Yes, the presence pulse can disrupt communication. That's one of the
reasons there is a built-in retry in owfs for read and write, and very
conservative settings for checking checksums and verifying writes.
As I read it, the presence pulse makes more sense in a microprocessor-based
system (like an embedded system) where sitting on the line waiting for a
presence pulse doesn't require so much overhead.
Some of the bus masters can register the presence pulse if it occurs when
they aren't otherwise communicating on the line. We don't use that
information in OWFS -- it doesn't fit with our scheme, and it's not
reliable. In theory we could use it to invalidate a cached directory, but
the tradeoff of extra communication and complexity isn't clear.
Paul Alfille
On Mon, Jun 18, 2012 at 6:37 PM, Eric Vickery <[email protected]>wrote:
>
> There is a way for slaves to announce their presence on the bus by
> providing a presence pulse when
> they are powered up. I'm not sure if all slaves to this or just some (or
> all) of the iButtons. Also
> I don't know which masters or software supports this, it is mainly used
> for things like door lock
> systems where the iButton is used as the security FOB.
>
> > That's pretty weird. That'd kill any communication initiated by the
> > master that is taking place when the iButton is powered up.
> >
> > Not saying I don't believe you, especially since I have no experience
> > with these devices, but I'm tempted to order a sample just to further my
> > knowledge of the protocol (and a possible extension to it like the one
> > being discussed) ;-)
> >
> > Cheers,
> >
> > Eloy Paris.-
> >
> Yes it is rather strange that they allow it. After this thread started the
> other day I did a Google
> search and found a few references in some Maxim documents and they talked
> about a communication
> failure because of a presence pulse arriving at the wrong time.
>
> Eric
>
>
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