CAN bus

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 electromagnetically 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. 125 kbit/s at 500 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.

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 dominant recessive dominant dominant dominant recessive dominant recessive	Logical AND 0 1 0 0 0 1 0 1

So, if you are transmitting a recessive bit, and someone sends a dominant bit, you see a dominant bit, and you know there was a collision. (All other collisions are invisible.) The way this works is that a dominant bit is asserted by creating a voltage across the wires while a recessive bit is simply not asserted on the bus. If anyone sets a voltage difference, everyone sees it, hence, dominant.

Commonly when used with a differential bus, a Carrier Sense Multiple Access/Bitwise Arbitration (CSMA/BA) 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.

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 transmission of the identifier field. Each node starting to transmit at the same time sends an ID with dominant as binary 0, starting from the high bit. As soon as their ID is a larger number (lower priority) they'll be sending 1 (recessive) and see 0 (dominant), so they back off. At the end of ID transmission, all nodes bar one have backed off, and the highest priority message gets through unimpeded.

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:

• Base frame format: with 11 identifier bits
• Extended frame format: with 29 identifier bits

The CAN standard requires the implementation must accept the base frame format and may accept the extended frame format. But must tolerate the extended frame format.

Base frame format

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 bits (r0, r1)	2	Reserved bit but they must be set to dominant
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

One restriction placed on the identifier is that the first 7 bits cannot be all recessive bits.

Extended frame format

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 (r0, r1)	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.

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

Error frame consists of two different fields

The first field is given by the superposition of ERROR FLAGS contributed from different stations. The following second field is the ERROR DELIMITER.

There are two types of error flags

Active Error Flag

Transmitted by a node detecting an error on the network that is in error state "error active".

Passive Error Flag

Transmitted by a node detecting an active error frame on the network that is in error state "error passive".

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
• Well organized CAN educational page
• DeviceNet Official Website
• CANopen overview
• CAN-Wiki
• CAN in Automation (CiA) international users' and manufacturers' group
• Mercedes Benz CAN DATA BUS
• CAN-BUS over vehicle power-line
• CAN Bus Interface Description CANbus Pin Out, and Signal Names