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Re: Multi-master RS485

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Il 26/03/2021 16:54, antispam@math.uni.wroc.pl ha scritto:
> pozz <pozzugno@gmail.com> wrote:
>> Are there any *real* implementation of multi-master half-duplex RS485 bus?
>>
>> I often use half-duplex RS485 and I always use a master-slave polling
>> protocol. This choice dramatically reduce the complexity of the system
>> and it works well with a limited number of nodes.
>>
>> When the nodes are 10 or 20 and they usually send only a few bytes in a
>> minuted, the polling scheme is inefficient. The bus is alway busy for
>> polling, but the real data are very small.
> 
> What do you really want?  That is what is your "efficiency"
> criterion.  For embedded system a lot of folks consider
> worst case delay.  For normal design worst case is when
> all nodes have data to transmit.  Clearly, "last node"
> have to wait on all previous nodes.  So, best "efficiency"
> would be sum of transmit times for data.  On top ot that
> you have various overhead: bus turnaround times, time
> for polling or token passing, possible timeout in case
> of fixed time slots.  AFAICS polling messages from master
> to slave may be very small, so they should at most
> double time: you have twice as many messages compared
> to case when each node "magically" knows when to
> transmit.  You can play some tricks, like group poll,
> when nodes are ordered (numbered) and each node in
> the group is supposed to send its data, starting from
> first.  Of course, it means that each node must be
> capable of detecting end of transmission from
> lower numbered node.  Token passing may be slightly
> more efficient, as data messages may serve double
> duty of passing token to next node: so you save
> cost of polling messages.
> 
> But you basically fight with constant factor of 2.
> If you need more improvement, then you need bigger
> change.  Say faster links or star topology.  In
> star topology intermediate nodes may aggregate
> messages, so you can significantly decrease number
> of messages going to maser node.  And it is easier
> to provide fast links to limited number of "hub"
> nodes, than to have fast links everywhere.  But
> as downside, there is more complexity and potentially
> lower reliability...
> 

Suppose you have a bus with 20-30 nodes: one master is the central unit, 
the other are slaves that get a code from a keypad. When a code is typed 
on the keypad, it is transmitted to the master. If it is ok, master 
sends an ack to the slave that get the code so it enabled an electric 
lock and the door is unlocked.

With a polling scheme, master must sends a poll request continuously and 
the the slaves answer, most of the time, with no data. It appears to me 
a very inefficient approach.

What's the worst case, i.e. the delay between pressing OK button on the 
keypad and the door opening? It is the sum of 30 (number of slaves) 
polling requests and replies. If a single request takes 100ms, the worst 
case should be around 3 seconds.
I'd like to reduce this time to 100ms.

It is impossible with polling approach. However the bus is idle (without 
real data) most of the time. I could avoid polling at all and instruct 
the slave to transmit when it wants. I think the probability of a 
conflict with another slave is very low and I could reach the 
requirement of 100ms above. However I sholud design the behaviour with a 
conflict, even rare, happens.

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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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