lorakiss/PROJECT.md

Micro controller LoRa KISS modem

This project is a LoRa KISS modem for common LoRa capable development boards, such as created by vendors like Heltec, RAK, Seeed, etc.

It is using the USB-serial interface (optionally hardware serial interface) to allow the user to send and receive LoRa packets using the KISS modem protocol.

KISS Modem

KISS Framing

KISS (Keep It Simple, Stupid) is a standard TNC protocol for packet radio. Frames use special bytes for delimiting:

Symbol	Value	Meaning
FEND	0xC0	Frame end/begin delimiter
FESC	0xDB	Frame escape
TFEND	0xDC	Transposed FEND (follows FESC)
TFESC	0xDD	Transposed FESC (follows FESC)

A frame has the structure:

FEND <type byte> <data bytes...> FEND

The type byte encodes the port number (upper nibble) and command (lower nibble):

type = (port << 4) | cmd

For host-to-modem data frames, cmd = 0x00. Any FEND or FESC bytes appearing in the data payload must be escaped as FESC TFEND or FESC TFESC respectively.

Ports

The KISS modem uses three ports for communication:

Port	Direction	Purpose
0	bidirectional	Raw LoRa packet data
1	TNC → host	Signal quality (SNR + RSSI)
2	bidirectional	Radio configuration commands

Port 1 — Signal Quality

Before each received LoRa packet delivered on port 0, the modem emits a signal quality frame on port 1. The payload is 3 bytes, big-endian:

Offset	Size	Type	Field	Description
0	1	int8	SNR	Signal-to-noise ratio in dB (rounded)
1	2	int16	RSSI	Received signal strength in dBm

Example: FEND 0x11 [-7] [0xFF 0x99] FEND = SNR = -7 dB, RSSI = -103 dBm

Configuration Commands (Port 2)

Configuration commands allow the host to query and control the modem's LoRa parameters. Each configuration frame's data payload (after the KISS type byte) begins with a 1-byte command byte. The modem responds to every SET_* command with either RES_OK (0x01) or RES_ERROR (0x02) on port 2.

Commands:

Cmd	Name	Dir	Payload
0x00	Reserved	—	—
0x01	RES_OK	←	(none)
0x02	RES_ERROR	←	(none)
0x10	GET_RADIO	→	(none)
0x10	resp	←	freq_kHz, bw_hz, sf, cr, power
0x11	SET_RADIO	→	freq_kHz, bw_hz, sf, cr, power
0x12	GET_FREQUENCY	→	(none)
0x12	resp	←	freq_kHz
0x13	SET_FREQUENCY	→	freq_kHz
0x14	GET_BANDWIDTH	→	(none)
0x14	resp	←	bw_hz
0x15	SET_BANDWIDTH	→	bw_hz
0x16	GET_SF	→	(none)
0x16	resp	←	sf
0x17	SET_SF	→	sf
0x18	GET_CR	→	(none)
0x18	resp	←	cr
0x19	SET_CR	→	cr
0x1A	GET_POWER	→	(none)
0x1A	resp	←	power_dBm
0x1B	SET_POWER	→	power_dBm
0x1C	GET_SYNCWORD	→	(none)
0x1C	resp	←	syncword
0x1D	SET_SYNCWORD	→	syncword

Legend: → = host → TNC, ← = TNC → host

Units

All integer fields are transmitted in big-endian byte order.

Parameter	Type	Unit	Encoding	Example
Frequency	uint32	kHz	4 bytes	869.618 MHz → 869618
Bandwidth	uint32	Hz	4 bytes	62.5 kHz → 62500
SF	uint8	—	1 byte (5–12)	SF 7 → 0x07
CR	uint8	—	1 byte (5–8)	4/5 → 0x05
Power	int8	dBm	1 byte	22 dBm → 0x16
Sync word	uint8	—	1 byte	0x34 → public

LoRa

Hardware

Common LoRa hardware is supported via the RadioLib library:

Module	Frequency band	Notes
SX1262	868 / 915 MHz	Used on Heltec, RAK, Seeed boards
SX1268	433 / 470 MHz	433 MHz ISM band variants
LR1110	150–960 MHz	Semtech LR11xx family
LR1120	150–960 MHz, 2.4 GHz	Semtech LR11xx family
SX1276 / SX1278	137–1020 MHz	Older boards; supported via RadioLib

Bandwidths

All LoRa bandwidths supported by RadioLib are accepted. SX126x-based boards support:

Bandwidth	Wire value (uint32 Hz)
7.8 kHz	7800
10.4 kHz	10400
15.6 kHz	15600
20.8 kHz	20800
31.25 kHz	31250
41.7 kHz	41700
62.5 kHz	62500
125 kHz	125000
250 kHz	250000
500 kHz	500000

Note: SX1272/SX1273-based boards support only 125, 250, and 500 kHz. Not all bandwidths are legal for all frequency bands — consult local regulations.

Project Structure

loramodem/
├── platformio.ini          # Board environments and build configuration
├── PROJECT.md              # This file
├── src/
│   ├── main.cpp            # Entry point; Arduino setup() and loop()
│   ├── kiss.h / kiss.cpp   # KISS frame encoder/decoder
│   ├── radio.h / radio.cpp # RadioLib wrapper (init, TX, RX, config)
│   └── config.h            # Default radio parameters and pin definitions
├── include/                # Shared headers (if needed)
├── lib/                    # Local libraries (if needed)
├── variants/               # Board-specific hardware definitions
│   ├── heltec_wifi_lora_32_v3/pins.h
│   ├── rak4631/pins.h
│   └── [other boards]/pins.h
└── test/                   # PlatformIO Unity test suite

Building

Prerequisites: PlatformIO CLI or IDE extension.

Build for a specific board:

pio run -e heltec_wifi_lora_32_v3

Upload to a connected board:

pio run -e heltec_wifi_lora_32_v3 -t upload

Monitor serial output:

pio device monitor -b 115200

Note: KISS frames are binary. Use a KISS-capable TNC client (e.g., Direwolf, socat) to interact with the modem over serial.

Available board environments are defined in platformio.ini.

Configuration Defaults

The modem initializes with these LoRa parameters at power-on:

Parameter	Default value
Frequency	TBD MHz
Bandwidth	125 kHz
SF	TBD
CR	4/5
Power	TBD dBm
Sync word	0x34 (LoRa public)

These defaults may be overridden at compile time via config.h. Parameters changed via KISS port 2 commands take effect immediately but are not persisted across power cycles unless persistent storage (EEPROM/NVS) is implemented.

Framework

This project uses PlatformIO as the build system with RadioLib as the LoRa driver library. Board-specific pin assignments and hardware configurations are isolated in the variants/ directory, allowing the core KISS modem logic to remain board-agnostic.

The USB-CDC serial interface is used by default for KISS communication. On supported boards, a hardware UART may be selected instead via compile-time configuration in config.h.