APRS Mobile Tracker / single port computer
This tracker set-up is to allow the tracker to be used as a stand-alone tracker
(GPS/TNC/Radio - no computer) OR to allow a laptop computer to be plugged 
in - without changing the TNC programming or cabling.  It only works with the  
KPC3+ TNC with 8.2 or later firmware.  The tracker will track you, or any other 
APRS station.  You will see all posits as well as your own.  You will be able to   
send, receive, and ACK messages.
So here's the basics of how the system works.

1.  The GPS receiver talks to the TNC via the radio port.  It keeps the TNC
updated with current position information

2.  The TNC sends out position packets as a stand-alone tracker.

3.  The TNC listens to the radio, decodes position packets, and sends them to the computer.

4.  The Computer  (Which may or may not be there) is running your APRS program - any
version you want. 

5.  The computer displays all the incoming packets (including yours).

That's it.