# 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 [arguments...] Commands: start [-d ] Select Serial Device values: , default: /dev/ttyS6 [-t ] Scanning timeout [s] (default: no timeout) default: 0 [-m ] 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 [arguments...] Commands: start Start a new instance [-d ] Select Serial Device values: , default: /dev/ttyS1 [-b ] Baudrate (can also be p:) default: 57600 [-r ] Maximum sending data rate in B/s (if 0, use baudrate / 20) default: 0 [-p] Enable Broadcast [-u ] Select UDP Network Port (local) default: 14556 [-o ] Select UDP Network Port (remote) default: 14550 [-t ] Partner IP (broadcasting can be enabled via -p flag) default: 127.0.0.1 [-m ] Mode: sets default streams and rates values: custom|camera|onboard|osd|magic|config|iridium|minimal| extvision|extvisionmin|gimbal|uavionix, default: normal [-n ] wifi/ethernet interface name values: [-c ] 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] 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 ] Select Mavlink instance via local Network Port [-d ] Select Mavlink instance via Serial Device values: status Print status for all instances [streams] Print all enabled streams stream Configure the sending rate of a stream for a running instance [-u ] Select Mavlink instance via local Network Port [-d ] Select Mavlink instance via Serial Device values: -s Mavlink stream to configure -r 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 [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 [ []] topic(s) to match (implies -a) ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://px4.gitbook.io/px4-user-guide/development/modules_main/modules_communication.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.