TNC-Pi is a special version of TNC-X designed to interface directly with the Raspberry Pi computer. It can connect to the Pi either via the Pi's serial port, or via the I2C protocol. In the latter case, a single Pi can support multiple TNC-Pi's at the same time, since each TNC-Pi can be given a unique I2C address.
The firmware for TNC-Pi was based on the original TNC-X open source code by John Hansen, W2FS as modified by John Wiseman, G8BPQ to support the I2C protocol as well as the original serial port connection.
There are lots of potential applications for TNC-Pi. Here you see the APRS program Xastir running on the Pi driving a TNC-Pi via the serial connection. With this setup you've got most of the functionality of a D700/Avmap combination at less than a quarter the cost.
A Linux version of the WinLink program Paclink is currently under development. Imagine how small your WinLink station can be if you build it around a Raspberry Pi/TNC-Pi combination?
Imagine running a pair of TNC-Pi's with a single Raspberry Pi to create a dual frequency digipeater. G8BPQ has created a new version of his venerable BPQ multi-port packet program that will support this.
TNC-Pi is a fun, easy to build kit. All parts are through-hole; there's absolutely no surface mount soldering involved. You can view a copy of the manual here.