qCAN - a simple CAN protocol #

qCAN (“kwɪ-kæn” or “kju-kæn” ) is a standard based on the CAN protocol. The main goal of the standard is to provide a minimalistic but efficient robotic communication protocol for fast development and simple debugging.

qCAN extends bare CAN with the following features:

• Each device sends periodically a message indicating that the device is accessible
• Each message contains data about Source and Target devices
• CAN frame is a message one of the following types:
  • 🔵 Data - e.g. sensor data
  • 🔴 Command
  • 🟡 Presence - each node says to the network that the node is connected

Data messages can be sent to the entire CAN network or to a specific device.

Commands can be send similarly to all devices or a particular device.

Presence messages must be sent at least once in 3 seconds by each device and can be addressed only to the entire CAN network.

Base CAN configuration:

• Bitrate: 125 kbits/s
• ID: 11 bits (standard mode) / 29 bits (extended mode)

Addressing and Message Descriptor #

Each device in the network has its unique Device ID. For the Standard Mode each device can have ID from 1 to 7, for the Extended Mode from 1 to 254.

The ID field of the CAN frame is used to describe the message. The descriptor contains:

• Device ID of the sender
• Device ID of the target (0 for broadcasting)
• Message Type

This information is mapped in the ID bits in the way allowing easy reading in HEX.

On the picture bellow you can see a Standard CAN frame and a qCAN ID structure

An Extended CAN frame allows qCAN to put more information into the ID field:

Message Types #

Examples of Message IDs:

• 0x200 - a Presence message of the device with ID 0x2
• 0x52F - a Command message of the device 0x5 to 0x2
• 0x701 - a Data message of the device 0x7 for everybody
• 0xAA00000 - a Presence message of the device with ID 0xAA
• 0x01020FF - a Command message of the device 0x1 to 0x2
• 0x0C00001 - a Data message of the device 0xC for everybody

Presence Messages #

A device is online if it declares its presence at least once per 3 seconds.

To declare being online the device should send a CAN frame with following content:

• Identifier:
  • Source ID: ID of the device declaring the presence
  • Target ID: 0 - broadcast message
  • Message Type: 0x0 - presence message
• Data:
  • Data0: qCAN Standard Major Version
  • Data1: qCAN Standard Minor Version
  • Data2: qCAN Standard Patch Version

Versioning #

Note: Until version 1.0.0 this section can be violated

The version consists of 3 numbers: Major.Minor.Patch (e.g. 2.3.9)

• All devices using the same Major should be able to interact with other devices at least partly
• All devices using the same Minor should be able to fully interact with each other and have to have the same feature set

Compatible libraries #

Libraries bellow are developed inside the Zakhar project and support ZakharCAN.

• Arduino: https://github.com/Zakhar-the-Robot/lib_arduino_canbus
• ESP-IDF: https://github.com/Zakhar-the-Robot/lib_espidf_canbus
• STM32: https://github.com/Zakhar-the-Robot/lib_stm32_canbus