Jeff King wrote:

> What about PSP? It has the R/W lines also brought out also.

  Sorry, maybe I'm just tired, but what's PSP in this context?

>> it is a dumb beacon which merely transmits.

> Not true. You might take a look around the site before you make that
> statement... there are both receive and transmit versions of the
> circuit.  Granted, the receive versions are using external serial
> SRAM/FRAM, but its being done on a PIC.

  Sounds pretty kludgy to me...  Receive versions without the RAM are
pretty basic, regurgitate small packets (APRS beacons) only.

> Also, a KISS version is almost complete.

  Yes, I think that is a good summary.  A PIC can implement a reasonable
HDLC chip as I proposed, very effective and simple interfacing to a
parallel port in "nybble" mode, or you can make it work with a serial
port, and you can even stick some RAM in there to make it a KISS-modem.
Which is great, but it's not a TNC, can't handle protocol.

  That was always the limitation of KISS - using a full TNC to do this
was a waste.  So, it it's dirt-cheap *and* does KISS, it could be quite
useful to mate a PC (or Palm).

  You can make it digipeat at this level, but you are pretty much
stretching its abilities and you are encountering complexity.

  Beyond, to build a TNC you really need to look at a data-processing
chip which *might* include the parallel "expandable" PICs, but these now
lose their Harvard advantages and compete on an even footing with 8031
and 68HC11.  Suddenly you hit performance limits, start considering
extra hardware to compensate, prices climb and you end up with - a TNC.

  I think however, the ATMega looks promising...
--
  Cheers,
        Paul B.