Simple data bus over DC Power Wires

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Hi Guys,

I'm in the process of implementing some simple control nodes (just
switches) and looking for the simplest (cheapest) network protocol to
use.  To date, LIN bus seems to be the best candidate.  I need very
low speed, high reliability and low off-board distance (a few up to
10s of feet).

Ideally, something that could be two wire with power and data combined
would be ideal, so that the power could be distributed to power the
slave nodes.  LIN is good, but it requires a 4 wire (or 3 I suppose)
interface.

I looked at ASI (Acuator Sensor Interface), but not sure how practical
it is.

Anything else on the radar screen?

Paul

Re: Simple data bus over DC Power Wires


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There just may be:
http://www.interfacebus.com/Design_Connector_Field_Buses.html





Re: Simple data bus over DC Power Wires

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 I looked there, but couldn't see anything related
 to multi-channel control over the DC supply wires.

 I have a pal who (in later life) is resurrecting
 his interest in model railways. Apparently there
 is now some reasonably standardised method of
 multi-device control via the DC supply through
 the rails.  Trains, points and signals can all be
 controlled via one DC supply.  It is supposed to
 be popular in the US model engineering market,
 but he hasn't been able to find out any technical
 details yet.

--
Tony Williams.

Re: Simple data bus over DC Power Wires

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I thought someone would mention this eventually. It's called
Digital Command Control (DCC) and is maintained by the
National Model Railroad Association. The NMRA standards are at
http://www.nmra.org/standards/consist.html#standards-DCC .
There are a lot of manufacturers world-wide that make the
various mobile and staionary decoders, booster/command stations,
hand-held throttles, computer interfaces, etc. Note that the
standard only applies to the booster to decoder side, while the
booster / throttle / computer network tends to be proprietary
to each manufacturer. Most of the modelers I know use Digitrax
http://www.digitrax.com /.

Mike




Re: Simple data bus over DC Power Wires
In article

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 Thanks for the information.  I've bounced your reply
 to my pal.

--
Tony Williams.

Re: Simple data bus over DC Power Wires


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I don't think that DCC is suitable because it looks they reverse the
polarity on the rails or some crazy thing. The Siemens ASI seems to be
it. It uses a simple unshielded two wire data+power cable and the
network can be up to 64 slaves each of which can be multiple I/O. Noise
immunity is achieved by superimposing a narrowband sinewave that is
modulated by something called APM= Alternate Pulse Modulation- of just
exactly what-phase, amplitude, or something else-I don't know, but the
slave receivers require crystal timebases. The ASI has been around for
10 years now so that it is well-developed and supported- you can tie it
into almost any other industrial control bus, or PCI, or VME, or
whatever- and there are plenty of chipsets available for building a
product from scratch too. It has quite a bit of protocol overhead so
that custom development would not make sense unless you're building a
compatible product for market- an in-house actuator control bus should
use readily available off-the-shelf modules from any one of dozens of
manufacturers.


Re: Simple data bus over DC Power Wires

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 Yes, I was quite surprised to see that.  I had
 imagined some sort of dc rail, modulated with
 an mf two-tone (or something).  Using a full
 bipolar switching scheme seems a hard way to
 go about it.

 However it seems that the model railway market
 is all going DCC, so that's the way my pal will
 have to go as well.

--
Tony Williams.

Re: Simple data bus over DC Power Wires

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Try <http://www.nmra.org/standards/dccbasic.html

Paul Burke


Re: Simple data bus over DC Power Wires
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You want cheap:


               L1
DC source ----))))))))----------------------- To others
or load               !               !
                      \ R1           --- C1
                      /              ---
                      \               !
                      !    R4         / R2
                      !   -/\/\/---   \
!--------------!      !  !         !  / R3
! Micro     TXD!---!!-   !    /+!------/\/\/-- Vcc/2
!              !   C2    !   /  !
!           RXD!------------    ! U1
!--------------!             \  !
                              \-!---- Vcc/2


C2, R1 and L1 turn the RS232 output of the micro into positive and
negitive going glitches.

L1 blocks the AC from going into the supply or load.  It is in parallel
with all the others.  The total inductance interacts with C2 to define the
width of the glitches.

R1 damps the L1 C2 combination to make the responce not overshoot.

C1 picks the glitches off the power line and applies them to U1.  U1 is
configured to have enough positive feedback that it doesn't chatter.

Now all you need is software.

This method has been done over 1000 foot cables.




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snipped-for-privacy@rahul.net   forging knowledge


Re: Simple data bus over DC Power Wires

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what would be the disadvantage of removing U1 and all connected parts on
receiving side and connecting C1 directly to RXD grounded through input
impedance resistor ? Now that's inexpensive :)

As for the protection of RXD from spikes, I would consider the built-in
clamp diodes (some microcontrollers have them) together with R2.

Roman



Re: Simple data bus over DC Power Wires
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If you do that, the micro gets something like this on its input:

    !-
    ! \
    !  \
    !   \
----       -------      ------------------------
                  !    /
                  !   /
                  !  /
                  !-


U1 turns that into:

       -----------
       !         !
-------!         !----------------------------

Which is very like the other unit's TXD signal.


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If the micro has an ADC you could perhaps do without the U1.

--
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snipped-for-privacy@rahul.net   forging knowledge


Re: Simple data bus over DC Power Wires
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How about the following circuit

                L1
 DC source ----))))))))----------o-------------- To others
 or load                         !
                                --- C1
                                ---
                                 !
                                 /
                                 \ R2
 !--------------!                /
 ! Micro     TXD!---!            !
 !              !   !            !     R3
 !           RXD!---o------------o----/\/\/----! GND
 !--------------!

The high pass filter R2,R3,C should work with reasonable (R2+R3)*C time
constant, i.e. should carry over everything from first harmonic, which
is BR/N, where N = databits+stopbits+startbits.

Let's consider 2 extreme cases, one with all bits ones and one with all bits
zeroes for 8N1.

In case of all zeroes (8N1):

   !-!                 !-!                 !-!
   ! !                 ! !                 ! !
-!-! !-!-!-!-!-!-!-!-!-! !-!-!-!-!-!-!-!-!-! !-
  x S                 x S                 x S

S = Start Bit
x = Stop Bit

DC component of 1/10th signallig voltage will be stripped
1st harmonic of BR/10 and everything above will be carried over.
Microcontroller will receive correct data. The recessive state will be
slightly negative.

In case of all ones:

   !-!-!-!-!-!-!-!-!-! !-!-!-!-!-!-!-!-!-! !-!-
   !                 ! !                 ! !
-!-!                 !-!                 !-!
  x S                 x S                 x S

DC component of 9/10th signallig voltage will be stripped (too bad)
1st harmonic of BR/10 and everything above will be carried over.
The recessive state will be negative, the dominant state will be slightly
over ground. Microcontroller would not receive correct data unless DC
component is restored.

As for restoring DC component, one example I can think of is if R2 << R3 and
micro would turn the high on TXD or RXD for long enough to discharge C1
after each byte received.

This design is only theoretical, being inspired by Ken's post. I did not
build it.

Roman



Re: Simple data bus over DC Power Wires
Interesting Ken,

When you say, this has been used up to 1000 ft, you actually implemented it?

If so, how reliable was it and what kind of data rates did you get?

Thanks for the feedback,

Paul


snipped-for-privacy@violet.rahul.net (Ken Smith) wrote in message
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Re: Simple data bus over DC Power Wires
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Yes and sold it to someone.  The circuit was a little different because we
used an LT1081 to make the signal from the micro bigger and stronger and
attempted to cancel the self signal (the micro hearing its self).


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It was about 100% for one side only transmitting.  The self signal
cancelation was a bad idea.  It is better, if you can, just to only have
one side talk.

--
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snipped-for-privacy@rahul.net   forging knowledge


Re: Simple data bus over DC Power Wires
Interesting Ken,

When you say, this has been used up to 1000 ft, you actually implemented it?

If so, how reliable was it and what kind of data rates did you get?

Thanks for the feedback,

Paul


snipped-for-privacy@violet.rahul.net (Ken Smith) wrote in message
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Re: Simple data bus over DC Power Wires
Just my opinion, but AS-i works great and is easy to set up. I just
have one node, my first trial. 12 valves, solenoid operated with open
and close proximity switches, one repeater.

On 19 Jun 2004 07:04:41 -0700, bushie snipped-for-privacy@hotmail.com (Paul W.) wrote:

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Re: Simple data bus over DC Power Wires
Any idea what it would cost per node to implemented ASI, and how complex is it?

Now sure how much the ASI chips cost.  Siemens makes them I think.

Paul

snipped-for-privacy@home.com wrote in message
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Re: Simple data bus over DC Power Wires
I used a pre-packaged interface card for Fisher Delta-V and field
hardware from peperyl-fuchs. The valves didn't cost any more with ASI
vs solenoids and limit switches. The other expense was the repeater
for long distance ~$500

On 22 Jun 2004 14:58:31 -0700, bushie snipped-for-privacy@hotmail.com (Paul W.) wrote:

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Re: Simple data bus over DC Power Wires
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You could always go analog.
Each switch has a series LC network, and the control circuit
just imposes a small AC frequency over the network, and looks at
the impedance.
The plusses of this are of course you only need one real circuit,
the others can practically be built onto a chocolate block.


Re: Simple data bus over DC Power Wires
responding to
http://www.electrondepot.com/embedded/simple-data-bus-over-dc-power-wires-16986-.htm

Hello,
You could also look into a company called Yamar (
http://www.yamar.com/sig60.php/ ) who makes chips that communicate over
the DC power lines using the LIN bus. Soon there will be modules that can
implement 250K CAN bus. They are not very expensive. You can follow the
electric car charging standard. Once this is final then I imagine more
suppliers will be making parts. If you want data and power then look into
a company called PowerByProxy out of NZ. This is more expensive.
http://www.powerbyproxi.com /


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