2.5 KiB
This document describes the protocol Klipper uses to communicate over 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).