CAN Protocol

The CAN (Controller Area Network) protocol is a serial-based digital communication protocol developed by BOSCH. It was initially developed for use in the automotive industry. It makes use of priority-based message arbitration. The voltage is not part of the standard, and operating voltages of 5V or 12V are commen.

A alternative communications protocol used in similar applications is the LIN protocol.

Bit Rate And Transmission Distances

The following equation can be used as a rule-of-thumb to calculate the maximum transmission speed for distances larger than 50m.

$BR\times L\leq 60$

where:
$BR$ = bit rate (in Mbit/s)
$L$ = length (in m)

A table of common distances/transmission rates is shown below:

Speed Distance
1Mbit/s 25m
800kBit/s 50m
500kBit/s 100m
250kBit/s 250m
125kBit/s 500m
50kBit/s 1000m
20kBit/s 2500m
10kBit/s 5000m

Arbitration

The CAN network uses priority-based message arbitration. The drivers to the CAN line(s) are open-drain. This means that if a node writes a 0 (dominant), it will over-write a 1 (recessive). This is also called a “wired AND” configuration.

Bit Stuffing

Any sequential sequence of 5 bits of the same type requires the transmitter to insert a bit of the opposite polarity. Consequentially, the receiver has to remove this bit from the incoming data stream.

Message Lengths

There are two different message lengths supported by the CAM protocol.

  • CAM Base Frame (CAN2.0A)
  • CAM Extended Frame (CAN2.0B)

Errors

There are 5 different types of errors:

  • Bit Error: The transmitter monitors the bus level as it sends bits. If the level is not the same as what it is transmitting, a bit error occurs. Physical layer error.
  • Stuff Error: If 6 or more consecutive bits of the same type are found. Physical layer error.
  • Format Error: Data-link layer error.
  • CRC Error: When the computed CRC does not match the one received in the message packet. Data-link layer error.
  • Acknowledge (ACK) Error. Data-link layer error.

Standards

ISO11898

Widely followed basic CAN standard, defining parts of the physical and data link layers.

CANopen

CANopen was developed for embedded devices in automation systems . It defines the OSI network layers that the basic CAN standards leaves unspecified, which includes the network layer and above.

NEMA 2000

A communication protocol for ships which is based on the CAN standard.

SAE J1939-11

Uses a shielded twisted pair. Used in trucks, agricultural and industrial equipment.

IC’s

Atmel T89C51CC01 Microcontroller. 8-bit 8051 architecture, with CAN interface. Supports bootloading from the CAN protocol

MCP2515: Microchip CAN Controller.

MCP2551: Microchip Highspeed CAN Transceiver

The Freescale MC9SO8D range of microcontrollers have built-in support for both CAN and LIN communication protocols. The CAN peripheral block is called an MSCAN.