Controller Area Network
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Controller 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. It can be even more robust against noise if twisted pair wire is used. Although initially created for automotive purposes (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 Mbit/s are possible at networks length below 40 m. Decreasing the bit rate allows longer network distances (e.g. 250 kbit/s at 250 m).
The CAN data link layer protocol is standardized in ISO 11898-1 (2003). This standard describes mainly the data link layer — composed of the 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.
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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 transmits a dominant bit and another node transmits a recessive bit then the dominant bit "wins" (a logical AND between the two).
| Truth tables for dominant/recessive and logical AND | |||||||||||||||||||
| Bus state with two nodes transmitting
| Logical AND
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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
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
The CAN standard requires the implementation must accept the standard format and may accept the extended format.
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 (0-8 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 |
| Substitute remote request (SRR) | 1 | Must be recessive |
| Identifier extension bit (IDE) | 1 | Must be recessive |
| Identifier B | 18 | Second part of the (unique) identifier for the data |
| Remote transmission request (RTR) | 1 | Must be dominant |
| Reserved bits (r1 & r0) | 2 | Reserved |
| Data length code (DLC) | 4 | Number of bytes of data (0-8 bytes) |
| Data field | 0-8 bytes | Data to be transmitted (length dictated by DLC field) |
| CRC | 15 | Cyclic redundancy check |
| CRC delimiter | 1 | Must be recessive |
| ACK slot | 1 | Transmitter sends recessive and any receiver can assert a dominant |
| ACK delimiter | 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 two types of error frames
- Active Error Frame
Transmitted by a node detecting an error on the networkthat is in error state "error active".
- Passive Error Frame
Transmitted by a node detecting an active error frame on the network that is in error state "error passive".
Overload frame
Bit stuffing
In CAN frames a bit of opposite polarity is inserted after five consecutive bits of the same polarity. This practice is called bit stuffing, and is due to the "Non Return to Zero" (NRZ) coding adopted. The "stuffed" data frames are destuffed by the receiver. Since bit stuffing is used, six consecutive bits of the same type (111111 or 000000) are considered an error. Bit stuffing implies that sent data frames could be larger than one would expect by simply enumerating the bits shown in the tables above.
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
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, many implementations of higher layer protocols were created. Among these are DeviceNet, CANopen, SDS, J1939 and CAN Kingdom.
External links
- Bosch's Controller Area Network homepage (http://www.can.bosch.com/)
- Well organized CAN educational page (http://www.kvaser.com/can/protocol/index.htm)
- DeviceNet Official Website (http://www.odva.org/)
- CANopen overview (http://www.can-cia.org/canopen/)
- CAN-Wiki (http://www.can-wiki.info/)
- CAN in Automation (CiA) international users' and manufacturers' group (http://www.can-cia.org/)
- Mercedes Benz CAN DATA BUS (http://www.launch-techs.com/Support/Info/can-bus.htm)de:Controller Area Network