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klipper/docs/CANBUS.md

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CANBUS

This document describes Klipper's CAN bus support.

Device Hardware

Klipper currently only supports CAN on stm32 chips. In addition, the micro-controller chip must support CAN and it must be on a board that has a CAN transceiver.

To compile for CAN, run make menuconfig and select "CAN bus" as the communication interface. Finally, compile the micro-controller code and flash it to the target board.

Host Hardware

In order to use a CAN bus, it is necessary to have a host adapter. There are currently two common options:

  1. Use a Waveshare Raspberry Pi CAN hat or one of its many clones.

  2. Use a USB CAN adapter (for example https://hacker-gadgets.com/product/cantact-usb-can-adapter/). There are many different USB to CAN adapters available - when choosing one, we recommend verifying it can run the candlelight firmware. (Unfortunately, we've found some USB adapters run defective firmware and are locked down, so verify before purchasing.)

It is also necessary to configure the host operating system to use the adapter. This is typically done by creating a new file named /etc/network/interfaces.d/can0 with the following contents:

auto can0
iface can0 can static
    bitrate 500000
    up ifconfig $IFACE txqueuelen 128

Note that the "Raspberry Pi CAN hat" also requires changes to config.txt.

Terminating Resistors

A CAN bus should have two 120 ohm resistors between the CANH and CANL wires. Ideally, one resistor located at each the end of the bus.

Note that some devices have a builtin 120 ohm resistor (for example, the "Waveshare Raspberry Pi CAN hat" has a soldered on resistor that can not be easily removed). Some devices do not include a resistor at all. Other devices have a mechanism to select the resistor (typically by connecting a "pin jumper"). Be sure to check the schematics of all devices on the CAN bus to verify that there are two and only two 120 Ohm resistors on the bus.

To test that the resistors are correct, one can remove power to the printer and use a multi-meter to check the resistance between the CANH and CANL wires - it should report ~60 ohms on a correctly wired CAN bus.

Finding the canbus_uuid for new micro-controllers

Each micro-controller on the CAN bus is assigned a unique id based on the factory chip identifier encoded into each micro-controller. To find each micro-controller device id, make sure the hardware is powered and wired correctly, and then run:

~/klippy-env/bin/python ~/klipper/scripts/canbus_query.py can0

If uninitialized CAN devices are detected the above command will report lines like the following:

Found canbus_uuid=11aa22bb33cc

Each device will have a unique identifier. In the above example, 11aa22bb33cc is the micro-controller's "canbus_uuid".

Note that the canbus_query.py tool will only report uninitialized devices - if Klipper (or a similar tool) configures the device then it will no longer appear in the list.

Configuring Klipper

Update the Klipper mcu configuration to use the CAN bus to communicate with the device - for example:

[mcu my_can_mcu]
canbus_uuid: 11aa22bb33cc

USB to CAN bus bridge mode

Some micro-controllers support selecting "USB to CAN bus bridge" mode during "make menuconfig". This mode may allow one to use a micro-controller as both a "USB to CAN bus adapter" and as a Klipper node.

When Klipper uses this mode the micro-controller appears as a "USB CAN bus adapter" under Linux. The "Klipper bridge mcu" itself will appear as if was on this CAN bus - it can be identified via canbus_query.py and configured like other CAN bus Klipper nodes. It will appear alongside other devices that are actually on the CAN bus.

Some important notes when using this mode:

  • The "bridge mcu" is not actually on the CAN bus. Messages to and from it do not consume bandwidth on the CAN bus. The mcu can not be seen by other adapters that may be on the CAN bus.

  • It is necessary to configure the can0 (or similar) interface in Linux in order to communicate with the bus. However, Linux CAN bus speed and CAN bus bit-timing options are ignored by Klipper. Currently, the CAN bus frequency is specified during "make menuconfig" and the bus speed specified in Linux is ignored.

  • Whenever the "bridge mcu" is reset, Linux will disable the corresponding can0 interface. Generally, this may require running commands such as "ip up" to restart the interface. Thus, Klipper FIRMWARE_RESTART commands (or regular RESTART after a config change) may require restarting the can0 interface.