In a message dated 96-12-09 12:03:43 EST, Norm writes:

<< [SNIP] I am working on a design using soemthing similar.  In my
application, there is a master processor on the "network" who controls when
each PIC can talk.
 All PIC's listen all of the time.  The master polls the PIC's for data and
 when they have data, they answer within a time window.  The plan is to use
 one line as the data line and only allow half duplex comms.  The PC fits
 into my network just like one of the PIC's (it could be the master).  Note
 that the idle state of the serial port must be a logic high.  This means an
 inverter for the output and inputs to the serial port (if I remember
 correctly).  Each device pulls the line low as it sends the data.  The
 outputs must be open collector.
>>

I've been using the RS-485 protocol for this.  For the PICs it works great!!
 But---using Visual Basic for Windows, my PC end is problematic as far as
timing goes.  I encounter very variable delays - up to 250 msec.) in getting
the PC to respond to the bus.  Have you found a good technique for getting a
narrower time window?

Regards,
Shel Michaels