TNC (KISS / TNC2) Functionality

Our firmware includes a built-in Terminal Node Controller (TNC) that bridges the LoRa radio network with APRS applications running on your computer or smartphone. It turns your device into a modem that can send and receive APRS packets.

The TNC is available over three independent interfaces, and each interface can use one of two framing modes: KISS or TNC2. You enable the interfaces and choose their modes in the “TNC Settings” section of the web configuration panel.

➡️ For the configuration fields, see: Configuration → TNC Settings

Framing modes: KISS vs TNC2

Each interface can run in one of two modes. The mode determines how packets are encoded on the wire — it does not change which interface you connect to.

  KISS TNC2
Frame format AX.25 binary, SLIP-framed Raw APRS text, terminated with \n
Callsign length Max 6 characters (AX.25 strict) Longer callsigns accepted
SSID Numeric 0–15 only Alphanumeric -XX SSIDs accepted
Validation Strict — frames violating AX.25 are rejected Lenient — passes raw text through
Best for Standard KISS apps (APRSIS32, APRSDroid, etc.) Tools that need long/alphanumeric callsigns or simple text framing

Key difference: KISS follows AX.25 strictly, so any address longer than 6 characters or an SSID outside the numeric range 0–15 is rejected. TNC2 sends the raw APRS text frame and therefore accepts alphanumeric -XX SSIDs that KISS cannot represent.

If you use tactical callsigns or any callsign with an alphanumeric SSID, you must use TNC2 — KISS will reject those frames.

Interfaces: which one, when

Interface Transport Connection Modes Notes
TNC Server TCP/IP (Wi-Fi) Port 8001, up to 4 simultaneous clients KISS / TNC2 Most convenient; no cable needed
USB Serial TNC USB serial COM port @ 115200 bps KISS / TNC2 Direct wired connection to your computer
Hardware Serial TNC GPIO serial Configurable RX/TX pins @ 115200 bps KISS / TNC2 For wiring to external hardware/microcontrollers

All three interfaces can be enabled at the same time, each with its own independent mode.

TCP/IP TNC Server

When the device is connected to your Wi-Fi network, it can run a TNC server over TCP/IP. This lets you connect APRS software wirelessly over your local network.

This is the most convenient method for connecting modern applications without a physical cable.

USB Serial TNC

The TNC can also be accessed via the device’s USB serial port. When connected to your computer, the device appears as a COM port that your APRS software can open. This provides a direct, wired connection.

Hardware Serial TNC

A separate hardware (GPIO) serial port can also act as a TNC, intended for wiring the device directly to external hardware or another microcontroller.

Accept Own Frames

The Accept Own Frames option (in TNC Settings) makes the TNC also process packets sent by the device itself, instead of ignoring them. Useful when you want your APRS software to see everything the device transmits.

Compatible Software

The TNC is compatible with a wide range of APRS software. Classic KISS applications like APRSIS32 (Windows) or APRSDroid (Android) work with the KISS mode, but we highly recommend modern, web-based clients for the best experience.

  1. APRS TNC Web

    • A modern, browser-based TNC web server that runs on any device on your local network. It requires no installation—simply open the web page and connect to your device’s IP address from within your local network.
    • Link: https://github.com/SQ2CPA/aprs-tnc-web

  2. LoRa APRS Mobile App

    • A dedicated Android App designed for a seamless mobile experience. It allows you to easily send and receive LoRa APRS messages, view packets on a map, and interact with the network directly from your smartphone.
    • Link: https://app.lora-aprs.pl