APRS TNC Project

In the amateur radio world, there are various forms of packet radio used throughout the bands. When looking at the higher bands such as 2 meter, as far as popularity goes, he Automatic Packet Reporting System (APRS) takes the cake. It is useful for relaying geographic positions, as well as other telemetry and short messages. It was first developed in the 1980s by Bob Bruninga, US Navy researcher, and was gradually adopted, and embraced by the amateur radio community.


Because of its early adoption by the amateur radio community, APRS has been incorporated in many models of radio which can make its use by amateurs facile. Initially though, communication over the APRS network had to be done using a Terminal Node Controller (TNC), a special modem like piece of equipment. This allows communication through the use of a computer, and radio, with the TNC acting as a type of intermediary. Because of the limitations of APRS implementations in consumer grade radios, or lack of a built in solution; a TNC, or computer sound card based system can be preferable or necessary.


Having become interested in APRS relatively early on during my amateur radio career, I did not possess any APRS capable transceiver. Purchase of a cable meant to interface between the my inexpensive handheld radio and sound card of a smart phone did not yield any useful results. I decided that perhaps a TNC was the logical next course of action. At the time, the TNCs available either looked ancient, or far too expensive for my hobbyist tastes. It didn’t help that given recent world evens, there was a chip shortage. I decided that a possible avenue could be constructing a TNC from available plans.


After a brief search, I located a promising open source project MicroAPRS that I could construct on a solderless breadboard, and which used a microcontroller I had a nice stock of. Because I had all of the components necessary, the only thing I had to lose was some time if things didn’t work.


But, it did work. So it was time to get everything off the breadboard, and on to a printed circuit board. The nice thing about this project, is that the user has the choice of two different versions of firmware; one that uses the KISS protocol, which is useful when interfacing with existing APRS software, and the other version that incorporates a basic toolbox of commands for a terminal interface, opening the door for many different kinds of control options.