QuadMeUp_Crossbow/README.md

QuadMeUp Crossbow

QuadMeUp Crossbow is a DIY project that gives 5km (at least) of RC link for UAV (airplanes and drones) for a price below $40. I uses SX1278 (LoRa 868MHz/915MHz) compatible (like HopeRF RFM95W) radio modules connected to Arduino compatible boards. It can be regular Arduino connected via SPI to SX1278 or dedicated board like Adafruit Feather 32u4 RFM LoRa or LoRa32u4 II

Current state

Works:

• Getting data from OpenTX radio using SBUS protocol
• Transmitting 10 channels to RX modules
• Frequency hopping
• Getting basic telemetry from RX module
• Sending 10 channels using SBUS to flight controller

Needs implementation:

• Binding
• TX module configuration
• Allowing to use OLED on TX to get basic data
• RX configuration from TX module
• Sending telemetry from TX to OpenTX radio

Protocol

Byte	Description	Notes
1	Frame type & used radio channel	bits 7-5 defines frame, bits 4-0 current radio channel
2 - 33	Payload	lenghth defined by frame type
payload length + 2	CRC	using crc8_dvb_s2 method

CRC

CRC is computed using crc8_dvb_s2 method. Initial CRC value for each frame CRC is equal to CRC of 4 bind bytes (unique for transmitter module).

Frame types

Value	Value hex	Description	Direction	Payload length
0000	0x0	RC channels data RC_DATA	TX -> RX	9
0001	0x1	Receiver health and basic telemetry RX_HEALTH	RX -> TX	6
0010	0x2	Request receiver configuration	TX -> RX	no used
0011	0x3	Receiver configuration	RX -> TX	no used
0100	0x4	Set receiver configuration	TX -> RX	no used
0101	0x5	PING frame, uses 9 byte payload	TX -> RX	4
0110	0x6	PONG frame, the same payload as PING	RX -> TX	4

RC_DATA frame format

Protocol allows to send 10 RC channels in total encoded as following

• channels 1 to 4 encoded using 10 bits each (5 bytes)
• channels 5 to 6 encoded using 8 bits each (2 bytes)
• channels 7 to 10 encoded using 4 bits per channel (2 bytes)

Total length of RC_DATA payload is 9 bytes

RX_HEALTH frame format

Byte	Description
1	RX RSSI
2	RX SNR
3	RX supply volatage, sent in 0,1V
4	RX analog input 1 sent in 0,1V
5	RX analog input 2 sent in 0,1V
6	Flags

Flags

Bit	Meaning
00000001	Device in Failsafe mode

PING and PONG frames

PING and PONG frames are to determine packet roundrip between TX and RX module. TX sends PING frame with curent micros. If RX receives PING frame, it respons its payload as PONG frame.

OpenTX setup

OpenTX 2.2.1 or newer is required with enabled External RF SBUS.

• Mode: SBUS
• Channel Range: 1-16
• Refresh rate: 12.0ms or 14.0ms normal

Tested with:

• FrSky X9D Plus

TX module connection diagram

RX module connection diagram