Simon Harding wrote:

> 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.

  This sounds like a description of Ethernet.  You use time-domain
rather than frequency domain multiplexing, i.e., your low-rate data is
sent infrequently, but very fast.

  Advantages:  Use mature, off-the-shelf technology.  Expansion path
fully developed.

> I have looked into embedded modem devices but the problem seems to be
> the conversion from asynchronous to synchronous data.

  Twin-chip solutions use parallel interface.  Forget synchronous/
asynchronous, just send bytes.  This may seem overkill, but you're going
to have to address line drivers and *isolation* sooner or later, I
suspect!  When you do, any initial cost savings in a custom system will
vanish.

  Aside: The pulp electronic press seems to have a fixation on how
"unreliable" co-ax (10base-2) is compared to twin pair (10Base-T).  I'm
ROTFL.  Use decent connectors as found in all professional video
installations, should be reliable.  OTOH, should your hub wall-wart
(usually a non-standard voltage) fail, you're history!
--
  Cheers,
        Paul B.