MAVLink Peripherals (GCS/OSD/Companion)

Ground Control Stations (GCS), On-Screen Displays (OSD), Companion Computers, ADS-B receivers, and other MAVLink peripherals interact with PX4 using separate MAVLink streams, sent via different serial ports. These communication channels are configured using the MAVLink parameters.

In order to assign a particular peripheral to a serial port we use the (abstract) concept of a MAVLink instance.

Each instance can represent a particular set of streamed messages (see MAV_X_MODE below); parameters are used to define the set of messages, the port used, data rate, etc.

At time of writing three MAVLink instances are defined, which correspond to the 0, 1, 2 in the parameters listed below.

The parameters for each instance are:

  • MAV_X_CONFIG - Set the serial port (UART) for this instance "X", where X is 0, 1, 2. It can be any unused port, e.g.: TELEM2, TELEM3, GPS2 etc. For more information see Serial Port Configuration.
  • MAV_X_MODE - Specify the telemetry mode/target (the set of messages to stream for the current instance and their rate). The default values are:

    • Normal: Standard set of messages for a GCS.
    • Custom or Magic: Nothing (in the default PX4 implementation). Modes may be used for testing when developing a new mode.
    • Onboard: Standard set of messages for a companion computer.
    • OSD: Standard set of messages for an OSD system.
    • Config: Standard set of messages and rate configuration for a fast link (e.g. USB).
    • Minimal: Minimal set of messages for use with a GCS connected on a high latency link.
    • ExtVision or ExtVisionMin: Messages for offboard vision systems (ExtVision needed for VIO).
    • Iridium: Messages for an Iridium satellite communication system.

    If you need to find the specific set of message for each mode search for MAVLINK_MODE_ in /src/modules/mavlink/mavlink_main.cpp.

    The mode defines the default messages and rates. A connected MAVLink system can still request the streams/rates that it wants using MAV_CMD_SET_MESSAGE_INTERVAL.

  • MAV_X_RATE - Set the maximum data rate for this instance (bytes/second).
    • This is the combined rate for all streams of individual message (the rates for individual messages are reduced if the total rate exceeds this value).
    • The default setting will generally be acceptable, but might be reduced if the telemetry link becomes saturated and too many messages are being dropped.
    • A value of 0 sets the data rate to half the theoretical value.
  • MAV_X_FORWARD - Enable forwarding of MAVLink packets received by the current instance onto other interfaces. This might be used, for example, to transfer messages between a GCS and a companion computer so that the GCS can talk to a MAVLink enabled camera connected to the companion computer.

Next you need to set the baud rate for the serial port you assigned above (in MAV_X_CONFIG).

You will need to reboot PX4 to make the parameter available (i.e. in QGroundControl).

The parameter used will depend on the assigned serial port - for example: SER_GPS1_BAUD, SER_TEL2_BAUD, etc. The value you use will depend on the type of connection and the capabilities of the connected MAVLink peripheral.

The TELEM 1 port is almost always used for the GCS telemetry stream.

To support this there is a default serial port mapping of MAVLink instance 0 as shown below:


For example, to use a companion computer on TELEM 2 you might set parameters as shown:

© PX4 Dev Team. License: CC BY 4.0            Updated: 2020-10-23 00:56:02

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