RE: Re: RS485 - XPL on the wire

["Ian Lowe" <ian@xxxxxxxxxxxxxxxxxx>]

Well...

In a peer to peer environment (like xPL) any device needs to be able to
send a message.

If we adopt a master with multiple slaves model, then the master needs
to poll the slaves regularly (with a nasty overhead!). If instead we
adopt a token passing model...

Then a "token" is passed from station to station. It's like the
safety
key used when crossing single track railway bridges etc - if you don't
hold the key, you can't talk.

IBM built a very resilient network system called Token Ring which
implemented this same mechanism.

Using the shared access medium (the twisted pair network) we could have
multiple stations all able to send a message, with a token passing from
node to node giving them the right to speak...

You have a control system in place on the "master" device - if a
token
hasn't been seen for a second or so, it will send a special message
around that clears the ring and starts again...

Ian.

-----Original Message-----
From: Frank Mc Alinden [mailto:fmcalind@xxxxxxx]
Sent: 22 October 2004 10:12
To: ukha_xpl@xxxxxxx
Subject: Re: [ukha_xpl] Re: RS485 - XPL on the wire


Hi Ian

> One station can talk, the others listen - easy enough, but how can you
> *tell* someone else is talking?

AFAIK  you cant ......each node only responds to an Addressed message
from a Master .....so thererfore not interested in whos talking ....or
are you interested in whos talking ????

Frank


----- Original Message -----
From: "Ian Lowe" <ian@xxxxxxx>
To: <ukha_xpl@xxxxxxx>
Sent: Friday, October 22, 2004 7:05 PM
Subject: RE: [ukha_xpl] Re: RS485 - XPL on the wire


>
> One of the biggest gotcha's for me was simply the technical aspects of
> RS485 on the wire...
>
> One station can talk, the others listen - easy enough, but how can you
> *tell* someone else is talking?
> I guess I'm thinking in terms of a CSMA/CD situation like ethernet and
> couldn't find any RS485 chip that gave you Carrier feedback to
indicate
> that the media was in use.
>
> I guess you'd have to implement a token passing network system like
> token ring...
>
> Ian.
>
> -----Original Message-----
> From: Frank Mc Alinden [mailto:fmcalind@xxxxxxx]
> Sent: 22 October 2004 08:07
> To: ukha_xpl@xxxxxxx
> Subject: Re: [ukha_xpl] Re: RS485 - XPL on the wire
>
>
> Hi Lehane / Ian
>
>             Like Ian  I am also interested in xpl 485 enabling
> stuff...But to develope stuff we need to get more feedback from xPL
> Users on the sort of devices that they would like....Would be nice if
> several rs485 developements going on at the same time by different
> people.....I think there would need to be a RS232 / 485 convertor made
> which means one side would have to be bit banged , not aware of any
pics
> with  2 uarts ??or a commercially available unit ?? .............Have
> done a little bit on rs485 and used the chksum as you
> decribed.............Im more than happy to get involved in
> rs485 projects if  others are and there is enough interest as
well......
>
> This link mentions the rs485 stuuf i was playing with
> http://skybax.rapidhoster.co.uk/~armagh/page10.html
>
> Frank
>
>
> ----- Original Message -----
> From: "g8kmh" <lehane@xxxxxxx>
> To: <ukha_xpl@xxxxxxx>
> Sent: Friday, October 22, 2004 1:49 AM
> Subject: [ukha_xpl] Re: RS485 - XPL on the wire
>
>
> >
> >
> > OK,
> > I'd missed xpl_devcom in my searches, oops!
> >
> > LRC-8 is a simple checksum on the message. XOR everything after
STX
> > including ETX and the result is the checksum. No big deal to do
and
> > offers some protection against message corruption, which since
the
> > RS485 is going to be around mains cabling (even taking into
account
> > normal separations) I think is a reasonable precaution.
> >
> > Be interested in the PIC implementations....
> >
> > Thanks,
> > Lehane
> >
> > <snipped>
> >
> >
> >
> >
> >
> >
> > xPL Links: http://www.xplproject.org.uk http://www.xplhal.com
> http://www.xpl.myby.co.uk
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>
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