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Controller Area NetworkController Area Network (CAN) is a multicast shared serial bus standard originally developed in the 1980s by Robert Bosch GmbH for connecting electronic control units (ECUs). CAN was specifically designed to be robust in noisy environments and can utilize a differential balanced line like RS-485 and can be even more robust against noise if twisted pair wire is used. Although initially created for automotive (as a vehicle bus) nowadays it is used in many embedded control applications (e.g., industrial) that may be subject to noise. Bit rates up to 1 Mbps are possible at networks length below 40 m. Decreasing the bit rate allows longer network distances (e.g. 250 kbps at 250 m). The CAN data link layer protocol is standarized in ISO 11898-1 (2003). This standard describes mainly the data link layer — comprised by logical link control (LLC) sublayer and the Media Access Control (MAC) sublayer — and some aspects of the physical layer of the ISO/OSI Reference Model. All the other protocol layers are left to the network designer's choice. Data transmission CAN transmits data through a binary model of "dominant" bits and "recessive" bits where dominant is a logical 0 and recessive is a logical 1. If one node tranmits a dominant bit and another node transmits a recessive bit then the dominant bit "wins" (a logical AND between the two). | olspan="2"| Truth tables for dominant/recessive and logical AND | Bus state with two nodes transmitting {| align="center" border="1" cellpadding="10" | | dominant | recessive | | dominant | dominant | dominant | | recessive | dominant | recessive | } | | Logical AND {| align="center" border="1" cellpadding="10" | | 0 | 1 | | 0 | 0 | 0 | | 1 | 0 | 1 | } | Commonly when used with a differential bus, a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) scheme is implemented: if two or more devices start transmitting at the same time, there is a priority based arbitration scheme to decide which one will be granted permission to continue transmitting. Specifically, a dominant bit is asserted by creating a voltage across the wires while a recessive bit is simply not asserted on the bus. During arbitration, each transmitting node monitors the bus state and compares the received bit with the transmitted bit. If a dominant bit is received when a recessive bit is transmitted then the node stops transmitting (i.e., it lost arbitration). Arbitration is performed during the transmition of the identifier field. Frames All frames (aka messages) begin with a start-of-frame (SOF) bit that, obviously, denotes the start of the frame transmission. CAN has four frame types: - Data frame: a frame containing node data for transmission
- Remote frame: a frame requesting the transmission of a specific identifier
- Error frame: a frame transmitted by any node detecting an error
- Overload frame: a frame to inject a delay between data and/or remote frames
The CAN standard requires the implementation must accept the standard format and may accept the extended format. Data frame The data frame is the only frame for actual data transmission. There are two message formats: - Standard: with 11 identifier bits
- Extended: with 29 identifier bits
Standard The frame format is as follows: | Field name | Length (bits) | Purpose | | Start-of-frame | 1 | Denotes the start of frame transmission | | Identifier | 11 | A (unique) identifier for the data | | Remote transmission request (RTR) | 1 | Must be dominant | | Identifier extension bit (IDE) | 1 | Must be dominant | | Reserved bit (r0) | 1 | Reserved | | Data length code (DLC) | 4 | Number of bytes of data (0-7 bytes) | | Data field | 0-8 bytes | Data to be transmitted (length dictated by DLC field) | | CRC | 15 | Cyclic redundancy check | | CRC delimeter | 1 | Must be recessive | | ACK slot | 1 | Transmitter sends recessive and any receiver can assert a dominant | | ACK delimeter | 1 | Must be recessive | | End-of-frame (EOF) | 7 | Must be recessive | One restriction placed on the identifier is that the first 7 bits cannot be all recessive bits. Extended The frame format is as follows: | Field name | Length (bits) | Purpose | | Start-of-frame | 1 | Denotes the start of frame transmission | | Identifier A | 11 | First part of the (unique) identifier for the data | | Remote transmission request (RTR) | 1 | Must be dominant | | Identifier extension bit (IDE) | 1 | Must be recessive | | Identifier B | 18 | Second part of the (unique) identifier for the data | | Reserved bits (r1 & r0) | 2 | Reserved | | Data length code (DLC) | 4 | Number of bytes of data (0-7 bytes) | | Data field | 0-8 bytes | Data to be transmitted (length dictated by DLC field) | | CRC | 15 | Cyclic redundancy check | | CRC delimeter | 1 | Must be recessive | | ACK slot | 1 | Transmitter sends recessive and any receiver can assert a dominant | | ACK delimeter | 1 | Must be recessive | | End-of-frame (EOF) | 7 | Must be recessive | The two identifier fields (A & B) combined form a 29-bit identifier. One restriction placed on identifier A is that the first 7 bits cannot be all recessive bits. Remote frame The remote frame is identical to the data frame except: - the RTR bit set to recessive
- data length contains the number of bytes that are required from the data frame
Error frame There are tow types of error frames - Active Error Frame Transmitted by a node detecting an error on the network - Passive Error Frame Transmitted by a node detecting an active error frame on the network Overload frame ISO standards There are several CAN physical layer standards: - ISO 11898-2: CAN high-speed
- ISO 11898-3: CAN fault-tolerant (low-speed)
- ISO 11992-1: CAN fault-tolerant for truck/trailer communication
- SAE J2411: Single-wire CAN (SWC)
ISO 11898-2 uses a two-wire balanced signaling scheme. It is the most used physical layer in car powertrain applications and industrial control networks. The ISO 11898-4 standard defines the time-triggered communication on CAN (TTCAN). It is based on the CAN data link layer protocol providing a system clock for the scheduling of messages. Application layer implementations Also, as the CAN standard does not include tasks of application layer protocols, such as flow control, device addressing and transportation of data blocks larger than one message, there were created many implementations of higher layer protocols. Among these are DeviceNet, CANopen, SDS, J1939 and CAN Kingdom. External links
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