In message  <961210030758_808636133@emout16.mail.aol.com> PICLIST@MITVMA.MIT.EDU
 writes:
> 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?

Visual BASIC isn't a proper compiled language, and might be a bit slow.
Also, Windoze 3.1 and 95 are somewhat indeterminate in terms of timing,
I've heard. We are using Windoze NT at work for this reason.

Leon
--
Leon Heller, G1HSM                | "Do not adjust your mind, there is
E-mail leon@lfheller.demon.co.uk  |  a fault in reality": on a wall
Phone: +44 (0)118 9471424         |  many years ago in Oxford.