A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow ISO specifies the CAN physical layer for transmission rates up to 1 Mbit/s for use within road vehicles. It describes the medium access. Find the most up-to-date version of ISO at Engineering ISO Road vehicles — Controller area network (CAN) — Part 5: High- speed medium access unit with low-power mode.
|Published (Last):||28 April 2018|
|PDF File Size:||15.91 Mb|
|ePub File Size:||11.54 Mb|
|Price:||Free* [*Free Regsitration Required]|
A Controller Area Network CAN bus is a robust vehicle bus standard 118998-5 to allow microcontrollers and devices to communicate with each other in applications without a host computer.
It is a message-based protocoldesigned originally for multiplex electrical wiring within automobiles to save on copper, but is also used in many other contexts. Released in the Mercedes-Benz W was the first production vehicle to feature a CAN-based multiplex wiring system.
This specification has two parts; part A is for the standard format with an bit identifier, and part B is for the extended format with a bit identifier. These standards are freely available from Uso along with other specifications and white papers.
These standards may be purchased from the ISO. Bosch is still active in extending the CAN standards. 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. The EOBD standard has been mandatory for all petrol vehicles sold in the European Union since and all diesel vehicles since The modern automobile may have as many as 70 electronic control units ECU for various subsystems.
Some of these form independent subsystems, but communications among others are 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.
In recent years, the LIN bus 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. Two or more nodes are required on the CAN network to communicate. The node may also be a gateway allowing a general purpose computer such as a laptop to communicate over a USB or Ethernet port to the devices on a CAN network.
CAN bus – Wikipedia
All nodes are connected to each other through a two wire bus. Designating “0” as dominant gives the nodes with the lower ID numbers priority on the bus. The dominant common mode voltage must be within 1. ISOalso called low speed or fault tolerant CAN Kbpsuses 11898–5 linear bus, star bus or multiple star buses connected by a linear bus and is terminated at each node by a fraction of the overall termination resistance. The dominant differential voltage must be greater than 2. With both high speed and low speed CAN, the speed of the transition is faster when a recessive to dominant transition occurs since the CAN wires are being actively driven.
The speed of the dominant to recessive transition depends primarily on the length of the CAN network and the capacitance of the wire used.
High speed CAN is usually used in automotive and industrial applications where the bus runs from one end of the environment to the other. Fault tolerant CAN is often used where groups of nodes need to be connected together.
The specifications require the bus iao kept within a minimum and maximum common mode bus voltage, but do not define how to keep the bus within this range. The CAN bus must be terminated. The termination resistors are needed ios suppress reflections as well as return the bus to its recessive or idle state. Low speed CAN uses resistors at each node. A terminating bias circuit provides power and ground in addition to the CAN signaling on a four-wire cable. This provides automatic electrical bias and termination at each end of each bus segment.
Each node is able to send and receive messages, but not simultaneously. A message or Frame consists primarily of the ID identifier11889-5 represents the priority of the message, and up to eight data bytes. The message is transmitted serially onto the bus using a non-return-to-zero NRZ format and may be received by all nodes. The devices that are connected by a CAN network are typically sensorsactuatorsand other control devices.
CAN data transmission uses a lossless bitwise arbitration method of contention resolution. This arbitration method requires all nodes on the CAN network to be synchronized to sample every bit on the CAN network at the same time.
This is why some call CAN synchronous. Unfortunately the term synchronous is imprecise since the data is transmitted without a clock signal in an asynchronous format. The CAN specifications use the terms “dominant” bits and “recessive” bits where dominant is a logical 0 actively driven to a voltage by the transmitter and recessive is a logical 1 passively returned to a voltage by a resistor.
The idle state is represented by the recessive level Logical 1. If one node transmits a dominant bit and another node transmits a recessive bit then there is a collision and the dominant bit “wins”.
This means there is no delay to the higher-priority message, and the node transmitting the lower priority message automatically attempts to re-transmit six bit clocks after the end of the dominant message.
This makes CAN very suitable as a real time prioritized communications system. The exact voltages for a logical 0 or 1 depend on the physical layer used, but the basic principle of CAN requires that each node listens to the data on the CAN network including the transmitting node s itself themselves. If a logical 1 is transmitted by all transmitting nodes at the same time, then a logical 1 is seen by all of the nodes, including both the transmitting node s and receiving node s. If a logical 0 is transmitted by all transmitting node s at the same time, then a logical 0 is seen by all nodes.
If a logical 0 is being transmitted by one or more nodes, and a logical 1 is being transmitted by one or more nodes, then a logical 0 is seen by all nodes including the node s transmitting the logical 1. When a node transmits a logical 1 but sees a logical 0, it realizes that there is a contention and it quits transmitting. By using this process, any node that transmits a logical 1 when another node transmits a logical 0 “drops out” or loses the arbitration. A node that loses arbitration re-queues its message for later transmission and the CAN frame bit-stream continues without error until only one node is left transmitting.
This means that the node that transmits the first 1 loses arbitration. Since the 11 or 29 for CAN 2. For example, consider an bit ID CAN network, with two nodes with IDs of 15 binary representation, and 16 binary representation, If these two nodes transmit at the same time, each will first transmit the start bit then transmit the first six zeros of their ID with no arbitration decision being made.
BS ISO 11898-5:2007
When this happens, the node with the ID of 16 knows it transmitted a 1, but sees a 0 and realizes that there is a collision and it lost arbitration. Node 16 stops transmitting which allows the node with ID of 15 to continue its transmission without any loss of data.
The node with the lowest ID will always uso the arbitration, and therefore has the highest priority.
Decreasing the bit rate allows longer network distances e. The improved CAN FD standard allows increasing the bit rate after arbitration and can increase the speed of the data section by a factor of up to ten or more of the arbitration bit rate. Message IDs must be unique on a single CAN bus, otherwise two nodes would continue transmission beyond the end of the arbitration field ID causing an error.
In the early s, the choice of IDs for ido was done simply on the basis of identifying the type of data and the sending node; however, as the ID is also used as the message priority, this led to poor real-time performance. All nodes on the CAN network must operate at the same nominal bit rate, but noise, phase shifts, oscillator tolerance and oscillator drift mean that the actual bit rate may not be the same as the nominal bit rate.
Synchronization is important during arbitration since the nodes in arbitration must be able to see is their 118985- data and 1189-85 other nodes’ transmitted data at the same time. Synchronization is also important to ensure that variations in oscillator 11988-5 between nodes do not cause errors. Synchronization starts with a hard synchronization on the first recessive to dominant transition after a period of bus idle the start bit.
Resynchronization occurs on every recessive to dominant transition during the frame. The CAN controller expects the transition to occur at a multiple of the nominal bit time. If the transition does not occur at the exact time the controller expects it, the controller adjusts the nominal bit time accordingly.
The adjustment is accomplished by dividing each bit into a number of time slices called quanta, and assigning some number of quanta to each of the four segments within the bit: The number of quanta the bit is divided into can vary by controller, and the number of quanta assigned to each segment can be varied depending on bit rate and network conditions. A transition that occurs before or after it is expected causes the controller to calculate the time difference io lengthen phase segment 1 or shorten 18198-5 segment 2 by this time.
This effectively adjusts the timing of the receiver to the transmitter to synchronize them. This resynchronization process is done 111898-5 at every recessive to dominant transition to ensure the transmitter and receiver stay in sync. Continuously resynchronizing reduces errors induced by noise, and allows a receiving node that was synchronized to a node which lost arbitration to resynchronize to the node which won arbitration.
The CAN protocol, like many networking protocols, can be decomposed into the following abstraction 11898–5. 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, acknowledgement, error detection and signaling, and fault confinement.
The electrical aspects of the physical layer voltage, current, number of conductors were specified in ISO 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:. 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. 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 vehicles lines can go up to 24V while truck systems can go as high as 36V. Noise immunity on ISO However, when dormant, a low-impedance bus such as CAN draws more current and power than other voltage-based signaling busses.
Best practice determines that CAN bus balanced pair signals be carried in twisted pair wires in a shielded cable to minimize RF emission and reduce interference susceptibility in the already noisy RF environment of an automobile.
Also, in the de facto mechanical configuration mentioned above, a supply rail is included to distribute power to each of the transceiver nodes. The design provides a common supply for all the transceivers.