Last day´s i continued working on my HDLC/AX25 KISS TNC software. It is far away from beeing finished but may be useful for someone already.
What´s it ?: You can connect an RF modem with NRZ or NRZI coded output. I just tested 1k2 and 9k6 bps AX25 modems. The microcontroller (currently Atmega644) decodes the frames and forwards it to a host PC via a serial KISS interface. This interface supports plain KISS, SMACK and RMNC-CRC-KISS. What´s not in this version is the CRC check for the received packets. This will be one of the next steps.
What it should be somewhen: A complete KISS/SMACK TNC which uses an external modem. Most probably this will not work on a Atmega644 because of the RAM limitations. I try to get it into an Atmega8515 with external SRAM. Another option might be a Atmega1281 or 1284 without external memory. For 1k2 (not sure for 9k6) is would also be possible to add the modem functionality directly to the microcontroller and therefore to safe the external modem.