Hello Mercedes,
If you want to build the ham packet radio TNC with the modem in 1200 baud, you need to check the Bell 202 standard. There is a chip call TC3105 or T?3105 which is used in Baycom project/product. If you want to go to 9600 baud, I suggest you just forget it.
About the controller, there are projects and c source code out there, try to search them in the net. The AX25 protocol is rather long so if you want to implement it purly by software, you get lots of work to do. Those old TNC use the Zilog SCC or SIO to implement it because they got the build hardware to handle th HDLC protocol. Search all those things from the Google.
73,Li.