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Re: Two-wires RS485 and the driver enable problem

From pozzugno <pozzugno@gmail.com>
Newsgroups comp.arch.embedded
Subject Re: Two-wires RS485 and the driver enable problem
Date 2014-10-14 09:28 +0200
Organization A noiseless patient Spider
Message-ID <m1ijai$que$1@dont-email.me> (permalink)
References <m1geor$bt4$1@dont-email.me> <m1gqn4$fvt$1@speranza.aioe.org>

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Il 13/10/2014 17:17, Don Y ha scritto:
> On 10/13/2014 4:58 AM, pozzugno wrote:

> First, you need to *know* that it is *you* that has been granted access to
> the bus/resource.  If this requires you to perform some analysis of the
> ENTIRE MESSAGE (to verify that the "address field" is, in fact, intact!),
> then you probably DON'T want to try to ride the coat-tails of the message,
> directly.
>
> As the front end of a message (including yours) is more likely to be
> corrupted
> (by a collision on the bus -- someone jabbering too long or too soon), you
> might consider designing a packet format that has one checksum on the
> address field "early" in the packet (before the payload) and another that
> handles the balance of the message.
 >
 > This allows you to capture the address information and it's closely
 > following
 > checksum, verify that it is *you* that are being addressed and prepare
 > for your
 > acquisition of the bus before the message has completely arrived.

I have just one checksum at the end of the message, but the address 
field is at the beginning.  Anyway I look at the address field as it 
arrives to decide if it's a frame for me.

I know the address field could be corrupted, but IMHO it's not 
important.  If the address field is corrupted and appear for me, but the 
master wanted to talk to another node, I store the message till the end, 
but the checksum will be wrong, so the frame will be discarded.  If the 
address field is corrupted and it doesn't appear for me, but the master 
really wanted to talk with me, I discard early the message.

IMHO, adding a new checksum at the beginning of the frame, only to 
protect the address field, doesn't add more robustness to the final 
performance.


> You can also arrange to access the bus in "timeslots" referenced to some
> easily recognizable event (e.g., a beacon sent by the master).  So,
> you do all of your timing off of that single event (see "some special
> point" in the beacon, start timer, wait to transmit until "your slot").
>
> Note this also works if you just assume the "next slot" after the master's
> message is the slot you should use (you just limit the length of the
> master's
> message so it fits in that fixed delay).  Similarly, the master can "know"
> that your reply will never exceed a fixed duration so it won't issue
> another
> beacon/request until that has expired.
>
> Hopefully, this makes sense.  Sorry, I'm off for a pro bono day so no time
> here...  :-/

I think I'll have to think more deeply about this beacon approach.  Any 
useful info on the Internet?

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Thread

Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 13:58 +0200
  Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 14:29 +0200
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 14:38 +0200
      Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 15:28 +0200
        Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 07:21 -0700
          Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 16:46 +0200
            Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:02 -0700
        Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 16:59 +0200
          Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:08 -0700
            Re: Two-wires RS485 and the driver enable problem pozz <pozzugno@gmail.com> - 2014-10-14 07:48 +0200
              Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 23:28 -0700
                Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 09:20 +0200
                Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-14 06:09 -0700
              Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-14 10:36 +0200
          Re: Two-wires RS485 and the driver enable problem Wouter van Ooijen <wouter@voti.nl> - 2014-10-13 18:14 +0200
      Re: Two-wires RS485 and the driver enable problem Tim Wescott <seemywebsite@myfooter.really> - 2014-10-13 16:30 -0500
  Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-13 14:47 +0200
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 14:56 +0200
      Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-13 16:13 +0200
      Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:06 -0700
        Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 08:50 +0200
          Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-14 14:34 -0700
  Re: Two-wires RS485 and the driver enable problem Richard Damon <Richard@Damon-Family.org> - 2014-10-13 09:11 -0400
  Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-13 10:53 -0400
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-13 17:03 +0200
  Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-13 15:20 +0000
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-16 09:01 +0200
      Re: Two-wires RS485 and the driver enable problem Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2014-10-16 10:19 +0300
      Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 05:02 -0400
  Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:17 -0700
    Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 08:19 -0700
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 09:28 +0200
      Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-14 06:09 -0700
        Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-14 14:28 -0400
    Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-14 04:36 -0400
  Re: Two-wires RS485 and the driver enable problem Les Cargill <lcargill99@comcast.com> - 2014-10-13 12:35 -0500
    Re: Two-wires RS485 and the driver enable problem langwadt@fonz.dk - 2014-10-13 15:02 -0700
      Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 17:36 -0700
        Re: Two-wires RS485 and the driver enable problem Les Cargill <lcargill99@comcast.com> - 2014-10-13 21:53 -0500
          Re: Two-wires RS485 and the driver enable problem Don Y <this@is.not.me.com> - 2014-10-13 20:24 -0700
  Re: Two-wires RS485 and the driver enable problem upsidedown@downunder.com - 2014-10-13 22:02 +0300
    Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-13 19:09 +0000
      Re: Two-wires RS485 and the driver enable problem glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-13 20:58 +0000
        Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-13 21:08 +0000
          Re: Two-wires RS485 and the driver enable problem langwadt@fonz.dk - 2014-10-13 14:40 -0700
    Re: Two-wires RS485 and the driver enable problem Dave Nadler <drn@nadler.com> - 2014-10-14 09:58 -0700
      Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-14 14:20 -0400
        Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-14 23:35 +0200
          Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-15 02:00 -0400
            Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-15 09:47 +0200
              Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 01:30 -0400
                Re: Two-wires RS485 and the driver enable problem David Brown <david.brown@hesbynett.no> - 2014-10-16 20:46 +0200
                Re: Two-wires RS485 and the driver enable problem Dave Nadler <drn@nadler.com> - 2014-10-16 12:00 -0700
            Re: Two-wires RS485 and the driver enable problem upsidedown@downunder.com - 2014-10-15 11:01 +0300
            Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-15 14:24 +0000
              Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 01:17 -0400
                Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-16 08:36 +0200
                Re: Two-wires RS485 and the driver enable problem rickman <gnuarm@gmail.com> - 2014-10-16 05:05 -0400
                Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-16 12:54 +0200
                Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-16 14:16 +0000
    Re: Two-wires RS485 and the driver enable problem upsidedown@downunder.com - 2014-10-15 09:09 +0300
      Re: Two-wires RS485 and the driver enable problem Grant Edwards <invalid@invalid.invalid> - 2014-10-15 14:26 +0000
  Re: Two-wires RS485 and the driver enable problem glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-13 20:55 +0000
  Re: Two-wires RS485 and the driver enable problem Tim Wescott <seemywebsite@myfooter.really> - 2014-10-13 16:37 -0500
    Re: Two-wires RS485 and the driver enable problem pozzugno <pozzugno@gmail.com> - 2014-10-14 09:37 +0200
  Re: Two-wires RS485 and the driver enable problem Arlet Ottens <usenet+5@c-scape.nl> - 2014-10-14 20:44 +0200

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