donglora/docs/PROTOCOL.md

DongLoRa USB Protocol Specification

DongLoRa communicates over USB CDC-ACM (virtual serial port) using binary messages. No baud rate configuration is needed — USB handles the transport.

USB identifiers: VID 1209, PID 5741

Framing

Each message is serialized with postcard and framed with COBS:

[COBS-encoded postcard bytes] [0x00 sentinel]

COBS guarantees that 0x00 never appears in the encoded data, so the sentinel byte unambiguously marks the end of each frame. Maximum frame size is 512 bytes. USB packets are chunked at 64 bytes.

Serialization (postcard)

Postcard is a compact binary format. The subset used by DongLoRa:

Type	Encoding
u8	Raw byte
u16, u32	Varint (LEB128: 7 data bits per byte, MSB = continuation)
i8, i16	Zigzag then varint ((n << 1) ^ (n >> bits-1))
Enum variant	Varint of 0-based variant index, then variant fields
Option<T>	0x00 = None, 0x01 + T = Some
Vec<u8, N>	Varint length, then raw bytes
Struct	Fields in declaration order, no delimiters

Commands (Host → Firmware)

Index	Command	Fields	Description
0	Ping	—	Health check. Returns Pong.
1	GetConfig	—	Request current radio config. Returns Config or Error(NotConfigured).
2	SetConfig	RadioConfig	Set radio parameters. Validated against hardware limits. Returns Ok or Error(InvalidConfig).
3	StartRx	—	Enter continuous receive mode. Returns Ok or Error.
4	StopRx	—	Return to idle. Returns Ok.
5	Transmit	Option<RadioConfig>, Vec<u8, 256>	Transmit a packet. Optional per-packet config override. Returns TxDone or Error. Automatically resumes RX if it was active.
6	DisplayOn	—	Turn on the OLED display.
7	DisplayOff	—	Turn off the OLED display.

Responses (Firmware → Host)

Index	Response	Fields	Description
0	Pong	—	Reply to Ping.
1	Config	RadioConfig	Current radio configuration.
2	RxPacket	rssi: i16, snr: i16, payload: Vec<u8, 256>	Received packet with signal quality.
3	TxDone	—	Transmission complete.
4	Ok	—	Command succeeded (SetConfig, StartRx, StopRx).
5	Error	ErrorCode	Command failed.

RadioConfig

Field	Type	Wire encoding	Valid range	Description
freq_hz	u32	varint	150,000,000 – 960,000,000	Frequency in Hz
bw	Bandwidth	varint (0–9)	see table	Signal bandwidth
sf	u8	varint	5 – 12	Spreading factor
cr	u8	varint	5 – 8	Coding rate denominator (5 = CR 4/5)
sync_word	u16	varint	any	LoRa sync word (e.g. 0x3444)
tx_power_dbm	i8	zigzag varint	-9 – +22	TX power in dBm

Bandwidth

Wire value	Bandwidth
0	7.8 kHz
1	10.4 kHz
2	15.6 kHz
3	20.8 kHz
4	31.25 kHz
5	41.7 kHz
6	62.5 kHz
7	125 kHz
8	250 kHz
9	500 kHz

ErrorCode

Wire value	Error	Meaning
0	InvalidConfig	Radio config validation failed
1	RadioBusy	Radio is busy (e.g. RX restart failed)
2	TxTimeout	Transmission timed out
3	CrcError	CRC error on received packet
4	NotConfigured	Command requires SetConfig first
5	NoDisplay	Display command sent but no display attached

Worked Example

Configure the radio for 915 MHz, 125 kHz BW, SF7, CR 4/5, sync word 0x3444, 14 dBm TX power.

1. Build the RadioConfig

Field	Value	Varint bytes
freq_hz	915,000,000	80 84 AF B4 03
bw	7 (125 kHz)	07
sf	7	07
cr	5 (CR 4/5)	05
sync_word	0x3444	C4 E8 00
tx_power_dbm	14 → zigzag 28	1C

2. Build the SetConfig command

Prepend the variant index for SetConfig (index 2):

02 80 84 AF B4 03 07 07 05 C4 E8 00 1C

3. COBS-encode and send

COBS encoding wraps the bytes so 0x00 never appears in the data, then appends a 0x00 sentinel:

[COBS-encoded bytes] 00

Write the resulting bytes to the USB serial port. Read back a COBS frame, decode it, and the first byte is the response variant index (4 = Ok).

Host Implementation Checklist

1. Open the USB serial port (find by VID:PID 1209:5741)
2. Implement varint and zigzag encoding/decoding
3. Implement COBS encode/decode (use a library — available in every language)
4. Build SetConfig with your desired radio parameters
5. Send it, read back Ok
6. Send StartRx, read back Ok
7. Loop: read frames, decode RxPacket responses
8. To transmit: send Transmit with payload, read back TxDone

See examples/ for working Python implementations.