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What is CAN bus communication protocol

The scope of this article is to understand the basics of the classic CAN bus protocol.

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Unofficial CAN bus logo by CiA
Unofficial CAN bus logo by CiA
By David Ampurua
4 min read
What is CAN bus communication protocol

Table of contents

Definition

The CAN bus (Controller Area Network) is a field bus communication protocol. The bus is composed of nodes, called ECUs (Electronic Control Units), and allows them to share data easily, without needing separate connections. A node broadcasts information through two wires: CAN High and CAN Low. Every other node receives the data and decides whether to use it or ignore it.

Why use CAN bus?

  • Simple & low cost: ECUs communicate through one CAN system instead of complicated individual connections, reducing errors, weight, wiring, and costs.
  • Centralized control but decentralized communication: The CAN bus lets you access all network ECUs from a single point, making diagnostics, data logging, and configuration easier. And nodes can communicate without requiring a central host computer.
  • Highly reliable: The system is robust towards electric disturbances and electromagnetic interference -> ideal for safety critical applications. CAN bus has robust error detection mechanisms and automatic retransmission for corrupted messages.
  • Efficient: CAN frames are prioritized by ID, ensuring high-priority information reaches the system without delays or interruptions of other frames.

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CAN protocol on OSI model

CAN bus doesn’t fully map to all layers of the OSI model because its primary design focuses on embedded systems’ real-time communication requirements, rather than generic networking. CAN bus fit into the first two layers of OSI model:

CAN data link and physical sub-layers in relation to the OSI model CAN data link and physical sub-layers in relation to the OSI model

Standard frame composition

Standard CAN bus frame (without stuff bits) Standard CAN bus frame (without stuff bits) Standard CAN bus frame (without stuff bits)

Frame field explained:

  • SOF (Start of Frame): This marks the beginning of the CAN frame. A “dominant 0” signals to tell other nodes that a CAN node wants to start communication.
  • ID (Identifier): Sets the message’s priority. The lower the number -> the higher the priority.
    • Base frame format, CAN 2.0A, with 11 identifier bits (used by CANopen).
    • Extended frame format, CAN 2.0B, have 29 identifier bits (used by J1939).
  • RTR (Remote Transmission Request): Shows whether the node is sending data or requesting data from another node.
  • Control: Holds two important elements
    • IDE (Identifier Extension Bit): A “dominant 0” indicates an 11-bit identifier.
    • DLC (Data Length Code): 4 bits specifying the number of data bytes to be sent (from 0 to 8).
  • Data: Contains the payload. This is where signals are placed for extraction and decoding.
  • CRC (Cyclic Redundancy Check): Used to ensure data integrity.
  • ACK: Shows that a node has successfully received and acknowledged the data.
  • EOF (End of Frame): Marks the end of the CAN frame. Must be “recessive 1”.
  • IFS (Inter-frame spacing): Must be “recessive 1”.

Take into account that: “dominant” state = CAN-H > CAN-L and “recessive” state = CAN-H ≤ CAN-L

Frame types

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 or remote frame.

CAN topologies

There are two main topologies depending on user requeriments:

  • High-speed CAN (ISO 11898-2) works up to 1 Mbit/s (regular CAN) and 5 Mbit/s (CAN-FD). It uses a linear bus with a 120 Ω resistor at each end.
  • Low-speed or fault-tolerant CAN (ISO 11898-3), works up to 125 kbit/s. Uses different wiring options: linear bus, star bus, or a mix of both. Each node (device) is connected with part of the total termination resistance; the total should be close to (but not less than) 100 Ω.

CAN bus linear and star topologies CAN bus linear and star topologies Basic CAN topologies

CAN history

gantt
    title CAN bus history milestones
    dateFormat  YYYY-MM-DD
    axisFormat %Y
    todayMarker off

    Bosch developed the CAN protocol                    :milestone, 1986-01-01, 0d
    Bosch released the CAN 2.0                          :milestone, 1991-01-01, 0d
    The CAN protocol is standardized (ISO 11898)        :milestone, 1993-01-01, 0d
    ISO 11898 becomes a standard series                 :milestone, 2003-01-01, 0d
    Bosch released the CAN FD 1.0 (Flexible Data-Rate)  :milestone, 2012-01-01, 0d
    The CAN FD protocol is standardized (ISO 11898-1)   :milestone, 2015-01-01, 0d
    The CAN XL protocol is standardized (ISO 11898-1 and ISO 11898-2)   :milestone, 2024-01-01, 0d

CAN standard overview

  • ISO 11898 series. Road vehicles — Controller area network (CAN):
    • Part 1: Data link layer and physical signalling
    • Part 2: High-speed medium access unit
    • Part 3: Low-speed, fault-tolerant, medium-dependent interface
    • Part 4: Time-triggered communication
    • Part 5: High-speed medium access unit with low-power mode
    • Part 6: High-speed medium access unit with selective wake-up functionality
  • ISO 16845 series. Road vehicles — Controller area network (CAN) conformance test plan:
    • Part 1: Data link layer and physical signalling
    • Part 2: High-speed medium access unit — Conformance test plan

References

Useful tools

Field bus, Communication
what is can
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