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.

In the end we delivered a complete working system using Slotted Aloha for
the estimated cost of developing just one Station using CAN.

Just sometimes, it's worth going back to the 60's in order to go forward
into a profitable future    8-)

In case you were wondering, the cost of the MCP2510 Development Kit was
included in the cost of the system.  The one thing that troubles me is that
the customer was just a little too happy when he was presented with the
Bill, I'm sure we undercharged him.

Regards

Chris Carr

----- Original Message -----
From: "Patrick J" <info@DATECH.SE>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, July 21, 2001 4:35 AM
Subject: Re: [PIC]:Thoughts on RS485 collision detection


> OK, I might be totally off but how about _avoiding_ collision instead of
> detecting it ?
>
> Say all pics on the bus have ID:s #1..#5, and that also represent their
priority
> on the bus.
> Now, for a pic to be allowed to send it must wait the number of mS that is
equal
> to its
> ID number (2mS for pic with ID=2 in this example) AFTER the LAST detected
> transmission
> on the bus, before it starts to send.
>
> This would mean that pics w high priority (low ID and therefor short wait
time)
> can
> 'sneak' in before a low priorty pic which is still waiting before send.
> In principle a low priority pic can be 'starved' (no data) but if one can
asume
> the
> high priority pics can behave and not take up all of the bus it should
work.
>
>
> > Olin Lathrop wrote:
> > > Multi drop busses usually either have a single master like IEEE-488,
IIC,
> > > USB (at the broad overview level) and most RS-485 implementations
(although
> > > that is not part of the standard), or collision detection with random
> > > backoff like ethernet, or collision detection with a priority scheme
like
> > > CAN.
>
> --
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>
>

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