PX4-Autopilot/docs/en/modules/modules_communication.md

# Modules Reference: Communication

## frsky_telemetry

Source: [drivers/telemetry/frsky_telemetry](https://github.com/PX4/PX4-Autopilot/tree/main/src/drivers/telemetry/frsky_telemetry)

FrSky Telemetry support. Auto-detects D or S.PORT protocol.

### Usage {#frsky_telemetry_usage}

```
frsky_telemetry <command> [arguments...]
 Commands:
   start
     [-d <val>]  Select Serial Device
                 values: <file:dev>, default: /dev/ttyS6
     [-t <val>]  Scanning timeout [s] (default: no timeout)
                 default: 0
     [-m <val>]  Select protocol (default: auto-detect)
                 values: sport|sport_single|sport_single_invert|dtype, default:
                 auto

   stop

   status
```

## mavlink

Source: [modules/mavlink](https://github.com/PX4/PX4-Autopilot/tree/main/src/modules/mavlink)

### Description

This module implements the MAVLink protocol, which can be used on a Serial link or UDP network connection.
It communicates with the system via uORB: some messages are directly handled in the module (eg. mission
protocol), others are published via uORB (eg. vehicle_command).

Streams are used to send periodic messages with a specific rate, such as the vehicle attitude.
When starting the mavlink instance, a mode can be specified, which defines the set of enabled streams with their rates.
For a running instance, streams can be configured via `mavlink stream` command.

There can be multiple independent instances of the module, each connected to one serial device or network port.

### Implementation

The implementation uses 2 threads, a sending and a receiving thread. The sender runs at a fixed rate and dynamically
reduces the rates of the streams if the combined bandwidth is higher than the configured rate (`-r`) or the
physical link becomes saturated. This can be checked with `mavlink status`, see if `rate mult` is less than 1.

**Careful**: some of the data is accessed and modified from both threads, so when changing code or extend the
functionality, this needs to be take into account, in order to avoid race conditions and corrupt data.

### Examples

Start mavlink on ttyS1 serial with baudrate 921600 and maximum sending rate of 80kB/s:

```
mavlink start -d /dev/ttyS1 -b 921600 -m onboard -r 80000
```

Start mavlink on UDP port 14556 and enable the HIGHRES_IMU message with 50Hz:

```
mavlink start -u 14556 -r 1000000
mavlink stream -u 14556 -s HIGHRES_IMU -r 50
```

### Usage {#mavlink_usage}

```
mavlink <command> [arguments...]
 Commands:
   start         Start a new instance
     [-d <val>]  Select Serial Device
                 values: <file:dev>, default: /dev/ttyS1
     [-b <val>]  Baudrate (can also be p:<param_name>)
                 default: 57600
     [-r <val>]  Maximum sending data rate in B/s (if 0, use baudrate / 20)
                 default: 0
     [-p]        Enable Broadcast
     [-u <val>]  Select UDP Network Port (local)
                 default: 14556
     [-o <val>]  Select UDP Network Port (remote)
                 default: 14550
     [-t <val>]  Partner IP (broadcasting can be enabled via -p flag)
                 default: 127.0.0.1
     [-m <val>]  Mode: sets default streams and rates
                 values: custom|camera|onboard|osd|magic|config|iridium|minimal|
                 extvision|extvisionmin|gimbal|onboard_low_bandwidth|uavionix|lo
                 w_bandwidth|distance_sensor, default: normal
     [-n <val>]  wifi/ethernet interface name
                 values: <interface_name>
     [-c <val>]  Multicast address (multicasting can be enabled via
                 MAV_{i}_BROADCAST param)
                 values: Multicast address in the range
                 [239.0.0.0,239.255.255.255]
     [-F <val>]  Sets the transmission frequency for iridium mode
                 default: 0.0
     [-f]        Enable message forwarding to other Mavlink instances
     [-w]        Wait to send, until first message received
     [-x]        Enable FTP
     [-z]        Force hardware flow control always on
     [-Z]        Force hardware flow control always off

   stop-all      Stop all instances

   stop          Stop a running instance
     [-u <val>]  Select Mavlink instance via local Network Port
     [-d <val>]  Select Mavlink instance via Serial Device
                 values: <file:dev>

   status        Print status for all instances
     [streams]   Print all enabled streams

   stream        Configure the sending rate of a stream for a running instance
     [-u <val>]  Select Mavlink instance via local Network Port
     [-d <val>]  Select Mavlink instance via Serial Device
                 values: <file:dev>
     -s <val>    Mavlink stream to configure
     -r <val>    Rate in Hz (0 = turn off, -1 = set to default)

   boot_complete Enable sending of messages. (Must be) called as last step in
                 startup script.
```

## uorb

Source: [systemcmds/uorb](https://github.com/PX4/PX4-Autopilot/tree/main/src/systemcmds/uorb)

### Description

uORB is the internal pub-sub messaging system, used for communication between modules.

### Implementation

The implementation is asynchronous and lock-free, ie. a publisher does not wait for a subscriber and vice versa.
This is achieved by having a separate buffer between a publisher and a subscriber.

The code is optimized to minimize the memory footprint and the latency to exchange messages.

Messages are defined in the `/msg` directory. They are converted into C/C++ code at build-time.

If compiled with ORB_USE_PUBLISHER_RULES, a file with uORB publication rules can be used to configure which
modules are allowed to publish which topics. This is used for system-wide replay.

### Examples

Monitor topic publication rates. Besides `top`, this is an important command for general system inspection:

```
uorb top
```

### Usage {#uorb_usage}

```
uorb <command> [arguments...]
 Commands:
   status        Print topic statistics

   top           Monitor topic publication rates
     [-a]        print all instead of only currently publishing topics with
                 subscribers
     [-1]        run only once, then exit
     [<filter1> [<filter2>]] topic(s) to match (implies -a)
```