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Re: Multi-master RS485

From Don Y <blockedofcourse@foo.invalid>
Newsgroups comp.arch.embedded
Subject Re: Multi-master RS485
Date 2021-03-25 09:57 -0700
Organization A noiseless patient Spider
Message-ID <s3ifec$1me$1@dont-email.me> (permalink)
References <s3idlk$dm7$1@dont-email.me> <s3ie0u$ldm$1@dont-email.me> <s3iead$p3k$1@reader1.panix.com>

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On 3/25/2021 9:38 AM, Grant Edwards wrote:
> On 2021-03-25, Don Y <blockedofcourse@foo.invalid> wrote:
>> On 3/25/2021 9:27 AM, pozz wrote:
> 
>>> Are there any *real* implementation of multi-master half-duplex RS485 bus?
> 
>> Sure!  An easy way is to time-division-multiplex the bus and assign
>> specific slots to specific nodes.
>>
>> You can also use a token-passing protocol to select the next master.
>>
>> You have to address start-up issues to initialize the first
>> set of accesses.  And, you have to include provisions to handle
>> the case where things go wonky and resynchronization becomes
>> necessary.
>>
>> But, you *can* handle things like a device failing to pass the token
>> or skipping its timeslot in the "normal" protocol.
>>
>> Note that any approach requires pre-configuration; you can't just
>> arbitrarily add nodes to the bus and expect them to arbitrate
>> access (like CD/MA).
>>
>> [This is a minor lie -- but, autoconfiguration is not trivial]
> 
> Since the OP didn't specify async UARTs, I might mention that you can
> CAN controllers with a half-duplex RS485 physical layer but it's a bit
> of hack and easier to just use real CAN transceivers.

CAN is considerably more flexible, in that regard.

The OP can (if the application lends itself) just create one
big "frame" initiated by the master and let all of the slaves
slip their contributions in at the appropriate places
(without having to formally take ownership of the bus) as
well as peeling-off any data intended for their consumption.

But this seems like a design decision that should have been
made early on; not shoehorned in after the design has begun!

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Thread

Multi-master RS485 pozz <pozzugno@gmail.com> - 2021-03-25 17:27 +0100
  Re: Multi-master RS485 Don Y <blockedofcourse@foo.invalid> - 2021-03-25 09:33 -0700
    Re: Multi-master RS485 Grant Edwards <invalid@invalid.invalid> - 2021-03-25 16:38 +0000
      Re: Multi-master RS485 Don Y <blockedofcourse@foo.invalid> - 2021-03-25 09:57 -0700
      Re: Multi-master RS485 David Brown <david.brown@hesbynett.no> - 2021-03-25 20:28 +0100
    Re: Multi-master RS485 pozz <pozzugno@gmail.com> - 2021-03-25 17:57 +0100
      Re: Multi-master RS485 Don Y <blockedofcourse@foo.invalid> - 2021-03-25 10:04 -0700
  Re: Multi-master RS485 upsidedown@downunder.com - 2021-03-25 19:02 +0200
  Re: Multi-master RS485 antispam@math.uni.wroc.pl - 2021-03-26 15:54 +0000
    Re: Multi-master RS485 pozz <pozzugno@gmail.com> - 2021-03-28 00:37 +0100
      Re: Multi-master RS485 Richard Damon <Richard@Damon-Family.org> - 2021-03-27 20:14 -0400
        Re: Multi-master RS485 upsidedown@downunder.com - 2021-03-28 07:32 +0300
          Re: Multi-master RS485 Richard Damon <Richard@Damon-Family.org> - 2021-03-28 07:20 -0400
            Re: Multi-master RS485 upsidedown@downunder.com - 2021-03-28 14:59 +0300
              Re: Multi-master RS485 Richard Damon <Richard@Damon-Family.org> - 2021-03-28 08:09 -0400
      Re: Multi-master RS485 antispam@math.uni.wroc.pl - 2021-03-28 02:01 +0000
      Re: Multi-master RS485 Don Y <blockedofcourse@foo.invalid> - 2021-03-27 19:57 -0700
      Re: Multi-master RS485 Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2021-03-28 15:52 +0200
        Re: Multi-master RS485 Richard Damon <Richard@Damon-Family.org> - 2021-03-28 13:06 -0400
      Re: Multi-master RS485 pozz <pozzugno@gmail.com> - 2021-03-31 00:34 +0200
        Re: Multi-master RS485 Don Y <blockedofcourse@foo.invalid> - 2021-03-30 18:51 -0700
        Re: Multi-master RS485 upsidedown@downunder.com - 2021-03-31 06:42 +0300
          Re: Multi-master RS485 Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> - 2021-03-31 09:50 +0200
        Re: Multi-master RS485 David Brown <david.brown@hesbynett.no> - 2021-03-31 10:23 +0200
          Re: Multi-master RS485 pozz <pozzugno@gmail.com> - 2021-03-31 12:27 +0200
            Re: Multi-master RS485 David Brown <david.brown@hesbynett.no> - 2021-03-31 14:43 +0200
          Re: Multi-master RS485 pozz <pozzugno@gmail.com> - 2021-03-31 13:39 +0200
            Re: Multi-master RS485 David Brown <david.brown@hesbynett.no> - 2021-03-31 14:46 +0200
            Re: Multi-master RS485 Don Y <blockedofcourse@foo.invalid> - 2021-03-31 06:20 -0700
          Re: Multi-master RS485 upsidedown@downunder.com - 2021-03-31 15:29 +0300
            Re: Multi-master RS485 David Brown <david.brown@hesbynett.no> - 2021-03-31 14:49 +0200

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