At 21:32 15.10.97 +1000, you wrote:
>I'm working on a simple serial control project and need some ideas please.
>My application requires that a master unit transmit and recieve from as
>many as 30 slave units.
>The messages are only a byte long and the master will pole all slaves. I
>need a way of powering and communicating with the slaves via a 3 pin mic
>lead.(sheilded twisted pair). I would also like to use the system on
>exsisting audio lines thus leaving only one wire for comms.(and a ground).
>The final problem is the comms need to be inaudible(out of the audio
>range.) Natsemi have a powerline modem chip whick will operate on a carrier
>up to 50 Khz however it needs the mains zero crossing for synchronising and
>im running dc. Any Ideas Huh?!!
>

My first idea would be to implement something like tha Dallas 1 wire uLAN
interface.  Though, I have no idea if this will be out of the audio range...

Havard.