On Sun, Jul 22, 2001 at 02:17:55PM -0400, Brandon Fosdick wrote:
> Chris Carr wrote:
> >
> > I have only been half following this thread so my apologies if I have got
> > hold of the wrong end of the stick.
> >
> > I have implemented a somewhat similar system on RS485 using a Slotted Aloha
> > Protocol. It works very well  on low throughput systems and is quick and
> > cheap to implement.
> >
> > I was originally going to use CAN for the system and even invested in a
> > MCP2510 Development Kit. Only then did we get the Standards and when we
> > started reading them it slowly became clear that CAN is only a transport
> > media same as RS485 (except it includes error checking, collision detection
> > etc etc) You still have to overlay it with a Communications Protocol. The
> > idea of using CAN was rapidly dropped as the complexity could not be
> > justified for that project.
>
> I'm not familiar with Slotted Aloha, can you provide more details?

That's what search engines are for. I plopped slotted aloha into google and
10 pages of entries popped out.

It's a collision detection system that forces each node to transmit on an
agreed upon boundary with fixed message sizes. In this way collision detection
can be guranteed because multiple transmitters will all transmit at the same
time.

The only question I had is how does EIA-485 transceivers guarantee reports of
collisions? I thought that multiple transmitters on the bus would not guarantee
a known state on the line.

BAJ

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