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.  


Normal use of an HSP cable requires that the TNC not be in the GPS mode, and 
the computer needs to be operational for the mobile to send out APRS packets.  If 
the computer battery dies, or you turn it off, you have to reconfigure the system to 
get the tracker operational again (Put the TNC in the GPS mode, and change the 
cable.)  I wanted the computer to be completely OPTIONAL.


I found that the KPC3 sends received packets down the RS232 line EVEN if it is in 
the GPS mode.  This allows the TNC to stay in the GPS mode and send my position 
packets all the time - AND at the same time send other peoples received position 
packets to the computer.  But I still want to see my own position.  There is a command
called MXMIT.  This allows you to montior your own transmitted packets - which puts
your location on your APRS map.

So here's the basics of how the system works.

1.  The GPS receiver talks to the TNC.  It keeps the TNC updated with current position 

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.  However the computer does not talk to the TNC.

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

6.  The computer THINKS it is sending out position packets (boy, do we have it fooled!)

7.  Since the computer isn't really sending out the position packets we can turn I toff at
any time without affecting the tracker operation.  (Shhhh - don't let the computer know - it
thinks it's in control!)

So this seems too simple - what am I missing?

Well, this isn't a perfect solution.  There are a few things I have to sacrafice.

1.  I can't send messages from the computer (it's still not talking to the computer)

2.  If a message gets sent to me I can't ack it.

3.  I can't change the path on the fly.


There was one last minute addition - the switch.  The switch was added to allow me to
program the TNC.  (see schematic)  I mounted the miniture toggle switch in the RS232
housing that is plugged into the TNC.  
In position A the computer talks to the TNC and listens to the TNC.  The GPS rcvr is out 
of the picture.  This lets me change paths , times , etc.  
Position B is the normal operating position.