Hello Simon,

A simple high frequency modulation, lets say something above 300kHz (to
allow more than 30 cycles per 9600bps bit time). One frequency to
transmit another to receive. The receivers would be tunned at the
opposite transmiters frequency, working as a simple narrow band
filters.  You don't need the transmit clock involved, since the output
of the receiver would be rebuilt as pure as the UART output. Just pay
attention to avoid harmonics involved.  You can try to use the carrier
frequencies as high as possible at the coax.  The use of modems would be
nice but not totally necessary. What you are looking for is simply a
"radio via wire"... :)  Remember those nice Intercoms using only AC
wiring?  You have a pure, shielded and clean coax cable... much better,
huh?

Wagner.



Simon Harding wrote:
>
> Hi
> I have a requirement to transmit low rate (9600bps) half duplex data from a PIC
> based slave device back and forth to a main controller via a single coax,
> preferably via FSK, GMSK etc as apposed to shifting voltage levels as this will
> allow us to multiplex other signals onto the cable at the same time.
>
> I have looked into embedded modem devices but the problem seems to be the
> conversion from asynchronous to synchronous data. As the UART within the PIC
> does not allow access to RX and TX clock lines it makes it difficult to hook up
> the single chip modems that I have been able to locate.
>
> There is always the bit banging method, but as the UART is there it would seem
> sensible to use it if possible.
>
> Has anybody ever found a solution to this? Any ideas or pointers would be
> greatly appreciated.
>
> Regards
>
> Simon