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[docs] CAN - update wiring/multi-CAN port details (#26080)

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docs/en/can/index.md
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@ -1,7 +1,13 @@
# CAN
# CAN (DroneCAN & Cyphal)
[Controller Area Network (CAN)](https://en.wikipedia.org/wiki/CAN_bus) is a robust wired network that allows drone components such as flight controller, ESCs, sensors, and other peripherals, to communicate with each other.
Because it is designed to be democratic and uses differential signaling, it is very robust even over longer cable lengths (on large vehicles), and avoids a single point of failure.
It is particularly recommended on larger vehicles.
## Overview
CAN it is designed to be democratic and uses differential signaling.
For this reason it is very robust even over longer cable lengths (on large vehicles), and avoids a single point of failure.
CAN also allows status feedback from peripherals and convenient firmware upgrades over the bus.
PX4 supports two software protocols for communicating with CAN devices:
@ -18,29 +24,35 @@ In 2022 the project split into two: the original version of UAVCAN (UAVCAN v0) w
The differences between the two protocols are outlined in [Cyphal vs. DroneCAN](https://forum.opencyphal.org/t/cyphal-vs-dronecan/1814).
:::
:::warning
PX4 does not support other CAN software protocols for drones such as KDECAN (at time of writing).
:::
## Wiring
The wiring for CAN networks is the same for both DroneCAN and Cyphal/CAN (in fact, for all CAN networks).
Devices are connected in a chain in any order.
Devices within a network are connected in a _daisy-chain_ in any order (this differs from UARTs peripherals, where you attach just one component per port).
:::warning Don't connect each CAN peripheral to a separate CAN port!
Unlike UARTs, CAN peripherals are designed to be daisy chained, with additional ports such as `CAN2` used for [redundancy](redundancy).
:::
At either end of the chain, a 120Ω termination resistor should be connected between the two data lines.
Flight controllers and some GNSS modules have built in termination resistors for convenience, thus should be placed at opposite ends of the chain.
Otherwise, you can use a termination resistor such as [this one from Zubax Robotics](https://shop.zubax.com/products/uavcan-micro-termination-plug?variant=6007985111069), or solder one yourself if you have access to a JST-GH crimper.
The following diagram shows an example of a CAN bus connecting a flight controller to 4 CAN ESCs and a GNSS.
It includes a redundant bus connected to `CAN 2`.
![CAN Wiring](../../assets/can/uavcan_wiring.svg)
The diagram does not show any power wiring.
Refer to your manufacturer instructions to confirm whether components require separate power or can be powered from the CAN bus itself.
::: info
For more information, see [Cyphal/CAN device interconnection](https://wiki.zubax.com/public/cyphal/CyphalCAN-device-interconnection?pageId=2195476) (kb.zubax.com).
While the article is written with the Cyphal protocol in mind, it applies equally to DroneCAN hardware and any other CAN setup.
For more advanced scenarios, consult with [On CAN bus topology and termination](https://forum.opencyphal.org/t/on-can-bus-topology-and-termination/1685).
:::
### Connectors
@ -54,7 +66,30 @@ However, as long as the device firmware supports DroneCAN or Cyphal, it can be u
DroneCAN and Cyphal/CAN support using a second (redundant) CAN interface.
This is completely optional but increases the robustness of the connection.
All Pixhawk flight controllers come with 2 CAN interfaces; if your peripherals support 2 CAN interfaces as well, it is recommended to wire both up for increased safety.
Pixhawk flight controllers come with 2 CAN interfaces; if your peripherals support 2 CAN interfaces as well, it is recommended to wire both up for increased safety.
### Flight Controllers with Multiple CAN Ports
[Flight Controllers](../flight_controller/index.md) may have up to three independent CAN ports, such as `CAN1`, `CAN2`, `CAN3` (neither DroneCAN or Cyphal support more than three).
Note that you can't have both DroneCAN and Cyphal running on PX4 at the same time.
::: tip
You only _need_ one CAN port to support an arbitrary number of CAN devices using a particular CAN protocol.
Don't connect each CAN peripheral to a separate CAN port!
:::
Generally you'll daisy all CAN peripherals off a single port, and if there is more than one CAN port, use the second one for [redundancy](redundancy).
If three are three ports, you might use the remaining network for devices that support another CAN protocol.
The documentation for your flight controller should indicate which ports are supported/enabled.
At runtime you can check what DroneCAN ports are enabled and their status using the following command on the [MAVLink Shell](../debug/mavlink_shell.md) (or some other console):
```sh
uavcan status
```
Note that you can also check the number of supported CAN interfaces for a board by searching for `CONFIG_BOARD_UAVCAN_INTERFACES` in its [default.px4board](https://github.com/PX4/PX4-Autopilot/blob/main/boards/px4/fmu-v6xrt/default.px4board#) configuration file.
## Firmware