CAN Bus (1)

Introduction to CAN bus

Controller Area Network was developed by BOSCH as a robust vehicle bus standard designed to allow microcontrollers and devices comminicate without a host computer.

Therefore CAN Bus targets distributed control systems including vehicles to auto factory industries.

History of CAN

CAN 2.0(Classic CAN, 1Mb/s, 8bytes payload) was introduced in 1983 by BOSCH with ISO 11898 specification. Then was further developed to CAN FD(2015, 5Mb/s, 64bytes payload), all the way to CAN SIC XL(2020, 12Mb/s, 2048 bytes payload)

CAN as communication protocol has ISO standards of Physical and Data Link Layer.

As seen above CAN released two parts of standard ISO 11898 :

ISO 11898-1 covers Data Link Layer and part of Physical Layer

ISO 11898-2 covers Physical Layer for high speed CAN

(* ISO 11898-3 was released and covers physical layer for low-speed for fault tolerant CAN but is not part of CAN 2.0 specification)

Basic Components of CAN

Each end of CAN bus are notified with 120 Ohm resistors. Multiple nodes are connected to CANH(high) and CANL(low), respectively.

Key Aspects of CAN bus

Message based Communication

CAN bus is not address based protocol thus it does not require address to find its destination.

The message includes ID and data where the ID specifies the meaning of data frame. Lower number in ID has higher priority(0x00 has greatest, 0xff has lowest priority).

Since CAN bus works on message based frame, it os easier to append new data without having to reconfigure address of existing nodes.

Bus Access (how the message is accessed)

CSMA/CD-CR

Carrier Sense Multiple Access / Collision Detection - Collision Resolution

• Carrier Sense : Every node must monitor the bus for no activity(IDLE state) before sending messge
• Multiple Access : Every node has equal access to the bus while the bus is IDLE. (notice that there’s no priority pver nodes)
• Collision Detection : If multiple nodes send data simultaneously, each nodes detect message collision.
• Collision Resolution : When collision is detected, the bus resolves it with non-Destructive Bitwise Arbitration.