Might be difficult to build a TNC without a special CPU chip and EPROM, most of these chips are "owned" by KAM or Timewave who took over the AEA PK TNC Line, and they are for replacement. Might look for a used TNC here on eHam net or other ham radio classified ad sites. Lot of TNC's are going cheap as VHF Packet operation in most areas of the USA are dead or pretty much dead. The TPAR may have some TNC kits you could build:
http://www.tapr.org/tapr/index.html
