A controller area network (CAN bus) is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other. It is a message-based protocol, designed originally for multiplex electrical wiring within automobiles to save on copper, but it can also be used in many other contexts. For each device, the data in a frame is transmitted serially but in such a way that if more than one device transmits at the same time, the highest priority device can continue while the others back off. Frames are received by all devices, including by the transmitting device.
History Development of the CAN bus started in 1983 at Robert Bosch GmbH. The protocol was officially released in 1986 at the Society of Automotive Engineers (SAE) conference in Detroit, Michigan. The first CAN controller chips were introduced by Intel in 1987, and shortly thereafter by Philips. Released in 1991, the Mercedes-Benz W140 was the first production vehicle to feature a CAN-based multiplex wiring system.
Bosch published several versions of the CAN specification. The latest is CAN 2.0, published in 1991. This specification has two parts. Part A is for the standard format with an 11-bit identifier, and part B is for the extended format with a 29-bit identifier. A CAN device that uses 11-bit identifiers is commonly called CAN 2.0A, and a CAN device that uses 29-bit identifiers is commonly called CAN 2.0B. These standards are freely available from Bosch along with other specifications and white papers.
In 1993, the International Organization for Standardization (ISO) released CAN standard ISO 11898, which was later restructured into two parts: ISO 11898-1 which covers the data link layer, and ISO 11898-2 which covers the CAN physical layer for high-speed CAN. ISO 11898-3 was released later and covers the CAN physical layer for low-speed, fault-tolerant CAN. The physical layer standards ISO 11898-2 and ISO 11898-3 are not part of the Bosch CAN 2.0 specification.
In 2012, Bosch released CAN FD 1.0, or CAN with Flexible Data-Rate. This specification uses a different frame format that allows a different data length as well as optionally switching to a faster bit rate after the arbitration is decided. CAN FD is compatible with existing CAN 2.0 networks so new CAN FD devices can coexist on the same network with existing CAN devices, using the same CAN 2.0 communication parameters. , Bosch was active in extending CAN standards.
The CAN bus is one of five protocols used in the on-board diagnostics (OBD)-II vehicle diagnostics standard. The OBD-II standard has been mandatory for all cars and light trucks sold in the United States since model year 1996. The EOBD standard has been mandatory for all petrol vehicles sold in the European Union since 2001 and all diesel vehicles since 2004.Building Adapter for Vehicle On-board Diagnostic , obddiag.net, accessed 2009-09-09
Applications Automotive The modern automobile may have as many as 70 electronic control units (ECUs) for various subsystems.Comparison of Event-Triggered and Time-Triggered Concepts with Regard to Distributed Control Systems A. Albert, Robert Bosch GmbH Embedded World, 2004, Nürnberg Usually the biggest processor is the engine control unit. Others are used for autonomous driving, advanced driver assistance system (ADAS), transmission, airbags, antilock braking/ABS, cruise control, electric power steering, audio systems, power windows, doors, mirror adjustment, battery and recharging systems for hybrid/electric cars, etc. Some of these form independent subsystems, but communication among others is essential. A subsystem may need to control actuators or receive feedback from sensors. The CAN standard was devised to fill this need. One key advantage is that interconnection between different vehicle systems can allow a wide range of safety, economy and convenience features to be implemented using software alone - functionality which would add cost and complexity if such features were hard wired using traditional automotive electrics. Examples include:
Auto start/stop: Various sensor inputs from around the vehicle (speed sensors, steering angle, air conditioning on/off, engine temperature) are collated via the CAN bus to determine whether the engine can be shut down when stationary for improved fuel economy and emissions. Electric park brakes: The hill hold functionality takes input from the vehicle's tilt sensor (also used by the burglar alarm) and the road speed sensors (also used by the ABS, engine control and traction control) via the CAN bus to determine if the vehicle is stopped on an incline. Similarly, inputs from seat belt sensors (part of the airbag controls) are fed from the CAN bus to determine if the seat belts are fastened, so that the parking brake will automatically release upon moving off. Parking assist systems: when the driver engages reverse gear, the transmission control unit can send a signal via the CAN bus to activate both the parking sensor system and the door control module for the passenger side door mirror to tilt downward to show the position of the curb. The CAN bus also takes inputs from the rain sensor to trigger the rear windscreen wiper when reversing. Auto lane assist/collision avoidance systems: The inputs from the parking sensors are also used by the CAN bus to feed outside proximity data to driver assist systems such as Lane Departure warning, and more recently, these signals travel through the CAN bus to actuate brake by wire in active collision avoidance systems. Auto brake wiping: Input is taken from the rain sensor (used primarily for the automatic windscreen wipers) via the CAN bus to the ABS module to initiate an imperceptible application of the brakes while driving to clear moisture from the brake rotors. Some high-performance Audi and BMW models incorporate this feature. Sensors can be placed at the most suitable place, and their data used by several ECUs. For example, outdoor temperature sensors (conventionally placed in the front) can be placed in the outside mirrors, avoiding heating by the engine, and data used by the engine, the climate control, and the driver display.
In recent years, the LIN bus (Local Interconnect Network) standard has been introduced to complement CAN for non-critical subsystems such as air-conditioning and infotainment, where data transmission speed and reliability are less critical.
3D Printing CAN bus is often used to connect 3D printer mainboards to expansion boards due to its high electrical noise tolerance, especially from ground noise generated by stepper motors, commonly used in 3D printers and other CNC applications. The CAN FD variant is used by many, such as the Duet3D controller family, as it has higher bandwidth needed to support the high command rates of 3D printing.
Other The CAN bus protocol has been used on the Shimano DI2 electronic gear shift system for road bicycles since 2009, and is also used by the Ansmann and BionX systems in their direct drive motor. The CAN bus is also used as a fieldbus in general automation environments, primarily due to the low cost of some CAN controllers and processors. Manufacturers including NISMO aim to use CAN bus data to recreate real-life racing laps in the videogame Gran Turismo 6 using the game's GPS Data Logger function, which would then allow players to race against real laps. Johns Hopkins University's Applied Physics Laboratory's Modular Prosthetic Limb (MPL) uses a local CAN bus to facilitate communication between servos and microcontrollers in the prosthetic arm. Teams in the FIRST Robotics Competition widely use CAN bus to communicate between the roboRIO and other robot control modules. The CueScript teleprompter range uses CAN bus protocol over coaxial cable, to connect its CSSC – Desktop Scroll Control to the main unit The CAN bus protocol is widely implemented due to its fault tolerance in electrically noisy environments such as model railroad sensor feedback systems by major commercial Digital Command Control system manufacturers and various open-source digital model railroad control projects. Shearwater Research have implemented the protocol as DiveCAN to integrate their dive computers into diving rebreathers from various manufacturers.
Architecture Layers The CAN protocol, like many networking protocols, can be decomposed into the following abstraction layers: Application layer Application-specific logic Object layer Message filtering (mailboxes) Message and status handling Transfer layer Most of the CAN standard applies to the transfer layer. The transfer layer receives messages from the physical layer and transmits those messages to the object layer. The transfer layer is responsible for bit timing and synchronization, message framing, arbitration, acknowledgment, error detection and signaling, and fault confinement. It performs: Fault confinement Error detection Message validation Acknowledgement Arbitration Message framing Transfer rate and timing Information routing Physical layer
CAN bus (ISO 11898-1:2003) originally specified the link layer protocol with only abstract requirements for the physical layer, e.g., asserting the use of a medium with multiple-access at the bit level through the use of dominant and recessive states. The electrical aspects of the physical layer (voltage, current, number of conductors) were specified in ISO 11898-2:2003, which is now widely accepted. However, the mechanical aspects of the physical layer (connector type and number, colors, labels, pin-outs) have yet to be formally specified. As a result, an automotive ECU will typically have a particular—often custom—connector with various sorts of cables, of which two are the CAN bus lines. Nonetheless, several de facto standards for mechanical implementation have emerged, the most common being the 9-pin D-sub type male connector with the following pin-out: pin 2: CAN-Low (CAN−) pin 3: GND (ground) pin 7: CAN-High (CAN+) pin 9: CAN V+ (power)
This de facto mechanical standard for CAN could be implemented with the node having both male and female 9-pin D-sub connectors electrically wired to each other in parallel within the node. Bus power is fed to a node's male connector and the bus draws power from the node's female connector. This follows the electrical engineering convention that power sources are terminated at female connectors. Adoption of this standard avoids the need to fabricate custom splitters to connect two sets of bus wires to a single D connector at each node. Such nonstandard (custom) wire harnesses (splitters) that join conductors outside the node reduce bus reliability, eliminate cable interchangeability, reduce compatibility of wiring harnesses, and increase cost.
The absence of a complete physical layer specification (mechanical in addition to electrical) freed the CAN bus specification from the constraints and complexity of physical implementation. However, it left CAN bus implementations open to interoperability issues due to mechanical incompatibility. In order to improve interoperability, many vehicle makers have generated specifications describing a set of allowed CAN transceivers in combination with requirements on the parasitic capacitance on the line. The allowed parasitic capacitance includes both capacitors as well as ESD protection (ESD against ISO 7637-3). In addition to parasitic capacitance, 12V and 24V systems do not have the same requirements in terms of line maximum voltage. Indeed, during jump start events light vehicle lines can go up to 24V while truck systems can go as high as 36V. New solutions are emerging, allowing the same component to be used for CAN as well as CAN FD (see ).