[aprssig] D700-external PIC processing

[James Jefferson Jarvis]

> A packet parser would be helpful, but I think the most useful thing  
> someone could do is reverse engineer the connection and protocol  
> between the display head and the radio.  If we could wedge a  
> microcontroller in there, we'd have something _really_ useful.  Given  
> their replacement cost, I'm not too keen on hacking at mine. (:

It's just a two wire serial connection from the main microcontroller on 
the D700 and the display head. Probably even asyncronous serial. There 
are no bus trasceivers, just a buffer chip, so there is a dedicated RX 
and a dedicated TX pin.

I had the privledge of fixing a D700 that the control cable had gotten 
shorted on. I had to desolder a TSSOP buffer chip and put a new one in 
place. Troubleshooting that problem was fun.

-Jim KB0THN