>
>   How does one do multidrop?  I assume that the pic has tri-state...
> How do you do addressing without extra lines?

With RS-485 one would use a RS-485 transceiver which have separate XMIT
and receive pins, or you could share. Two pin support is possible.

With multidrop you go to a 9 bit protocol. The ninth bit indicates if the
data is a address or data. UARTS with multidrop hardware support can be
set to listen for a particular address and ignore all other bytes. In this
mode all the receivers will wait for an address packet. When one is received
each will check against their own address. All negative matches will result
in the UART going back into address only mode. The one (or more) that matches
switch the UART to receive the data. PICS of course be programmed to emulate
this behavior but they'll have to eat the overhead of reading rach byte.

Hope this helps,

BTW I wanted everyone to know that I finally got David Tait's PIC 16C84
programmer to funtion properly under Linux. Thanks for all the suggestions.

BAJ