CAN (Controller area network)

Networking

Every car uses canbus.

  • Serial Communication

  • Bus Interface

  • Half duplex: Only one device can talk at a time.

  • Differential Signal

    • More reliable

  • Asynchronous

    • Doesn't use a shared clock source

Canbus connects ECU (Electrical Control Units). Canbus allows any ECU to communicate over a CAN interface

History

Before CANBus, BOSH released CAN in the 90s, and it became an international standard in '93. Every car uses CANbus.

OS Tools and Commands

ip link show- list network devices including CAN candump canxxx - dump the comms coming from interface canxx

CAN Bus Drop Length (Stub Length)

Definition

CAN bus drop length (also called stub length) is the short branch cable that connects a node (ECU, sensor, controller, etc.) to the main CAN backbone.

It is part of the physical layer architecture of a CAN network.

Drop length is not the total bus length — it is only the small branch from the main trunk to a device.

CAN Bus Physical Architecture

A proper high-speed CAN network uses a linear backbone topology:

[120Ω]───Main Backbone────────Main Backbone────────[120Ω]
             |                        |
           Stub                     Stub
           (short)                  (short)
             |                        |
           Node                     Node

Key Rules

  • One continuous twisted-pair backbone

  • Short drop lines (stubs) to each node

  • 120Ω termination resistors only at the two ends

  • No star topology for high-speed CAN

Why Drop Length Matters

CAN is a high-speed differential communication system.

If the stub is too long, it can cause:

  • Signal reflections

  • Ringing

  • Data corruption

  • Intermittent CAN errors

  • Reduced maximum reliable bitrate

Long stubs act like transmission lines and cause impedance mismatch.

Recommended Maximum Stub Lengths (High-Speed CAN – ISO 11898-2)

|Bitrate|Max Stub Length| |---|---| |1 Mbps|~0.3 m (30 cm)| |500 kbps|~0.6 m| |250 kbps|~1 m| |125 kbps|~2 m|

Rule of Thumb

Higher bitrate → shorter allowable stub length.

What NOT To Do (Star Topology)

          Node
            |
Node --- Node --- Node
            |
          Node

Star configurations cause reflections and are not suitable for high-speed CAN.

Best Practice Summary

  • Keep the backbone long

  • Keep the drops short

  • Terminate only at the two physical ends

  • Use proper twisted pair cable

  • Match bitrate to total network length

If needed, calculate limits based on:

  • Bitrate

  • Total backbone length

  • Number of nodes