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.
TNC-X – The Expandable TNC available from http://tnc-x.com/
Now onto GMSK modem for the Pi