Detailed Explanation of the Four Major Automotive Bus Technologies

The automotive bus we are most familiar with is CAN, while LIN and FlexRay may be somewhat unfamiliar to many. Next, let’s introduce these four types of automotive buses.

The automotive bus is the communication network that connects automotive devices or instruments at the lower layer of the vehicle’s onboard network.Currently, there are four mainstream automotive buses: CAN bus, LIN bus, FlexRay bus, and MOST bus.

Here’s a table to illustrate the differences between various buses.

1. The Birth of Automotive Buses

The birth of automotive buses is closely related to the development of automotive electronics. The degree of electronic integration in vehicles is also seen as an important indicator of the level of modern automobiles. Traditional automotive electronics mostly adopt point-to-point single communication methods, with little interconnection, inevitably leading to a massive wiring system. Statistics show that a luxury car using traditional wiring methods can have wire lengths reaching up to 2000 meters and electrical nodes up to 1500, and this number approximately doubles every 10 years.

Detailed Explanation of the Four Major Automotive Bus Technologies

This further exacerbates the contradiction between the bulky wiring harness and the limited available space in the vehicle. From both material costs and work efficiency perspectives, traditional wiring methods cannot accommodate the development of modern automobiles. Additionally, to meet the real-time requirements of various electronic systems, it is necessary to share public data of the vehicle (such as engine speed, wheel speed, throttle pedal position, etc.), and each control unit has different requirements for real-time performance. Therefore, traditional electrical networks can no longer adapt to the development of modern automotive electronic systems, leading to the emergence of new automotive bus technologies.

2. CAN Bus

CAN bus, also known as the automotive bus, stands for “Controller Area Network.” It is a serial communication network that effectively supports distributed control and real-time control. It connects various individual control units in some form (mostly star topology) to form a complete system.

CAN bus was initially developed by Bosch in Germany to solve the data exchange issues between numerous electronic control modules (ECUs) in modern vehicles. It has now been widely used in automotive electronic systems and has become the main industry standard in European automotive manufacturing, representing the mainstream development trend of automotive electronic control networks.

Many renowned automotive manufacturers in the world, such as Volkswagen, Benz, BMW, Porsche, and Rolls-Royce, have adopted the CAN bus for data communication within their automotive control systems.

3. LIN Bus

LIN is a new type of low-cost open serial communication protocol jointly launched by Motorola and Audi, mainly used for distributed electronic control systems within the vehicle, especially for digital communication scenarios involving intelligent sensors or actuators. It is mainly applied in controlling electric windows, seat adjustments, lighting, etc.

A typical LIN network can have up to 12 nodes. For example, in door control, there are door locks, window switches, window lift motors, and operation buttons, all of which can be interconnected through just one LIN network. Moreover, through a CAN gateway, the LIN network can exchange information with other vehicle systems to achieve more comprehensive functionality. LIN has now become an international standard accepted by most automotive manufacturers and component producers.

The cost savings of LIN compared to CAN are mainly due to the use of single-wire transmission, low implementation costs of hardware or software in silicon chips, and the absence of quartz or ceramic resonators in the subordinate nodes. These advantages come at the cost of lower bandwidth and limited single-master bus access methods.

LIN consists of one master node and one or more slave nodes. All nodes include a slave communication task that is decomposed into sending and receiving tasks, while the master node also includes an additional master sending task. In real-time LIN, communication is always initiated by the master task.

4. FlexRay Bus

FlexRay bus is a new communication standard jointly developed by BMW, Philips, Freescale, and Bosch, specifically designed for in-vehicle networking. It uses a time-triggered mechanism and features high bandwidth and good fault tolerance, offering certain advantages in real-time performance, reliability, and flexibility.

FlexRay is a high-speed, deterministic bus technology for automotive applications, featuring fault tolerance. It combines event-triggered and time-triggered methods, offering efficient network utilization and system flexibility, making it suitable as the backbone network for the next generation of in-vehicle networks.

FlexRay can be applied in passive bus and star network topologies, as well as in combinations 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, with a maximum data transfer rate of 10 Mbps for each channel. Currently, FlexRay is mainly used in safety-related drive-by-wire systems and power systems, and it is applied in high-end vehicles from BMW.

BMW first applied FlexRay technology in the electronic control shock absorber system of the 2007 X5 series. This vehicle uses Freescale’s microcontroller and NXP’s transceiver to monitor data related to vehicle speed, longitudinal and lateral acceleration, steering angle, body and tire acceleration, and driving height, achieving better ride comfort, safety during driving, and high-speed responsiveness. Additionally, it minimizes load fluctuations on the tires and vibrations in the chassis.

5. MOST Bus

MOST is a data bus technology developed specifically for in-vehicle use, serving multimedia applications. MOST stands for “Media Oriented Systems Transport.”

Since BMW’s 7 Series was the first to adopt MOST (Media Oriented Systems Transport) technology, the technology’s popularity has surged in recent years, enabling real-time transmission of audio and video to meet the demands of high-end automotive entertainment devices; it can also be used in onboard cameras and other driving systems.

Characteristics of the MOST bus:

(1) Achieves a data transfer rate of 24.8 Mbit/s at low cost.

(2) Can operate regardless of the presence of a master 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, providing a standard for MOST devices, facilitating a simple and efficient application system interface.

(7) By using MOST, not only can the weight of the wiring harness connecting various components be reduced, but noise can also be minimized, alleviating the burden on system developers, ultimately achieving centralized control of various devices for users.

(8) The optical fiber network is immune to electromagnetic radiation interference and grounding loops.

The MOST bus network connects various control units through a ring data bus, which transmits data in only one direction, meaning each 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.

Optical fiber plug: The optical fiber connects to the control unit using specialized optical plugs. An arrow indicating the signal direction (to the receiver) appears on the plug, while the plug’s shell forms the connection with the control unit. Light signals enter the control unit or are transmitted to the next bus user through the optical fiber wire and optical plug.

Schematic diagram of the MOST bus control unit structure.

Control unit power module: The electricity supplied by the electrical plug is distributed by an internal power device to each component, allowing for the independent shutdown of certain components within the control unit, thereby reducing static current.

Transceiver – Optical Fiber Transmitter (FOT): This device consists of a photodiode and a light-emitting diode, where incoming light signals are converted into voltage signals by the photodiode before being sent to the MOST transceiver.

MOST transceiver: The MOST transceiver consists of a transmitter and a receiver.

Control Unit (ECU): The internal control unit (ECU) contains a microprocessor for controlling all basic functions of the control unit.

Dedicated components: These components are used to control specific functions, such as CD players and radio tuners.

MOST bus optical fiber pull line diagram.

MOST bus uses optical pulses to transmit data. The MOST bus adopts a ring structure. Data can only be transmitted in one direction within the ring bus.

The transmission technology of MOST is similar to that of the Public Switched Telephone Network (PSTN), with defined designs for data channels and control channels, where the control channel is used to set how to use and transmit data channels. Once set, data continues to flow from the sender to the receiver without further packet processing, making it ideal for real-time audio and video streaming transmission.

MOST fully complies with the ISO/OSI 7-layer data communication protocol reference model, and in terms of network connection, MOST adopts a ring topology. However, for more stringent control applications, MOST also allows star (also known as radial) or dual-ring connection configurations. In addition, each MOST control network allows for a maximum of 64 devices (nodes) to be connected.

Planning: Youth League Committee of Erqi District

Editors: Team Xiao Er and Team Xiao Qi

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