This document describes the protocol Klipper uses to communicate over CAN bus. See CANBUS.md for information on configuring Klipper with CAN bus.

Micro-controller id assignment

Klipper uses only CAN 2.0A standard size CAN bus packets, which are limited to 8 data bytes and an 11-bit CAN bus identifier. In order to support efficient communication, each micro-controller is assigned at run-time a unique 1-byte CAN bus nodeid (canbus_nodeid) for general Klipper command and response traffic. Klipper command messages going from host to micro-controller use the CAN bus id of canbus_nodeid * 2 + 256, while Klipper response messages from micro-controller to host use canbus_nodeid * 2 + 256 + 1.

Each micro-controller has a factory assigned unique chip identifier that is used during id assignment. This identifier can exceed the length of one CAN packet, so a hash function is used to generate a unique 6-byte id (canbus_uuid) from the factory id.

Admin messages

Admin messages are used for id assignment. Admin messages sent from host to micro-controller use the CAN bus id 0x3f0 and messages sent from micro-controller to host use the CAN bus id 0x3f1. All micro-controllers listen to messages on id 0x3f0; that id can be thought of as a “broadcast address”.

CMD_QUERY_UNASSIGNED message

This command queries all micro-controllers that have not yet been assigned a canbus_nodeid. Unassigned micro-controllers will respond with a RESP_NEED_NODEID response message.

The CMD_QUERY_UNASSIGNED message format is: <1-byte message_id = 0x00>

CMD_SET_NODEID message

This command assigns a canbus_nodeid to the micro-controller with a given canbus_uuid.

The CMD_SET_NODEID message format is: <1-byte message_id = 0x01><6-byte canbus_uuid><1-byte canbus_nodeid>

RESP_NEED_NODEID message

The RESP_NEED_NODEID message format is: <1-byte message_id = 0x20><6-byte canbus_uuid>

Data Packets

A micro-controller that has been assigned a nodeid via the CMD_SET_NODEID command can send and receive data packets.

The packet data in messages using the node’s receive CAN bus id (canbus_nodeid * 2 + 256) are simply appended to a buffer, and when a complete mcu protocol message is found its contents are parsed and processed. The data is treated as a byte stream - there is no requirement for the start of a Klipper message block to align with the start of a CAN bus packet.

Similarly, mcu protocol message responses are sent from micro-controller to host by copying the message data into one or more packets with the node’s transmit CAN bus id (canbus_nodeid * 2 + 256 + 1).