What do you do with a microcontroller once you have it running? Implement a TNC, of course! This is a project in progress, so far I have only completed the initial research and some preliminary layout, so there are no useful results at this point to show.

The objective was to extend the learning exercise for the tiny basic board to include at least one amateur application. The simplest, which can be used right away, is to implement a TNC (terminal node controller), to modulate and demodulate FSK tones for packet radio.

This is nothing new; it has been around since the 1980’s. This implementation was done without the aid of any VLSI hardware devices, so that it would demonstrate how the data is encoded and decoded. Two versions were laid out; the first is a simple add-on board to the Tiny Basic SBC, the second included a version of the microcontroller as a standalone device.

The tone generation was done using a simple R-2R ladder to implement a digital to analog converter, followed by a buffer amplifier. Demodulation was done by first squaring the signal and then using a simple frequency counter to determine which tone was which. Somewhat crude, but it does the job and illustrates how packet radio works.

3D Rendering of the TNC adapter

The figure above is a 3D rendering from KiCAD, the package that I use for PCB layout. on the left is a connector that mates with the expansion connector on the SBC using a ribbon cable, on the right is a set of terminal to connect to the radio. The parts are all dual-inline packages (DIP), so they can be easily assembled by the average amateur. The PTT circuit uses a FET which can sink up to 60mA, which should be enough for most radios.

Below is a manual that describes the board and SBC implementation in more detail. I should have this up and running by the end of January next year. Stay tuned.