CRSF Telemetry (TBS Crossfire Telemetry)

CRSF is a telemetry protocol that can be used for both RC control and to get telemetry information from the vehicle/flight controller on a compatible RC transmitter.

The protocol was developed by Team BlackSheep for their Crossfire RC system, but is also used by ExpressLRS (ELRS) RC systems. It is a bidirectional protocol that only needs a single UART for communicating both RC and telemetry.

The supported telemetry messages are listed here, and include: flight mode, battery level, GPS data RC signal strength, speed, altitude, and so on.

INFO

If you don't need telemetry you can connect a TBS Crossfire to the RCIN port and configure the receiver to use S.BUS. Crossfire radio systems can also be used as Telemetry Radios.

WARNING

PX4 does not include the CRSF protocol support by default. The instructions below explain how to build and upload custom PX4 firmware that includes the required modules.

Radio System Setup

To use CRSF telemetry you will need a TBS Crossfire radio system or ExpressLRS radio system that includes an RC controller with a transmitter, and a receiver (from the same vendor).

INFO

An RC radio system historically consisted of a ground-based controller that transmitted to an on-vehicle receiver. Even though many radio systems are now bidirectional, the ground module may still be referred to as the transmitter, and the air unit may be called a receiver.

Generally you will need to separately setup and configure the transmitter and receiver, and then bind them together.

A transmitter might come as an integral part of an RC controller, or it might be a separate module that you plug into a controller. If it is a separate module then you may also need to update the module software on the transmitter to firmware that supports CRSF, such as OpenTX or EdgeTx. In both cases you will need to configure the transmitter to enable CRSF.

The receiver must be wired to a spare port (UART) on the Flight Controller. Then you can bind the transmitter and receiver together.

Instructions for the steps above are covered in

• TBS Crossfire Manual
• Express LRS: QuickStart

Wiring

The TX and RX on your selected Flight Controller UART should be connected to separate channels on the receiver. The signal is usually uninverted, and can be directly connected (no additional inverter logic is required in the cable). You should check the manual for your specific receiver though!

TBS Receiver Wiring

For TBS receivers you wire the FC UART and receiver as shown (this assumes the TBS Nano RX).

FC UART	Nano RX
TX	Ch2
RX	Ch1

ExpressLRS Receiver Wiring

For ExpressLRS receivers wire to the flight controller UART as shown below (wiring is covered in detail here):

FC UART	ExpressLRS
TX	RX
RX	TX
VCC	VCC
GND	GND

PX4 Configuration

Firmware Configuration/Build

CRSF telemetry support is not included in any PX4 firmware by default. To use this feature you must build and upload custom firmware that includes crsf-rc and removes rc_input.

The steps are:

1. Setup a development environment for building PX4.

   As part of this process you will have used git to fetch source code into the PX4-Autopilot directory.

2. Open a terminal and cd into the PX4-Autopilot directory.

   cd PX4-Autopilot

3. Launch the PX4 board config tool (menuconfig) for your make target using the boardconfig option (here the target is the ARK Electronics ARKV6X flight controller):

   make ark_fmu-v6x_default boardconfig

4. In the PX4 board config tool:

   • Disable the default rc_input module
     1. Navigate to the drivers submenu, then scroll down to highlight rc_input.
     2. Use the enter key to remove the * from rc_input checkbox.
   • Enable the crsf_rc module
     1. Scroll to highlight the RC submenu, then press enter to open it.
     2. Scroll to highlight crsf_rc and press enter to enable it.

   Save and exit the PX4 board config tool.

5. Build the PX4 source code with your changes (again assuming you are using ARKV6X):

   make ark_fmu-v6x_default

This will build your custom firmware, which must now be uploaded to your flight controller.

Firmware Upload

To upload the custom firmware, first connect your flight controller to the development computer via USB.

You can upload firmware as part of the build process using the upload options:

make ark_fmu-v6x_default upload

Alternatively you can use QGroundControl to install the firmware, as described in Firmware > Installing PX4 master, beta, or custom firmware.

Parameter Configuration

Find and set the following parameters:

1. RC_CRSF_PRT_CFG — Set to the port that is connected to the CRSF receiver (such as TELEM1).

   This configures the serial port to use the CRSF protocol. Note that some serial ports may already have a default serial port mapping or default MAVLink serial port mapping that you will have to un-map before you can assign the port to CRSF. For example, if you want to use TELEM1 or TELEM2 you first need to modify MAV_0_CONFIG or MAV_1_CONFIG to stop setting those ports.

   There is no need to set the baud rate for the port, as this is configured by the driver.

2. RC_CRSF_TEL_EN — Enable to activate Crossfire telemetry.

Radio Setup

Radio Control Setup explains how to map your RC controller's attitude control sticks (roll, pitch, yaw, throttle) to channels, and to calibrate the minimum, maximum, trim and reverse settings for all other transmitter controls/RC channels.

RC Controllers

A transmitter might come as an integral part of an RC controller, or it might be a separate module that you plug into a controller.

RC Controllers that support TBS Crossfire and ExpressLRS TX modules:

• FrSky Taranis X9D Plus has an external module bay that can be used with TBS or ExpressLRS transmitter modules that are "JR module bay" compatible. You will need to install OpenTX software, which supports CRSF, and enable the external module and CRSF.
• Radiomaster TX16S has an internal ExpressLRS transmitter module. It also has an external module bay that can be used with TBS or ExpressLRS transmitter modules that are "JR module bay" compatible. It runs both OpenTX and EdgeTx software, either of which can support CRSF.

TBS Radio Systems

TBS Crossfire Radio Systems are listed here. A few options are listed below.

Transmitter modules:

• TBS CROSSFIRE TX - LONG RANGE R/C TRANSMITTER

Receivers:

• TBS Crossfire Nano RX - designed for small quadcopters.

ExpressLRS Radio Systems

Express LRS provide Radio System guidance in the Hardware Selection page. A few tested options are listed below.

Transmitter modules:

• TBD

Receivers:

• ExpressLRS Matek Diversity RX.

  INFO

  This is used in the Reptile Dragon 2 Build Log. See sections ELRS Rx and Radio Setup.

Telemetry Messages

The supported telemetry messages and their source are listed below (this table is reproduced from the TBS Crossfire Manual: "Available sensors with OpenTX").

Datapoint	Description	Data source
1RSS	Uplink - received signal strength antenna 1 (RSSI)	TBS CROSSFIRE RX
2RSS	Uplink - received signal strength antenna 2 (RSSI)	TBS CROSSFIRE RX
RQly	Uplink - link quality (valid packets)	TBS CROSSFIRE RX
RSNR	Uplink - signal-to-noise ratio	TBS CROSSFIRE RX
RFMD	Uplink - update rate, 0 = 4Hz, 1 = 50Hz, 2 = 150Hz	TBS CROSSFIRE RX
TPWR	Uplink - transmitting power	TBS CROSSFIRE TX
TRSS	Downlink - signal strength antenna	TBS CROSSFIRE TX
TQly	Downlink - link quality (valid packets)	TBS CROSSFIRE TX
TSNR	Downlink - signal-to-noise ratio	TBS CROSSFIRE TX
ANT	Sensor for debugging only	TBS CROSSFIRE TX
GPS	GPS Coordinates	TBS GPS / FC
Alt	GPS Altitudes	TBS GPS / FC
Sats	GPS Satellites acquired	TBS GPS / FC
Hdg	Magnetic orientation	TBS GPS / FC
RXBt	Battery voltage	TBS GPS / FC/ CROSSFIRE RX/ CORE
Curr	Current draw	TBS GPS / FC// CORE
Capa	Current consumption	TBS GPS / FC/ CORE
Ptch	FC Pitch angle	FC
Roll	FC Roll angle	FC
Yaw	FC Yaw angle	FC
FM	Flight mode	FC
VSPD	Barometer	FC

See Also

• TBS Crossfire Manual
• ExpressLRS Documentation
• FrSky Telemetry
• Radio Control Setup
• Radio Control Systems