Radio control bridge

The radio control bridge, RadioControlBridge, lets a MAVLink-capable transmitter drive your vehicle: an ExpressLRS handset, an OpenTX or EdgeTX radio. It reads the receiver's channels from a UART and puts them on the ovcs CAN bus. It decides nothing: which channel steers, which one throttles and which one picks the control level is your VMS composer's choice.

#How the channels reach the VMS

transmitter ── ExpressLRS link ──> receiver ── UART, MAVLink v2 ──> RadioControlBridge.MavlinkForwarder
                                                                        │
                                       ovcs bus: 0x2A0 (channels 1-4), 0x2A1 (channels 5-8)
                                                                        │
                                                                        v
                     VMS: OVCS.RadioControl.Steering, .Throttle, .Direction, .RequestedControlLevel, .RequestedRosCommander

1. The receiver runs in MAVLink mode and sends MAVLink v2 RC_CHANNELS_OVERRIDE messages over a UART. express_lrs decodes them.
2. RadioControlBridge.MavlinkForwarder copies channels 1 to 8 unchanged onto two radio_control_channels frames, 0x2A0 for channels 1 to 4 and 0x2A1 for 5 to 8, each channel a little-endian uint16 in microseconds.
3. On the VMS, each OVCS.RadioControl.* component reads the channel your composer names and publishes its requested_* value. VmsCore.Managers.ControlLevel then decides whether the radio is in force. Control levels covers the switches, each reference vehicle's channel layout and the safe fallbacks.

Every five seconds the forwarder logs a summary of the MAVLink traffic it received, which tells a silent link from a link that carries no RC messages.

The bridge also has an MSP OSD path for video goggles, RadioControlBridge.MspOsdForwarder, that sends nothing yet. msp_osd provides the MSP DisplayPort transport for HDZero, Walksnail and DJI VTXs it will use.

#Configure it in your vehicle

Declare the bridge in an entry of bridge_firmwares/0 (see Bridge firmwares), then implement the RadioControlBridge behaviour on your vehicle's top-level module. Its one callback returns a %RadioControlBridge.Config{} whose components list says which features run. From the OVCS Mini reference vehicle:

@behaviour RadioControlBridge

@impl RadioControlBridge
def radio_control_bridge_config(:host),
  do: %RadioControlBridge.Config{components: []}

def radio_control_bridge_config(:target),
  do: %RadioControlBridge.Config{
    components: [{:mavlink_forwarder, uart_port: "ttySC0", uart_baud_rate: 460_800}]
  }

A bare atom is shorthand for {atom, []}. An unknown component name raises at boot, so a typo fails loudly instead of silently dropping a feature.

The :host arm usually lists no components, since a workstation has no receiver UART. The UART settings live with the component that uses them: the bridge's apply_runtime_config/2 copies the :mavlink_forwarder options into the :express_lrs application environment before ExpressLrs.Application starts its connector, which is why they are read once, at boot.

#Hardware

The receiver connects to a UART of the bridge's board. On the OVCS Mini, ttySC0 at 460 800 baud is a UART of an SPI UART expander; on a board with a free hardware UART, name that one. The receiver's link must be in MAVLink mode, which on ExpressLRS forces Hybrid switch mode: Channel layout explains what that means for which channels carry three-position switches. Hardware lists the boards and parts the reference vehicles use.

#Check it on the bench

With the bridge firmware running, the two frames appear on the ovcs bus as soon as the receiver is bound. From a workstation whose CAN adapter, can0 here, sits on that bus:

candump can0,2A0:7FF,2A1:7FF

Without a transmitter, synthesise them with cansend on virtual CAN, as Driving on the host bench does for the OVCS Mini's channel layout.

#Where next

• Control levels: how the VMS turns the channels into commands.
• Bridges: the bridge firmware that hosts this bridge.
• ROS bridge: the other command path, from a gamepad or a planner.

This page is docs/bridges/radio_control.md in the ovcs repository; edit it there.