Radio Control Systems

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This article provides an overview of the RC Transmitter and Receiver Systems that can be used with ArduPilot autopilots.

Compatible RC Protocols

ArduPilot autopilots are compatible with the following receiver output protocols:

  1. PPM-Sum receivers

  2. SBus receivers

  3. IBUS receivers

  4. FPort Receivers

  5. Spektrum SRXL2,DSM, DSM2, and DSM-X Satellite receivers

  6. Multiplex SRXL version 1 and version 2 receivers

  7. CRSF receivers (including ExpressLRS systems)

  8. Graupner SUM-D

  9. Parallel PWM outputs encoded to PPM-Sum using an external encoder (see below)

Connecting the Receiver

For all protocols above, ArduPilot auto-detects the protocol of the RC receiver system. However, depending on the protocol and autopilot type, the physical connection to the autopilot may differ.

Some protocols, most noticeably SRXL2, CRSF, and ELRS, require a full UART connection.

In addition other protocols that also provide telemetry, like FPort, would generally require a bi-directional half-duplex connection in order to obtain telemetry. For these protocols the TX output of the UART should be connected to the serial input of the receiver. It is also possible on F7 and H7 boards to connect to the UART RX input with some additional configuration.

PPM-Sum/SBus/IBus

These receivers are usually connected to the RCin or SBUS input pin on the autopilot.

To connect a PPM-Sum receiver or an SBus receiver to a Pixhawk, for example, plug the ground (black), power (red) and signal (usually white - orange in the diagram below) wires to the RC pins on the Pixhawk.

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Tip

The parameter to enable the SBus output from the PixHawk style autopilots is BRD_SBUS_OUT. This is only to pass SBus externally to other devices, like servos. Not to connect a receiver to RCin or SBus In.

DSM/DSM2/DSM-X/SRXL/SUM-D

For autopilots that do not provide a separate DSM input, these can be connected as above. However, for performance reasons on autopilots that use an IOMCU (The Pixhawk/Cube family), the autopilot’s DSM input connection is highly recommended.

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FPort/FPort2

FPort is a bi-directional protocol, using SBus RC in one direction, and serial telemetry in the other. The RC portion can be decoded when attached to an autopilot as if it were SBus, but the embedded telemetry would be lost. See the FPort setup documentation for details on connection to one of the autopilots Serial Ports.

SRXL2/CRSF/ELRS

These bi-directional protocols require the use of a Serial Port. See links below for setup and connections.

RC input to Serial Port

Note

any UART RX input will auto-detect all the protocols (except PPM, or SRXL2/CRSF/ELRS which also require connection of the UART’s TX pin), if the serial port protocol is set to 23 (for example, generally, SERIAL2_PROTOCOL for the TELEM2 UART if used). The exception to this is for SBUS attached to UARTs on F4 based autopilots. This requires an external inverter since SBUS is inverted and F4 autopilots do not have selectable inversion on their UART pins.

Note

The serial port baudrate is automatically set and controlled by the firmware when any serial RC protocol is detected.

Radio System Selection

Selection will depend on many factors: range,telemetry requirements, cost, compatibility with existing equipment, etc. Most manufacturers often many different models with differing capabilities. Many systems have been reverse-engineered and “cloned” by other manufacturers offering more economical versions of transmitters and receivers. Many Transmitters offer multiple protocols and are based on the OpenTX firmware which is extremely flexible and also allows the use of LUA scripts to display telemetry data on their LCD screens.

Range

RC control range varies greatly depending on system used, installation, antennas used, terrain, and even weather conditions. But in general, for discussion purposes here, RC systems can be categorized into Short Range (2km and under) , Medium (2-10km) and Long Range (>10km). In addition, they may offer uni-directional (Vehicle to Transmitter) or bi-directional (Vehicle to/from Transmitter) telemetry.

Telemetry

FrSky Horus Transmitter running Yaapu LUA script

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Some systems provide transparent radio modems to the telemetry from the autopilot, and others have proprietary protocols. Those with proprietary protocols often have means of displaying telemetry data on the transmitter display itself, like FRSky or other OpenTX based Transmitters.

Telemetry speeds vary from 56K to 1-2K baud depending on protocol and, in some cases, distance. Often telemetry range will be less than radio control range.

Number of channels

ArduPilot requires at least 5 channels for most vehicles, however, 8 to 16 channels are commonly available in most systems and are very convenient for controlling other vehicle functions such as cameras or flight feature options. Many vehicles require 8 channels just for basic operation, such as many QuadPlanes.

Below is a table with some commonly available systems showing these elements. Note, not all versions of transmitters and/or receivers from a particular manufacturer may have these characteristics. Note: that many “clone” or “compatible” versions of this systems also exist, only the OEM system is listed.

Original Manufacturer

Range

Telemetry

Telem Speed

TX Display

RC Protocol

Notes

DragonLink

Long

Bi-dir

56K

via MTP/LUA

PPM_SUM/SBUS

1

CRSF

Long

Bi-Dir

Variable

via LUA

SBUS/CRSF

3

ELRS

Long

Limited

CRSF

4

FLYSKY

Short

No

IBus

FrSky X series

Short

Bi-dir

Medium

yes

PPM-SUM/SBUS/ FPort

2

FrSky R9 series

Medium

Bi-dir

Medium

yes

PPM-SUM/SBUS/ FPort

2

Futaba

Short

No

SBus

Graupner

Short

Yes

Medium

yes

SUM-D

Multiplex

Short

No

SRXL

Spektrum

Short

No

DSM/DSM2 DSM-X/ SRXL

Note 1: DragonLink provides a 56Kbaud transparent link for telemetry, allowing full MAVLink telemetry to/from the vehicle from the transmitter module. Dragonlink is an add-on module to the transmitter, such as an FRSky Taranis or RadioMaster T16. See DragonLink RC Systems. MTP (Mavlink to Passthru) converters are available to allow direct display of MAVLink Telemetry data on OpenTX transmitters using Yaapu Telemetry LUA Script.

Note 2: See Yaapu FrSky Telemetry Script for OpenTX. Future firmware versions will offer the ability to change parameters over FRSky telemetry from an Open TX compatible transmitter in addition to displaying the telemetry data. Most FRSky compatible transmitters use OpenTX. Note that R9 systems are not quite Long Range, but much further range than normal FRSky systems, themselves at the very high end of the Short Range category at 1.6-2km range.

Note 3: ArduPilot provides a means to send its telemetry data via CRSF such that it can be displayed on OpenTX transmitters using the Yaapu Telemetry LUA Script.

Note 4: ELRS (EpressLRS) is a system that uses the CRSF (TBS Crossfire) RC protocol with several minimizations to simplify the system. It has reduced features but it connects to ArduPilot just like CRSF, when CRSF RXs are attached using a full UART, instead of SBUS protocol to communicate to ArduPilot. See ExpressLRS site <https://www.expresslrs.org/2.0/> for more information.

PPM encoders

A PPM Encoder will allow you to use any older style RC receiver that has only PWM outputs for each channel instead of an SBUS or PPM output. See PPM encoders for more information.