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Automotive Data Communication Buses

The number of sensors, actuators, entertainment and navigation systems and their corresponding electronic control units in the typical automobile has been growing exponentially. Digital devices and systems must communicate via an electrical or optical signal employing a well defined protocol. These signals and protocols constitute a communications bus. This page provides an overview of the most common communication buses used in cars and trucks.

CAN (Controller Area Network)

Brief Description: Can bus is a serial bus originally developed by Robert Bosch GmbH in 1986 for in-vehicle networks in cars. CAN buses employing twisted wire pairs were specifically designed to be robust in electromagnetically noisy environments. The applications of CAN bus in automobiles include window and seat operation (low speed), engine management (high speed), brake control (high speed) and many other systems. CAN buses can also be found in other embedded control applications such as factory automation, building automation, and aerospace systems.

Maximum Data Rates: 1Mbps at 40m, 125Kbps at 500m, 50kbps at 1000m
Circuit Type: Differential
Physical Layer: Twisted Wire Pair, 9 pin D-Sub
Transmission Format: Asynchronous
Drive Voltage: High: 2.75v ~ 4.5v; Low: 0.5v ~ 2.25v; Differential: 1.5v ~ 3.0v
Network Topology: Point to Point
Standards: ISO 11898/11519

LIN (Local Interconnect Network)

Brief Description: LIN specifications are developed by the LIN consortium. Version 1.1 was released in 1999. The current version is 2.0. A primary advantage of this bus is that it can be implemented with a single wire (using the vehicle chassis as a current return path). A small and relatively slow in-vehicle communication and networking serial bus system, LIN bus is used to integrate intelligent sensors and actuators. LIN can also communicate over a vehicle's power distribution system with a DC-LIN transceiver.

Maximum Data Rates: 19.2Kbaud at 40m
Physical Layer: Single-Wire Implementation
Transmission Format: SCI (UART) Data Format
Operating Voltage: 12v over a Single Wire
Network Topology: Single Master / Multiple Slave (Up to 16 slaves)
Standards: Enhanced ISO 9141

FlexRay

Brief Description: FlexRay is a high-speed serial communication bus for in-vehicle networks. It is an extended protocol version of byteflight. The extended FlexRay has the performance features required for active safety, such as redundant transmission channels and a fault-tolerant synchronization mechanism. Applications for FlexRay include steer-by-wire and brake-by-wire systems.

Maximum Data Rates: 500 kbps ~ 10 Mbps
Communication Modes:Time-triggered, Event-triggered
Network Topology: Single-channel topologies, Dual-channel topologies

MOST (Media Oriented Systems Transport)

Brief Description: MOST was originally designed by Oasis SiliconSystems AG (now SMSC) in cooperation with BMW, Becker Radio, and DaimlerChrysler for multimedia applications in the automotive environment.  It was intended to be implemented on an optical fiber, so the bit rates of this bus system are much higher than previous automotive bus technologies. Since 1997, seventeen international automotive manufacturers and more than fifty key component suppliers including automotive electrical suppliers and audio-video manufacturers have contributed to this technology. MOST buses provide an optical solution for automotive peripherals like car radios, CD and DVD players, and GPS navigation systems.

Maximum Data Rate: 150 Mbps
Layers: All Seven Layers of the ISO/OSI Reference Model for Data Communication
Network Topology: Point to Point via a Ring Topology but Star Configurations
Other Feature: Plug and Play; 60 Channels, 15 MPEG1 Channels for user configuration
Standards: ISO 7498-1 (OSI Model)

Ethernet

Brief Description: Although Ethernet is not currently used in production vehicles, has many appealing features. Ethernet communication data rates range from 10 Mbps to 100 Gbps (much faster than existing automotive networks). Also, Ethernet is widely used, so parts and experienced developers are readily available. In November 2011, the OPEN (One Pair Ether-Net) Alliance Special Interest Group was founded to develop and promote the adoption of the Ethernet standard in automotive systems. A key part of this effort is a proprietary technology developed by Broadcom Corporation that helps to overcome EMI issues associated with transmitting 100 Mbps Ethernet over unshielded twisted wire pair (TWP) cable.

More Information:
http://www.opensig.org/

OBDII (On-Board Diagnostics II)

Brief Description: OBDII is the second generation of the OBD specification. Since on-board vehicle computers were introduced in the early 1980's, OBD systems have made it possible to give the vehicle owner or a technician access to information on the state of vehicle subsystems. Early implementations of OBD monitored a few emission related components and simply illuminated a malfunction indicator light. OBD II defines a communications protocol to provide a standardized series of diagnostic trouble codes (DTCs) via a standardized fast digital communications port. These codes allow a user to identify and remedy malfunctions within the vehicle.

Signal Protocols: There are 5 signaling protocols currently employed by the OBD II Interface (J1962 Connector);
SAE J1850 PWM (41.6 kbaud, Ford Motor Company Standard)
SAE J1850 VPW (10.4/41.6 kbaud, General Motors Standard)
ISO 9141-2 (10.4 kbaud, primarily used in Chrysler, European, and Asian vehicles)
ISO 14230 KWP2000 (Keyword Protocol 2000)
ISO 15765 CAN (250 kbps or 500 kbps).
Standards: ISO 11898, SAE J1962, J1850, J1939, J1978, J1979, J2012, J2178-1, J2178-2, J2178-3, J2178-4

SAE J1850

Brief Description: The SAE J1850 Bus, developed in 1994, is used for diagnostics and data sharing applications in vehicles. It can be found in OBD II connectors in some older makes of cars.

Maximum Data Rates: 41.6 kbps (PWM); 10.4 kbps (VPW)
Physical Layer: Two-wire, differential (PWM); Single-wire (VPW)
Standards: SAE J1850
More Information:
http://www.interfacebus.com/Automotive_SAE_J1850_Bus.html

SAE J1708

Brief Description: J1708 provides serial data communications between microcomputer systems in heavy-duty vehicle applications. J1708 defines the physical layer only.

Maximum Data Rates: 9600 bps
Physical Layer: 2-Wire 18 AWG Wire (maximum of 40m), RS485 bus as electrical layer
Standards: SAE J1708
More Information: http://www.sae.org/technical/standards/J1708_200408