by well-known companies such as Motorola and Audi. It is primarily used in in-vehicle distributed electronic control systems, especially for digital communication applications involving intelligent sensors or actuators. Its main applications include the control of power windows, seat adjustments, and lighting.
A typical LIN network can have up to 12 nodes. Taking door and window control as an example, a car door may have door locks, window switches, window motors, and control buttons; a single LIN network can connect them all. Furthermore, through a CAN gateway, the LIN network can exchange information with other automotive systems, enabling richer functionality. Currently, LIN has become an international standard and is accepted by most automakers and parts manufacturers.
The cost savings of LIN over CAN are primarily due to the use of single-wire transmission, lower hardware or software implementation costs in silicon, and the elimination of the need for quartz or ceramic resonators in slave nodes. These advantages come at the cost of lower bandwidth and a limited single-homed bus access method.
LIN consists of a host node and one or more slave nodes. All nodes contain a slave communication task that is broken down into send and receive tasks, while the host node also contains an additional host send task. In real-time LIN, communication is always initiated by the host task.
IV. Flexray Bus
The FlexRay bus is a new communication standard jointly developed by companies such as BMW, Philips, Freescale, and Bosch. It is designed specifically for in-vehicle networking, adopts a time-triggered mechanism, and features high bandwidth and good fault tolerance. It has certain advantages in terms of real-time performance, reliability, and flexibility.
Flexray is a high-speed, deterministic, fault-tolerant bus technology for automobiles. It combines event-triggered and time-triggered methods, offering high network utilization and system flexibility, and can serve as the backbone network for next-generation automotive internal networks.
Flexray can be applied to passive bus and star network topologies, as well as a combination of both. Both topologies support dual-channel ECUs, which integrate multiple system-level functions to save production costs and reduce complexity. The dual-channel architecture provides redundancy and doubles the available bandwidth. The maximum data transfer rate per channel reaches 10 Mbps. Currently, Flexray is mainly used in safety-critical drive-by-wire systems and powertrain systems, and is used in high-end BMW vehicles.
BMW first applied FlexRay technology to the electronically controlled damper system in the 2007 X5 series. This vehicle uses a Freescale-based microcontroller and an NXP transceiver to monitor data on vehicle speed, longitudinal and lateral acceleration, steering wheel angle, body and tire acceleration, and ride height. This results in improved ride comfort, driving safety, and high-speed responsiveness, while minimizing load variations on the tires and chassis vibrations.
V. MOST Bus
MOST is a data bus technology specifically developed for in-vehicle use, serving multimedia applications. MOST stands for "Multimedia Transmission System".
Since BMW's 7 Series first adopted MOST (System for Media Transport) technology, its adoption has accelerated rapidly in recent years, enabling real-time transmission of audio and video to meet the needs of high-end automotive entertainment systems; it can also be used in vehicle systems such as in-vehicle cameras.
Features of the MOST bus:
(1) Under the condition of ensuring low cost, a data transmission speed of 24.8 Mbit/s can be achieved.
(2) It can work regardless of whether there is a main control computer.
(3) Supports real-time processing of audio and compressed images.
(4) Supports synchronous and asynchronous data transmission.
(5) The transmitter/receiver is embedded with a virtual network management system.
(6) Supports multiple network connection methods, provides MOST device standards, and offers a convenient and simple application system interface.
(7) By adopting MOST, not only can the weight of the wiring harness connecting the components be reduced and the noise reduced, but the burden on system development technicians can also be reduced, and finally, centralized control of various devices can be achieved at the user's location.
(8) Fiber optic networks are not affected by electromagnetic radiation interference or grounding rings.
MOST bus network should be used in multimedia
The control units are connected by a ring data bus that transmits data in only one direction. This means that a control unit always has two optical fibers, one for the transmitter and the other for the receiver.
The MOST bus adopts a ring network structure.
Fiber Optic Connector: The optical fiber connects to the control unit using a specialized optical connector. A directional arrow on the connector indicates the input (to the receiver), and the connector housing forms the connection with the control unit. Optical signals travel through the fiber optic cable and connector to the control unit or to the next bus user.
MOST bus control unit structure diagram
Control unit power module: The power supplied by the electrical plug is then distributed to each component by the internal power supply device, so that a certain component in the control unit can be turned off individually, thereby reducing the static current.
Transceiver Unit 1: Optical Transmitter (FOT) : This device consists of a photodiode and a light-emitting diode. The incoming optical signal is converted into a voltage signal by the photodiode and then transmitted to the MOST transceiver.
MOST transceiver: A MOST transceiver consists of two components: a transmitter and a receiver.
Control Unit (ECU): The control unit (ECU) contains a microprocessor that operates all the basic functions of the control unit.
Specialized components: These components are used to control certain specialized functions, such as CD players and radio tuners.
MOST bus fiber optic cable diagram
The MOST bus uses optical pulses to transmit data. The MOST bus employs a ring structure. Within a ring bus, data can only be transmitted in one direction.
MOST's transmission technology is similar to the Public Switched Telephone Network (PSTN), with a defined data channel and control channel. The control channel is used to configure how the data channel is used for sending and receiving. Once configured, data flows continuously from the sender to the receiver without further packet processing. This operational mechanism is best suited for real-time audio and video streaming.
MOST is fully compliant with the ISO/OSI 7-layer data communication protocol reference model in its design. For network cable connections, MOST adopts a ring topology. However, for more demanding transmission and control applications, MOST also allows for star (also known as radial) or double-ring connection configurations. In addition, each MOST transmission and control network allows up to 64 devices (nodes) to be connected.
VI. Conclusion
CAN, as a reliable automotive bus, has been widely used in high-end vehicles. LIN, as a complement to CAN, has also become an international standard. Flexray, even more advanced than CAN, is developing rapidly. The development of automotive buses is unstoppable.
