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docs/CANBUS.md
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# CANBUS
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This document describes Klipper's CAN bus support.
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## Device Hardware
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Klipper currently only supports CAN on stm32 chips. In addition, the
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micro-controller chip must support CAN and it must be on a board that
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has a CAN transceiver.
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To compile for CAN, run `make menuconfig` and select "CAN bus" as the
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communication interface. Finally, compile the micro-controller code
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and flash it to the target board.
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## Host Hardware
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In order to use a CAN bus, it is necessary to have a host adapter.
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There are currently two common options:
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1. Use a
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[Waveshare Raspberry Pi CAN hat](https://www.waveshare.com/rs485-can-hat.htm)
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or one of its many clones.
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2. Use a USB CAN adapter (for example
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[https://hacker-gadgets.com/product/cantact-usb-can-adapter/](https://hacker-gadgets.com/product/cantact-usb-can-adapter/)). There
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are many different USB to CAN adapters available - when choosing
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one, we recommend verifying it can run the
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[candlelight firmware](https://github.com/candle-usb/candleLight_fw).
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(Unfortunately, we've found some USB adapters run defective
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firmware and are locked down, so verify before purchasing.)
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It is also necessary to configure the host operating system to use the
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adapter. This is typically done by creating a new file named
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`/etc/network/interfaces.d/can0` with the following contents:
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```
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auto can0
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iface can0 can static
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bitrate 500000
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up ifconfig $IFACE txqueuelen 128
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```
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Note that the "Raspberry Pi CAN hat" also requires
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[changes to config.txt](https://www.waveshare.com/wiki/RS485_CAN_HAT).
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## Terminating Resistors
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A CAN bus should have two 120 ohm resistors between the CANH and CANL
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wires. Ideally, one resistor located at each the end of the bus.
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Note that some devices have a builtin 120 ohm resistor (for example,
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the "Waveshare Raspberry Pi CAN hat" has a soldered on resistor that
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can not be easily removed). Some devices do not include a resistor at
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all. Other devices have a mechanism to select the resistor (typically
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by connecting a "pin jumper"). Be sure to check the schematics of all
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devices on the CAN bus to verify that there are two and only two 120
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Ohm resistors on the bus.
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To test that the resistors are correct, one can remove power to the
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printer and use a multi-meter to check the resistance between the CANH
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and CANL wires - it should report ~60 ohms on a correctly wired CAN
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bus.
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## Finding the canbus_uuid for new micro-controllers
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Each micro-controller on the CAN bus is assigned a unique id based on
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the factory chip identifier encoded into each micro-controller. To
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find each micro-controller device id, make sure the hardware is
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powered and wired correctly, and then run:
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```
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~/klippy-env/bin/python ~/klipper/scripts/canbus_query.py can0
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```
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If uninitialized CAN devices are detected the above command will
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report lines like the following:
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```
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Found canbus_uuid=11aa22bb33cc
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```
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Each device will have a unique identifier. In the above example,
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`11aa22bb33cc` is the micro-controller's "canbus_uuid".
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Note that the `canbus_query.py` tool will only report uninitialized
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devices - if Klipper (or a similar tool) configures the device then it
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will no longer appear in the list.
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## Configuring Klipper
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Update the Klipper [mcu configuration](Config_Reference.md#mcu) to use
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the CAN bus to communicate with the device - for example:
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```
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[mcu my_can_mcu]
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canbus_uuid: 11aa22bb33cc
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```
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## USB to CAN bus bridge mode
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Some micro-controllers support selecting "USB to CAN bus bridge" mode
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during "make menuconfig". This mode may allow one to use a
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micro-controller as both a "USB to CAN bus adapter" and as a Klipper
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node.
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When Klipper uses this mode the micro-controller appears as a "USB CAN
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bus adapter" under Linux. The "Klipper bridge mcu" itself will appear
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as if was on this CAN bus - it can be identified via `canbus_query.py`
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and configured like other CAN bus Klipper nodes. It will appear
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alongside other devices that are actually on the CAN bus.
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Some important notes when using this mode:
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* The "bridge mcu" is not actually on the CAN bus. Messages to and
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from it do not consume bandwidth on the CAN bus. The mcu can not be
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seen by other adapters that may be on the CAN bus.
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* It is necessary to configure the `can0` (or similar) interface in
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Linux in order to communicate with the bus. However, Linux CAN bus
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speed and CAN bus bit-timing options are ignored by Klipper.
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Currently, the CAN bus frequency is specified during "make
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menuconfig" and the bus speed specified in Linux is ignored.
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* Whenever the "bridge mcu" is reset, Linux will disable the
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corresponding `can0` interface. Generally, this may require running
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commands such as "ip up" to restart the interface. Thus, Klipper
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FIRMWARE_RESTART commands (or regular RESTART after a config change)
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may require restarting the `can0` interface.
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