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Arduino TNC and Arduino TNC Plus

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Arduino TNC is an open KISS TNC for AFSK 1200 whose history includes several stages of development and contributions from multiple amateur radio operators.

The project began with Robert Marshall KI4MCW, who created a software AFSK modem that did not require a dedicated modem IC.

Audio was sampled by the ADC of an AVR microcontroller, while AFSK demodulation and AX.25 decoding were performed in software.

Original KI4MCW project:

https://sites.google.com/site/ki4mcw/Home/arduino-tnc

Experiments and development by George Smart M1GEO:

https://www.george-smart.co.uk/arduino/arduino_tnc/

Polish hardware version:

https://hamspirit.pl/SQ9MDD/?p=337

Arduino TNC Plus:

https://hamspirit.pl/SQ9MDD/?p=541

Code developed by SQ9MDD and SQ5RWU:

https://github.com/SQ9MDD/arduino_tnc

Project idea

The basic idea was simple: use a popular AVR microcontroller as an AFSK modem without a specialised modem chip.

Functional diagram:

Radio
  |
audio + PTT
  |
Arduino
  |
AFSK / AX.25
  |
KISS
  |
computer / APRS application

Arduino handles:

  • audio sampling,
  • AFSK demodulation,
  • AX.25 decoding,
  • transmit signal generation,
  • PTT control,
  • KISS communication.

The external application handles higher-level APRS or Packet Radio functions.

Origins: KI4MCW

Robert Marshall KI4MCW began work on Arduino TNC around 2010.

The project was an experimental software APRS modem aimed mainly at AVR-based Arduino platforms.

One of its key goals was to perform as much work as possible directly in microcontroller code.

Instead of using a device such as TCM3105 or MX614, received audio was fed directly to the processor ADC.

Radio
  |
audio
  |
ADC
  |
demodulation algorithm
  |
AX.25

Later versions added, among other features:

  • AFSK 1200 reception,
  • KISS,
  • DCD,
  • automatic ADC bias correction,
  • packet transmission,
  • UART support.

The code was heavily optimised for the limited resources of AVR microcontrollers.

Software demodulation

The most interesting aspect of the original project was the absence of a dedicated analog modem.

Arduino sampled the audio signal and detected the tones:

1200 Hz
2200 Hz

used by classic Bell 202 AFSK in APRS.

This approach made it possible to build a complete modem with very few components.

At the same time it required carefully optimised software because the ATmega328P has limited processing resources.

M1GEO experiments

George Smart M1GEO tested the KI4MCW project in detail and experimented with his own input circuitry.

His work included:

  • audio input filtering,
  • ADC bias stabilisation,
  • reducing USB-related noise,
  • analysing sampling frequency,
  • testing different KI4MCW code versions.

M1GEO used version 0.14 and achieved good KISS TNC operation.

He also experimented with sending APRS data over Bluetooth to a phone and designed his own board.

This demonstrates an important feature of Arduino TNC: the project was open and simple enough to be adapted to different uses.

Polish Arduino TNC version

Based on the earlier work, a hardware version was documented by SQ9MDD.

The goal was to create a simple and inexpensive TNC interface that could be built from commonly available components.

Typical configuration:

Radio
  |
Arduino TNC
  |
USB / KISS
  |
Raspberry Pi / PC
  |
APRS software

The design was tested both with WA8LMF test material and on the live APRS channel.

It worked well as a small modem for Raspberry Pi, an iGate or a home station.

Software development by SQ5RWU

Łukasz SQ5RWU made an important contribution to later versions of the code.

The development history includes:

  • CRC checking of received frames,
  • stability fixes,
  • receive-path corrections,
  • transmit improvements,
  • code cleanup,
  • fixes required for stable operation.

Version:

0.15.3

contains a substantial set of SQ5RWU fixes and improvements.

It was followed by:

0.15.4

marked as the stable release.

At startup the device reports:

Arduino TNC v.0.15.4

KISS

Arduino TNC operates as a classic KISS TNC.

A typical setup is:

Radio
  |
Arduino TNC
  |
KISS
  |
APRSBox / Xastir / Linux AX.25 / other software

KISS is a simple standard interface between a TNC and an application.

This keeps the device independent of any one program.

AFSK 1200

The primary mode is:

AFSK 1200 Bd
1200 / 2200 Hz
Bell 202

the classic APRS modulation used on VHF.

Arduino generates the transmit audio in software and controls PTT.

Arduino TNC Plus

The next stage of the project was Arduino TNC Plus, or ATNC+.

It was developed as an extension of the earlier design.

The basic functionality was retained, but the radio interface was expanded.

The most important change was the addition of:

TCM3105

used as a signal regeneration stage according to a solution by Adam SP5RZP.

Functional diagram:

Radio
  |
audio path
  |
TCM3105
  |
Arduino
  |
KISS
  |
computer

The goal was to improve the signal presented to the digital section and increase decoding reliability under real RF conditions.

Hardware changes in ATNC+

In addition to TCM3105, other improvements were introduced.

These included:

  • DC-blocking capacitors at transformer inputs,
  • an improved audio path,
  • support for controlling handheld radios,
  • an additional configuration jumper,
  • a 2.2 kΩ resistor for selected PTT control methods.

The ATNC+ board is larger than the original version because of the additional modem circuitry.

Stable software for ATNC+

Testing of Arduino TNC Plus also showed the need for further software improvements.

A large part of this work was done by SQ5RWU.

The result was a stable software version used with ATNC+.

The code is available here:

https://github.com/SQ9MDD/arduino_tnc

This repository should be treated as a later, corrected development line, not as the original KI4MCW version.

Communication parameters

In the stable Arduino TNC version, the serial port operates at:

19200 bit/s

for the KISS interface.

Audio isolation

The design uses audio isolation transformers.

Their purpose is to electrically isolate the radio from the digital section.

This helps reduce:

  • ground loops,
  • computer noise,
  • audio-level problems,
  • USB-related interference.

PTT control

Arduino controls the transmitter automatically.

The PTT path uses isolation and a transistor.

Arduino
  |
PTT
  |
isolation
  |
Radio

This allows safe operation with different types of radios.

Applications

Arduino TNC and ATNC+ can be used as a modem for:

  • APRS iGate,
  • APRS home station,
  • Packet Radio,
  • Raspberry Pi,
  • Linux computers,
  • KISS-compatible applications,
  • custom experimental projects.

A typical installation can look like:

Radio
  |
Arduino TNC Plus
  |
USB
  |
Raspberry Pi
  |
APRSBox

Educational value

One of the greatest values of Arduino TNC is that it allows the complete packet path to be studied.

The project shows in practice:

  • how audio reaches the ADC,
  • how AFSK tones are detected,
  • how AX.25 frames are decoded,
  • how CRC works,
  • how KISS works,
  • how the TX signal is generated,
  • how PTT is controlled.

It is a very useful project for people interested in both APRS and AVR microcontrollers.

Project history

The development can be summarised as:

KI4MCW
  |
original software Arduino TNC
  |
M1GEO
  |
testing and radio-path development
  |
SQ9MDD
  |
Polish hardware version
  |
SQ5RWU
  |
code fixes and stabilisation
  |
Arduino TNC Plus

It is therefore not a single design by one author, but the result of successive experiments and improvements by several amateur radio operators.

Who is this project for?

Arduino TNC may be interesting for people who:

  • want to build their own KISS TNC,
  • experiment with Arduino and AVR,
  • want to learn AFSK 1200 in practice,
  • need a simple modem for Raspberry Pi,
  • are interested in AX.25,
  • want to understand how a software modem works.

Arduino TNC Plus is additionally interesting for those who want to use the expanded radio path with signal regeneration.

Documentation

Main sources: