Sbus

This library communicates with SBUS receivers and servos and is compatible with Arduino ARM and CMake build systems.

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Description

SBUS is a bus protocol for receivers to send commands to servos. Unlike PWM, SBUS uses a bus architecture where a single serial line can be connected with up to 16 servos with each receiving a unique command.

The SBUS protocol uses an inverted serial logic with a baud rate of 100000, 8 data bits, even parity, and 2 stop bits. The SBUS packet is 25 bytes long consisting of:

• Byte[0]: SBUS header, 0x0F
• Byte[1 -22]: 16 servo channels, 11 bits each
• Byte[23]
  • Bit 0: channel 17 (0x01)
  • Bit 1: channel 18 (0x02)
  • Bit 2: frame lost (0x04)
  • Bit 3: failsafe activated (0x08)
• Byte[24]: SBUS footer

Note that lost frame is indicated when a frame is lost between the transmitter and receiver. Failsafe activation typically requires that many frames are lost in a row and indicates that the receiver has moved into failsafe mode. Packets are sent approximately every 10 ms or 20 ms, depending on the system configuration.

Note on CH17 and CH18: Channel 17 and channel 18 are digital on/off channels. These are not universally available on all SBUS receivers and servos.

FrSky receivers will output a range of 172 - 1811 with channels set to a range of -100% to +100%. Using extended limits of -150% to +150% outputs a range of 0 to 2047, which is the maximum range acheivable with 11 bits of data.

Because SBUS is a digital bus format, it is an excellent means of receiving pilot commands from a transmitter and an SBUS capable receiver. If SBUS servos are used in the aircraft, SBUS is also an excellent means of sending actuator commands - servo commands can often be sent with lower latency and, by only using a single pin to command up to 16 servos, additional microcontroller pins are freed for other uses.

Inverted Serial

SBUS uses an inverted serial protocol, which is not commonly supported in Arduino. This library is able to use inverted serial for the following microcontrollers:

• Teensy 3.x
• Teensy 4.x
• Teensy LC
• STM32L496xx
• STM32L476xx
• STM32L433xx
• STM32L432xx
• ESP32

For all other microcontrollers, you must use a serial inverter.

Installation

Arduino

Simply clone or download and extract the zipped library into your Arduino/libraries folder. The library is added as:

#include "sbus.h"

An example is located in examples/arduino/sbus_example/sbus_example.ino. This library is tested with Teensy 3.x, 4.x, and LC devices and should work with other ARM devices. It is not expected to work with AVR due to the use of std::array.

CMake

CMake is used to build this library, which is exported as a library target called sbus. The header is added as:

#include "sbus.h"

The library can be also be compiled stand-alone using the CMake idiom of creating a build directory and then, from within that directory issuing:

cmake .. -DMCU=MK66FX1M0
make

This will build the library and example executable called sbus_example. The example executable source file is located at examples/cmake/sbus_example.cc. Notice that the cmake command includes a define specifying the microcontroller the code is being compiled for. This is required to correctly configure the code, CPU frequency, and compile/linker options. The available MCUs are:

• MK64FX512
• MK66FX1M0
• MKL26Z64
• IMXRT1062_T40
• IMXRT1062_T41

These are known to work with the same packages used in Teensy products. Also switching packages is known to work well, as long as it's only a package change.

The sbus_example target creates an executable for communicating with sbus receivers and servos. This target also has a _hex for creating the hex file and an _upload for using the Teensy CLI Uploader to flash the Teensy. Please note that the CMake build tooling is expected to be run under Linux or WSL, instructions for setting up your build environment can be found in our build-tools repo.

Namespace

This library is within the namespace bfs.

SbusRx

This class is used for receiving SBUS data from an SBUS capable receiver.

SbusRx(HardwareSerial *bus) Creates an SbusRx object. A pointer to the Serial object corresponding to the serial port used is passed. The RX pin of the serial port will receive SBUS packets.

bfs::SbusRx sbus(&Serial1);

void Begin() Initializes SBUS communication.

sbus.Begin();

(ESP-32 ONLY) void Begin(const int8_t rxpin, const int8_t txpin) Initialized SBUS communication, given the Serial RX and TX pins.

bool Read() Parses SBUS packets, returns true on successfully receiving an SBUS packet.

if (sbus.Read()) {
   // Do something with the received data
}

static constexpr int8_t NUM_CH() A constant defining the number of SBUS channels (i.e. 16), useful for defining arrays to read the data into.

std::array<int16_t, NUM_SBUS_CH_> ch() Returns an array of received channel data.

std::array<int16_t, bfs::SbusRx::NUM_CH()> rx_ch = sbus.ch();

bool ch17() Returns the value of channel 17.

bool ch17 = sbus.ch17();

bool ch18() Returns the value of channel 18.

bool ch18 = sbus.ch18();

bool lost_frame() Returns true if a frame has been lost.

bool lost_frame = sbus.lost_frame();

bool failsafe() Returns true if the receiver has entered failsafe mode.

bool failsafe = sbus.failsafe();

SbusTx

This class is used for transmitting SBUS data to SBUS capable servos.

SbusTx(HardwareSerial *bus) Creates an SbusTx object. A pointer to the Serial object corresponding to the serial port used is passed. The TX pin of the serial port will transmit SBUS packets.

bfs::SbusTx sbus(&Serial1);

void Begin() Initializes SBUS communication.

sbus.Begin();

(ESP-32 ONLY) void Begin(const int8_t rxpin, const int8_t txpin) Initialized SBUS communication, given the Serial RX and TX pins.

void Write() Writes an SBUS packet. The packet is written immediately, you should regulate timing of sending packets to servos to maintain a frequency of approximately 100 Hz or 50 Hz, depending on the setup of the SBUS system.

sbus.Write();

static constexpr int8_t NUM_CH() A constant defining the number of SBUS channels (i.e. 16), useful for defining arrays to write the data from.

void ch17(bool val) Sets the value of channel 17 to be transmitted.

sbus.ch17(true);

void ch18(bool val) Sets the value of channel 18 to be transmitted.

sbus.ch18(true);

void lost_frame(bool val) Sets whether to transmit the lost frame flag.

sbus.lost_frame(true);

void failsafe(bool val) Sets whether to transmit the failsafe flag.

sbus.failsafe(true);

void ch(const std::array<int16_t, NUM_SBUS_CH_> &cmd) Sets the channel data to be transmitted given an array of SBUS commands.

/* Set SBUS commands */
std::array<int16_t, bfs::SbusTx::NUM_CH()> cmd;
sbus.ch(cmd);

bool ch17() Returns the CH17 data to be transmitted.

bool ch17 = sbus.ch17();

bool ch18() Returns the CH18 data to be transmitted.

bool ch18 = sbus.ch18();

bool lost_frame() Returns the lost frame flag to be transmitted.

bool lost_frame = sbus.lost_frame();

bool failsafe() Returns the failsafe flag to be transmitted.

bool failsafe = sbus.failsafe();

std::array<int16_t, NUM_SBUS_CH_> ch() Returns the channel data to be transmitted.

std::array<int16_t, bfs::SbusRx::NUM_CH()> rx_ch = sbus.ch();