TNC with modem firmware



The objective of this project is to make a TNC behave like a telephone modem. Therefore it must be possible to 'dial in' to the modem using a standard SLIP or PPP connection. However, using a point-to-point protocol in a point-to-multipoint environment (radio frequency) would eventually cause unforeseen problems. Additionally HAMs should communicate AX.25 on the frequency.

The trick is to implement a bridge inside the TNC. The bridge can offer the correct data link protocol to the corresponding device, either PC or transceiver.

scheme representing the implementation of a bridge

HAMs can take advantage from this method, because it allows us to communicate over the radio as we are used to over the Internet using TCP/IP or other user friendly networking protocols.

Using this kind of TNC's encourages us into using servers which provides services that are identical to those on the Internet. That should not really be an obstacle as ON1BLU (Gert Leunen) clarified on his website.



The idea was implemented into the MCB152, a microcontroller based on an MCS51 derivate, design by ON4AWM (Walter Machiels). Everything is written in assembly language.

The initial, fully working versions are SLIP implementations which provide AX.25 UI connections in the air. These versions offer half duplex connections for Baycom modems for USCC cards.

Currently I am working on an extended version which should offer a full AX.25 v2.2 implementation, SLIP, PPP and a large variety of test modes.

Anyone interested on information about the sourcecode or willing to help programming is welcome to contact me personally.


Download sources

SLIP sources

release info, general include files
slip, slip, slip, slip, slip, slip

New implementation

most recent


Download HEX files



Related links


Contact me

Joachim Elen
Naamsesteenweg 333
B3800 Sint-Truiden
TEL: +32 11 674426

Acesses since september 2001