docs: Update CANBUS.md
Recommend USB to CAN adapter and no longer recommend using the waveshare rpi hat. Recommend using allow-hotplug for all USB adapters. Note bandwidth limitations when using USB to CAN bridge mode. Note that a USB to CAN mcu is not a USB serial device. Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
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Kevin O'Connor
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@ -4,7 +4,7 @@ This document describes Klipper's CAN bus support.
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## Device Hardware
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Klipper currently supports CAN on stm32, same5x, and rp2040 chips. In
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Klipper currently supports CAN on stm32, SAME5x, and rp2040 chips. In
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addition, the micro-controller chip must be on a board that has a CAN
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transceiver.
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@ -14,44 +14,35 @@ 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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In order to use a CAN bus, it is necessary to have a host adapter. It
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is recommended to use a "USB to CAN adapter". There are many different
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USB to CAN adapters available from different manufacturers. When
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choosing one, we recommend verifying that the firmware can be updated
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on it. (Unfortunately, we've found some USB adapters run defective
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firmware and are locked down, so verify before purchasing.) Look for
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adapters that can run Klipper directly (in its "USB to CAN bridge
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mode") or that run the
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[candlelight firmware](https://github.com/candle-usb/candleLight_fw).
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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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allow-hotplug can0
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iface can0 can static
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bitrate 500000
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bitrate 1000000
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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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Note that some devices have a builtin 120 ohm resistor that can not be
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easily removed. Some devices do not include a resistor at all. Other
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devices have a mechanism to select the resistor (typically by
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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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@ -95,22 +86,18 @@ canbus_uuid: 11aa22bb33cc
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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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during Klipper's "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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as if it was on this CAN bus - it can be identified via
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`canbus_query.py` and it must be configured like other CAN bus Klipper
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nodes.
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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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@ -119,12 +106,32 @@ Some important notes when using this mode:
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* Whenever the "bridge mcu" is reset, Linux will disable the
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corresponding `can0` interface. To ensure proper handling of
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FIRMWARE_RESTART and RESTART commands, it is recommended to replace
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`auto` with `allow-hotplug` in the `/etc/network/interfaces.d/can0`
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file. For example:
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FIRMWARE_RESTART and RESTART commands, it is recommended to use
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`allow-hotplug` in the `/etc/network/interfaces.d/can0` file. For
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example:
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```
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allow-hotplug can0
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iface can0 can static
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bitrate 500000
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bitrate 1000000
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up ifconfig $IFACE txqueuelen 128
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```
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* The "bridge mcu" is not actually on the CAN bus. Messages to and
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from the bridge mcu will not be seen by other adapters that may be
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on the CAN bus.
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* The available bandwidth to both the "bridge mcu" itself and all
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devices on the CAN bus is effectively limited by the CAN bus
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frequency. As a result, it is recommended to use a CAN bus frequency
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of 1000000 when using "USB to CAN bus bridge mode".
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Even at a CAN bus frequency of 1000000, there may not be sufficient
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bandwidth to run a `SHAPER_CALIBRATE` test if both the XY steppers
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and the accelerometer all communicate via a single "USB to CAN bus"
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interface.
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* A USB to CAN bridge board will not appear as a USB serial device, it
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will not show up when running `ls /dev/serial/by-id`, and it can not
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be configured in Klipper's printer.cfg file with a `serial:`
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parameter. The bridge board appears as a "USB CAN adapter" and it is
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configured in the printer.cfg as a [CAN node](#configuring-klipper).
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