On Fri, Feb 22, 2002 at 02:18:06PM -0500, Martin Peach wrote:
> > On Fri, Feb 22, 2002 at 10:31:32PM -0800, andre abelian wrote:
> > > Sherpa,
> > >
> > > Thanks for your replay,
> > > I have max1482 chips that I can use.
> > > A few things are not clear for me lets assume
> > > I got 30 slaves and 1 master if 2 slaves try to
> > > Communicate with master at the same time then what will happen?
> >
> > That doesn't happen by definition. The primary node directs the
> secondaries
> > as to when they may communicate. So if one secondary has been granted
> > access to the line, the others will not communcate on that line until
> directed
> > to do so.
>
> By who's definition? There is nothing in the RS-485 spec that prevents that.
> The spec says that multiple transmitters will not destroy each other by
> attempting to send at the same time, it does not say you can't do it. It's
> up to your implementation to watch out for that happening, usually by
> monitoring the output and comparing with the input, then retrying after a
> random delay, as in Ethernet collision detection or else by assigning a
> transmit window to each node.

That's why I included the second paragraph in my post. To quote:
-----
Also be very very aware that EIA485 (RS means recommended standard, EIA is
a actual standard) doesn't have any data link layer, so any communications
protocol you want to use you'll have to implement yourself.
-----

Since the primary/secondary relationship will have to be implemented in
software, it can (and in fact must), enforce those rules.

BAJ

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