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Multi-master RS485

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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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#30374 — Multi-master RS485

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.

I'm thinking to "improve" this bus with a multi-master approach, but it 
doesn't appear simple. Do you know of real and working implementation of 
this type on half-duplex RS485 bus?

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#30375

On 3/25/2021 9:27 AM, pozz 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.
> 
> I'm thinking to "improve" this bus with a multi-master approach, but it doesn't 
> appear simple. Do you know of real and working implementation of this type on 
> 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]

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#30376

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.

--
Grant

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#30379

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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#30382

On 25/03/2021 17:38, 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.
> 

The first CAN transceivers were basically RS-485 transceivers with the
CAN TX line connected to the driver's transmit enable input, the
driver's receive enable always on, and the driver's data in line
connected to 0.

You can also use UARTs along with CAN transceivers.  (I worked with a
propriety protocol like that a good while ago - it was a slightly odd
bus that evolved gradually from a Dallas one-wire bus.)

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#30378

Il 25/03/2021 17:33, Don Y ha scritto:
> On 3/25/2021 9:27 AM, pozz 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.
>>
>> I'm thinking to "improve" this bus with a multi-master approach, but 
>> it doesn't appear simple. Do you know of real and working 
>> implementation of this type on half-duplex RS485 bus?
> 
> Sure!  An easy way is to time-division-multiplex the bus and assign
> specific slots to specific nodes.

This means assigning a predefined slot duration for each node. What 
happens if a node wants to transmit a big packet? It should split packet 
in multiple consecutive slots.

Moreover, is time-division-multiplex efficient? Now with master-slave 
polling mechanism I already have a time-division-multiplex: for each 
slave there's a time window in which master sends the token and slave 
reply with token (and probably data).
With a pure time-division-multiplex I will save only the time of the 
master request.


> 
> You can also use a token-passing protocol to select the next master.

Same, each node is an "active" node in every loop. I imagine a bus that 
is idle for most of the time and busy only when a node wants to transmit 
something.
In my case real data are very low. Anyway I have to face the event of a 
conflict.


> 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.

This is why I'm searching a "ready-to-use" protocol, if any.


> 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]

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#30381

On 3/25/2021 9:57 AM, pozz wrote:
> Il 25/03/2021 17:33, Don Y ha scritto:
>> On 3/25/2021 9:27 AM, pozz 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.
>>>
>>> I'm thinking to "improve" this bus with a multi-master approach, but it 
>>> doesn't appear simple. Do you know of real and working implementation of 
>>> this type on half-duplex RS485 bus?
>>
>> Sure!  An easy way is to time-division-multiplex the bus and assign
>> specific slots to specific nodes.
> 
> This means assigning a predefined slot duration for each node. What happens if 
> a node wants to transmit a big packet? It should split packet in multiple 
> consecutive slots.

Or, the next node defers claiming it's timeslot until it senses the bus has
gone idle.

You have to decide how much you want to put in the lower level protocol
and how much you're willing to put *above* it (e.g., packet reassembly)

> Moreover, is time-division-multiplex efficient? Now with master-slave polling 
> mechanism I already have a time-division-multiplex: for each slave there's a 
> time window in which master sends the token and slave reply with token (and 
> probably data).
> With a pure time-division-multiplex I will save only the time of the master 
> request.

It depends on how big your packets are, how high the bus utilization, etc.
Only *you* can make that determination, from *your* application.

>> You can also use a token-passing protocol to select the next master.
> 
> Same, each node is an "active" node in every loop. I imagine a bus that is idle 
> for most of the time and busy only when a node wants to transmit something.
> In my case real data are very low. Anyway I have to face the event of a conflict.

If there is nothing to transmit, a node just sends a "keep alive" packet.
This also lets other nodes know that the network (and other devices on it)
are intact.

[When should a node "get nervous" that it hasn't been polled, "recently"?]

>> 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.
> 
> This is why I'm searching a "ready-to-use" protocol, if any.

Dunno.  I always roll a protocol that is appropriate for the
application.  That lets me tailor it to get the best performance
for the cost invested.  (my designs are closed so I don't have
to worry about something being *added*, arbitrarily, at a later
date)

The exercise of thinking about the protocol often reveals
vulnerabilities in the design; "what if node X doesn't
take its time slot?  what does that mean for the system,
as a whole?  should the other nodes be able to independantly
detect this and bring the system to a safe/secure state
WITHOUT waiting for the master to command them to do so?"

(what does it *mean* when X is misbehaving?)

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#30380

On Thu, 25 Mar 2021 17:27:34 +0100, pozz <pozzugno@gmail.com> wrote:

>Are there any *real* implementation of multi-master half-duplex RS485 bus?

For instance Profibus-DP. Usually one master is used for real-time
data, while the other is mainly used for configuration.  A token is
passed between masters, so each master knows when it is allowed to
access the slaves.

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#30384

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...

-- 
                              Waldek Hebisch

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#30385

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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#30386

On 3/27/21 7:37 PM, pozz wrote:
> 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.

The Problem is that RS485 is not designed with this sort of architecture
in mind. Yes,you could build a protocol which the slave listens first
and makes sure no one else is talking, Then send the message (with good
error detection built in) and see if it gets a response (maybe first an
ACK and then later an order to go ahead). If you don't get the ack, you
presume that your message got garbled so you send again after some
random wait designed that you and whoever you conflicted with won't send
again at the same time. MAybe even the master sends a NAK that it got
garbage.

Much better would be use a driver that can reliably detect a conflict
with known results. You send your message and listen for conflicts. If
you get a conflict you stop sending immediately.

If the system uses a driver where say 0 always wins over a 1, and the
bit time is longer than the network transmission time, then if two
drivers start at the same time, whoever send the first 0 when the other
wants to send a 1 will win and get their message through. The other
backs off, waits for the message and ack to clear, then does their message.

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#30389

On Sat, 27 Mar 2021 20:14:54 -0400, Richard Damon
<Richard@Damon-Family.org> wrote:

>On 3/27/21 7:37 PM, pozz wrote:

>> 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.

Does the user have to press a single key or enter a multidigit access
code ?

In the latter case, do the access code processing locally including
feedback beeps and only send a complete sequence to the master. When
the user enters the first digit, answer to the master poll "data will
soon be arriving" i.e. there is a user at the slave. The master can
then poll those slaves more frequently which have users at the slave
and hence read the full access code with the frequent polls.

>> 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.

The wire network must be huge, if you are forced to use 300 bits/s:-)

If you can use 9600 bits/s  i.e. 1 ms character time and a 2 byte poll
and 1 byte Ack/Nak that takes 3 ms plus some additional time for Rx/Tx
and Tx/Rx delays. I would expect a 5 ms transaction time and 150 ms
poll cycle,

>> 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.
>
>The Problem is that RS485 is not designed with this sort of architecture
>in mind. Yes,you could build a protocol which the slave listens first
>and makes sure no one else is talking, Then send the message (with good
>error detection built in) and see if it gets a response (maybe first an
>ACK and then later an order to go ahead). If you don't get the ack, you
>presume that your message got garbled so you send again after some
>random wait designed that you and whoever you conflicted with won't send
>again at the same time. MAybe even the master sends a NAK that it got
>garbage.
>
>Much better would be use a driver that can reliably detect a conflict
>with known results. You send your message and listen for conflicts. If
>you get a conflict you stop sending immediately.
>
>If the system uses a driver where say 0 always wins over a 1, and the
>bit time is longer than the network transmission time, then if two
>drivers start at the same time, whoever send the first 0 when the other
>wants to send a 1 will win and get their message through. The other
>backs off, waits for the message and ack to clear, then does their message.

It sounds that you are trying to reinvent the CAN bus protocol ?

You can even use RS-485 transceivers chips (Data out to Transmit
enable, Tx input to fixed "0"). You need a CAN control chip, e.g.
SJA1000 for the master and some simpler for slaves. There are several
uCs with built in CAN controllers usable as slaves.

With CAN, there are some snags. The maximum net payload is 64 _bits_
(8 bytes) so if more data needs to be transmitted, you may have to use
some higher level protocols above CAN. The other snag is that all
stations must hear each other well within a single _bit_ time to
handle conflicts. This limits the length of the bus and/or maximum bit
rate that can be used e.g. 250 kbit/s with 250 m bus cable. At lower
speeds the range can be extended.

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#30390

On 3/28/21 12:32 AM, upsidedown@downunder.com wrote:
> On Sat, 27 Mar 2021 20:14:54 -0400, Richard Damon
>
>> The Problem is that RS485 is not designed with this sort of architecture
>> in mind. Yes,you could build a protocol which the slave listens first
>> and makes sure no one else is talking, Then send the message (with good
>> error detection built in) and see if it gets a response (maybe first an
>> ACK and then later an order to go ahead). If you don't get the ack, you
>> presume that your message got garbled so you send again after some
>> random wait designed that you and whoever you conflicted with won't send
>> again at the same time. MAybe even the master sends a NAK that it got
>> garbage.
>>
>> Much better would be use a driver that can reliably detect a conflict
>> with known results. You send your message and listen for conflicts. If
>> you get a conflict you stop sending immediately.
>>
>> If the system uses a driver where say 0 always wins over a 1, and the
>> bit time is longer than the network transmission time, then if two
>> drivers start at the same time, whoever send the first 0 when the other
>> wants to send a 1 will win and get their message through. The other
>> backs off, waits for the message and ack to clear, then does their message.
> 
> It sounds that you are trying to reinvent the CAN bus protocol ?
> 
> You can even use RS-485 transceivers chips (Data out to Transmit
> enable, Tx input to fixed "0"). You need a CAN control chip, e.g.
> SJA1000 for the master and some simpler for slaves. There are several
> uCs with built in CAN controllers usable as slaves.
> 
> With CAN, there are some snags. The maximum net payload is 64 _bits_
> (8 bytes) so if more data needs to be transmitted, you may have to use
> some higher level protocols above CAN. The other snag is that all
> stations must hear each other well within a single _bit_ time to
> handle conflicts. This limits the length of the bus and/or maximum bit
> rate that can be used e.g. 250 kbit/s with 250 m bus cable. At lower
> speeds the range can be extended.
> 

That methodology is used in CAN, it is also used in I2C, and I suspect
other systems. It is close to the method used for ethernet on coax (but
that uses a slightly different method because the line might be longer
than allowing collision detection in a single bit.

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#30391

On Sun, 28 Mar 2021 07:20:43 -0400, Richard Damon
<Richard@Damon-Family.org> wrote:

>On 3/28/21 12:32 AM, upsidedown@downunder.com wrote:
>> On Sat, 27 Mar 2021 20:14:54 -0400, Richard Damon
>>
>>> The Problem is that RS485 is not designed with this sort of architecture
>>> in mind. Yes,you could build a protocol which the slave listens first
>>> and makes sure no one else is talking, Then send the message (with good
>>> error detection built in) and see if it gets a response (maybe first an
>>> ACK and then later an order to go ahead). If you don't get the ack, you
>>> presume that your message got garbled so you send again after some
>>> random wait designed that you and whoever you conflicted with won't send
>>> again at the same time. MAybe even the master sends a NAK that it got
>>> garbage.
>>>
>>> Much better would be use a driver that can reliably detect a conflict
>>> with known results. You send your message and listen for conflicts. If
>>> you get a conflict you stop sending immediately.
>>>
>>> If the system uses a driver where say 0 always wins over a 1, and the
>>> bit time is longer than the network transmission time, then if two
>>> drivers start at the same time, whoever send the first 0 when the other
>>> wants to send a 1 will win and get their message through. The other
>>> backs off, waits for the message and ack to clear, then does their message.
>> 
>> It sounds that you are trying to reinvent the CAN bus protocol ?
>> 
>> You can even use RS-485 transceivers chips (Data out to Transmit
>> enable, Tx input to fixed "0"). You need a CAN control chip, e.g.
>> SJA1000 for the master and some simpler for slaves. There are several
>> uCs with built in CAN controllers usable as slaves.
>> 
>> With CAN, there are some snags. The maximum net payload is 64 _bits_
>> (8 bytes) so if more data needs to be transmitted, you may have to use
>> some higher level protocols above CAN. The other snag is that all
>> stations must hear each other well within a single _bit_ time to
>> handle conflicts. This limits the length of the bus and/or maximum bit
>> rate that can be used e.g. 250 kbit/s with 250 m bus cable. At lower
>> speeds the range can be extended.
>> 
>
>That methodology is used in CAN, it is also used in I2C, and I suspect
>other systems. It is close to the method used for ethernet on coax (but
>that uses a slightly different method because the line might be longer
>than allowing collision detection in a single bit.

The difference is that in coaxial Ethernet CSMA/CD collisions _both_
messages are lost and _both_ needs to be retransmitted at a later
time. In CAN bus, the higher priority message gets through despite a
collision, only the lower priority message needs to be retransmitted.
For this reason a much higher bus loading are possible in CAN, while
in Ethernet high collision rates will kill the traffic completely much
earlier.

The dominant/recessivebit  protocol used in CAN may also be used with
other transports than RS-485 or dedicated CANbus transceivers. One
could even use  with 20 mA current loop by putting all stations into
the sane current loop, but the optoisolator photo transistor Vce(max)
may limit the number of stations in the same loop to about a dozen.

[toc] | [prev] | [next] | [standalone]

#30392

On 3/28/21 7:59 AM, upsidedown@downunder.com wrote:
> On Sun, 28 Mar 2021 07:20:43 -0400, Richard Damon
> <Richard@Damon-Family.org> wrote:
> 
>> On 3/28/21 12:32 AM, upsidedown@downunder.com wrote:
>>> On Sat, 27 Mar 2021 20:14:54 -0400, Richard Damon
>>>
>>>> The Problem is that RS485 is not designed with this sort of architecture
>>>> in mind. Yes,you could build a protocol which the slave listens first
>>>> and makes sure no one else is talking, Then send the message (with good
>>>> error detection built in) and see if it gets a response (maybe first an
>>>> ACK and then later an order to go ahead). If you don't get the ack, you
>>>> presume that your message got garbled so you send again after some
>>>> random wait designed that you and whoever you conflicted with won't send
>>>> again at the same time. MAybe even the master sends a NAK that it got
>>>> garbage.
>>>>
>>>> Much better would be use a driver that can reliably detect a conflict
>>>> with known results. You send your message and listen for conflicts. If
>>>> you get a conflict you stop sending immediately.
>>>>
>>>> If the system uses a driver where say 0 always wins over a 1, and the
>>>> bit time is longer than the network transmission time, then if two
>>>> drivers start at the same time, whoever send the first 0 when the other
>>>> wants to send a 1 will win and get their message through. The other
>>>> backs off, waits for the message and ack to clear, then does their message.
>>>
>>> It sounds that you are trying to reinvent the CAN bus protocol ?
>>>
>>> You can even use RS-485 transceivers chips (Data out to Transmit
>>> enable, Tx input to fixed "0"). You need a CAN control chip, e.g.
>>> SJA1000 for the master and some simpler for slaves. There are several
>>> uCs with built in CAN controllers usable as slaves.
>>>
>>> With CAN, there are some snags. The maximum net payload is 64 _bits_
>>> (8 bytes) so if more data needs to be transmitted, you may have to use
>>> some higher level protocols above CAN. The other snag is that all
>>> stations must hear each other well within a single _bit_ time to
>>> handle conflicts. This limits the length of the bus and/or maximum bit
>>> rate that can be used e.g. 250 kbit/s with 250 m bus cable. At lower
>>> speeds the range can be extended.
>>>
>>
>> That methodology is used in CAN, it is also used in I2C, and I suspect
>> other systems. It is close to the method used for ethernet on coax (but
>> that uses a slightly different method because the line might be longer
>> than allowing collision detection in a single bit.
> 
> The difference is that in coaxial Ethernet CSMA/CD collisions _both_
> messages are lost and _both_ needs to be retransmitted at a later
> time. In CAN bus, the higher priority message gets through despite a
> collision, only the lower priority message needs to be retransmitted.
> For this reason a much higher bus loading are possible in CAN, while
> in Ethernet high collision rates will kill the traffic completely much
> earlier.
> 
> The dominant/recessivebit  protocol used in CAN may also be used with
> other transports than RS-485 or dedicated CANbus transceivers. One
> could even use  with 20 mA current loop by putting all stations into
> the sane current loop, but the optoisolator photo transistor Vce(max)
> may limit the number of stations in the same loop to about a dozen.
> 
> 

Yes, the Ethernet protocal loses the whole message because the bit time
isn't large comparared to the propagation delay across the system, so
you might not know of the collision until it is too late to get off.

CAN/I2C require a bit time longer than network propagation, so the
transmitter knows after the first collision that it has lost, and the
master that was sending the dominant bit gets control.

As you say, you can use dominant/recessive bit physical layer with many
protocals, even pure asynchronous serial (though that may need a bit of
external logic to get you off the bus before you corrupt the message,
like the 485 with data on the enable.

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#30387

pozz <pozzugno@gmail.com> wrote:
> 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.

100ms for round trip looks very long.  You should be able to
do with very short (3-4 bytes, maybe 8 if you add strong
cryptography) messages, so at 9600 even with delays for
turnaround you should be closer to 20-30 ms.

And I do not understand why you wait for OK?  Typing on
keypad takes some time and it is likely that node will be
polled before number is finished.  Then node can ask for
polling with shorter delay.  If only small number of nodes
asks for more frequent polling, then master can respect
this giving you shorter effective response time.  If
several (most of) keypads are busy, then clearly master
can not quickly serve all of them, but that is exactly
situation where nodes talking without polling would lead
to trouble.

> 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.

You can bring down average, but worst case will be bad:	in case
of conflict you need enough delay to significanlty reduce risk
of repeated conflict.  And detection of conflicts may be
problematic.  You may sell more, as during demo salesman
can arrange for no conflicts and clients will see best
case.  But you may get bad publicity, as ocasional, much
longer than usual delay will look like malfunction.

-- 
                              Waldek Hebisch

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#30388

On 3/27/2021 4:37 PM, pozz wrote:

> 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.

That's because you are envisioning a request/ack per slave.  So, the maximum
latency is O(Kn) and increases with increasing 'n'.

You can reduce K.  Or, you could design the system so that the master
is not involved in that decision making process.  E.g., the master can
*tell* each node whether it can respond to a local keypress by unlocking the
door -- and reporting that after-the-fact if the master needs to track that
info.

To reduce K, you can initiate a polling frame at t=0.  Each node watches the
bus for I*L time (L being the time in which any lower numbered node can
reply; I being the ID of the current node).

Any node wanting the master's attention (or, wanting to tell the master
"yes, *my* keypad was activated!") seizes the bus at its timeslot and
generates an appropriate response.  All nodes with "higher* IDs
add the time required for this response (which could be known /a priori/
if you can ensure replies to THAT type of polling request are always
a fixed duration) to the delay that they wait before taking *their*
turn.

You can also change the polling order (for individual polls *or* for
this sort of group poll) to reflect where you are most likely to
encounter slave requests.  E.g., if node X has seen keypad activity,
recently, it may inherently be less/more inclined to see activity in
the near future.

For example, requests to access the janitorial supply closet may be
infrequent during *normal* occupancy. Requests to access the CEOs
office may be infrequent "afterhours" -- when the janitor's
activities are more commonplace.

You seem to be wanting a generic solution -- and then complaining that
it isn't very efficient for YOUR application!

> 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.

Then you need to be able to detect collisions on the bus (e.g., CD/MA).
With EIA485, that's not always possible as the signal your node
"sees" isn't exactly the signal that every OTHER node is seeing.

At the very least, the master would have to be able to ensure the
bus returns to an idle condition that the protocol *enforces*...
so that it can then verify what it thinks just transpired.

And, other nodes (who may have competed for the wire -- and *apparently*
lost) can note that their messages were NOT heard (as evidenced by the
fact that the master replied to someone OTHER than them!)

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#30393

Am 28.03.2021 um 00:37 schrieb pozz:

> 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.

That will never be possible.  If 30 those slaves simultaneously trigger 
a request, then at least one of them will have to wait for the 
communication to all the others to be done before it can get its reply.

The worst case for a bus of 30 slaves cannot possibly be exactly as 
quick as the optimum case for a single slave.

The only way that could work is if there is no bus at all, but rather a 
star topology: each slave has its own, dedicated connection to the 
master. I'll assume that this untenable because of the cabling costs.

> It is impossible with polling approach. However the bus is idle (without 
> real data) most of the time. 

That makes no sense at all.  If the bus is so idle, why are you polling 
so slowly?  Why would you poll at any less than the maximum possible 
communication rate, while at the same being concerned about too long a 
response time?

But I'll second upsidedown's hint: you appear to be trying rather 
unnecessarily hard to reinvent CAN.

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#30394

On 3/28/21 9:52 AM, Hans-Bernhard Bröker wrote:
> Am 28.03.2021 um 00:37 schrieb pozz:
> 
>> 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.
> 
> That will never be possible.  If 30 those slaves simultaneously trigger
> a request, then at least one of them will have to wait for the
> communication to all the others to be done before it can get its reply.
> 
> The worst case for a bus of 30 slaves cannot possibly be exactly as
> quick as the optimum case for a single slave.
> 
> The only way that could work is if there is no bus at all, but rather a
> star topology: each slave has its own, dedicated connection to the
> master. I'll assume that this untenable because of the cabling costs.
> 
>> It is impossible with polling approach. However the bus is idle
>> (without real data) most of the time. 
> 
> That makes no sense at all.  If the bus is so idle, why are you polling
> so slowly?  Why would you poll at any less than the maximum possible
> communication rate, while at the same being concerned about too long a
> response time?
> 
> But I'll second upsidedown's hint: you appear to be trying rather
> unnecessarily hard to reinvent CAN.

On thing to note, if you are worried about WORSE case performance even
under full load, the synchronous polling is probably the best option. If
every keypad got the last key press at exactly the same time, the
shortest maximum delay would have the master go at full speed and
interrogate each slave and process it. This eliminates all the loses due
to conflicts and re-transmitting.

If you do use a CAN bus style arbitration, and can make the protocol not
lose frames in the case of a conflict, and the 'high priority' slaves
will wait long enough between mesages to give everyone a chance to send
their message, then that might get a bit faster as the amount thqt needs
to be sent is less, but the protocol overheads to wait to see the bus is
idle might eat that up.

The asyncronous messaging with arbitration is probably faster at
'average' delay under lighter loads. This is just the way these things
tend to work.

Another option is to use a dominate/recessive bus drive and the master
sense out a query request, and with a fixed timing any slave that has a
request replies with its address, and sees if someone else answer too,
and drops out on conflict. (This method is used on the SMB Bus). The
master can then poll at a base rate, and then if it gets a reply,
immediate ask for more requests until it gets a no request pending answer.

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#30395

Il 28/03/2021 00:37, pozz ha scritto:
> 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.

Many thanks to all of you that spent some time to answer my question.

Some additional details. The example I wrote is... just an example and 
not exactly the real application. The system has a central unit and many 
slaves installed in different places of a building (imagine a keypad, a 
sensor and so on). This is a home/building automation application.

After first connection the master sends many configuration data to 
slaves (around 2-4kB), so CAN bus is not ideal because of its small 
packets (however I can split data on different packets).
Then the bus stays idle for most of the time. The user could interact 
with a slave and only in that case the slave has something to tell to 
the master.

Yes, you are right when you say that putting a known limit on the worst 
case can be done with a polling approach. However my goal is not limit 
the worst case, but minimizing the "normal"/mean delay in the 
slave->master->slave transaction.
I know this mean of delays can be reduced a lot, because the bus is idle 
most of the time, so the probability of a conflict is very very low.

Even number of slaves was just an example. I can think of systems with 
50 or 100 slaves and in those cases the polling approach performance 
would be very bad.

Do you know of real wired cable used in home/building automation? I 
heard about KNX on twisted pair, but I think the complexity and cost of 
the system would increase very fast.

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