sbus-arduino

This library communicates with SBUS receivers and servos and is built for use with the Arduino IDE.

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 7: channel 17 (0x80)
- Bit 6: channel 18 (0x40)
- Bit 5: frame lost (0x20)
- Bit 4: failsafe activated (0x10)
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 several 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.

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

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

Installation

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

#include "sbus.h"

AVR Boards: you will also need to clone or download the Embedded Template Library into your Arduino/libraries folder. This provides etl::array functionality as an alternative to std::array.

Methods

Receiving SBUS

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.

SbusRx sbus(&Serial1);

void Begin() Initializes SBUS communication.

sbus.Begin();

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

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

std::array<uint16_t, 16> rx_channels() Returns the array of received channel data.

std::array<uint16_t, 16> sbus_data = sbus.rx_channels();

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();

Writing SBUS

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.

SbusTx sbus(&Serial1);

void Begin() Initializes SBUS communication.

sbus.Begin();

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();

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 tx_channels(const std::array<uint16_t, 16> &val) Sets the channel data to be transmitted.

sbus.tx_channels(sbus_tx_data);

bool ch17() Returns the value of channel 17 to be transmitted.

bool ch17 = sbus.ch17();

bool ch18() Returns the value of channel 18 to be transmitted.

bool ch18 = sbus.ch18();

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

bool lost_frame = sbus.lost_frame();

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

bool failsafe = sbus.failsafe();

std::array<uint16_t, 16> tx_channels() Returns the array of channel data to be transmitted.

std::array<uint16_t, 16> sbus_tx_data = sbus.tx_channels();