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Simple data bus over DC Power Wires

P

Paul W.

Jan 1, 1970
0
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
 
F

Fred Bloggs

Jan 1, 1970
0
Paul said:
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?

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

Ken Smith

Jan 1, 1970
0
Hi Guys,

I'm in the process of implementing some simple control nodes (just
switches) and looking for the simplest (cheapest) network protocol to

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.
 
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.
 
I

Ian Stirling

Jan 1, 1970
0
In sci.electronics.design Paul W. said:
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).

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.
 
T

Tony Williams

Jan 1, 1970
0
Fred Bloggs said:

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.
 
M

Mike Lamond

Jan 1, 1970
0
Tony Williams said:
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.

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
 
T

Tony Williams

Jan 1, 1970
0
Mike Lamond said:
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/.

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

Roman Ziak

Jan 1, 1970
0
You want cheap:


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

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
 
F

Fred Bloggs

Jan 1, 1970
0
Tony said:
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.

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.
 
T

Tony Williams

Jan 1, 1970
0
Fred Bloggs said:
I don't think that DCC is suitable because it looks they reverse
the polarity on the rails or some crazy thing.

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.
 
P

Paul W.

Jan 1, 1970
0
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
 
P

Paul W.

Jan 1, 1970
0
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
 
P

Paul W.

Jan 1, 1970
0
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
 
K

Ken Smith

Jan 1, 1970
0
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 :)

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.

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

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

Ken Smith

Jan 1, 1970
0
Interesting Ken,

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

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).

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

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.
 
R

Roman Ziak

Jan 1, 1970
0
Ken Smith said:
If you do that, the micro gets something like this on its input:

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

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
 
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
 
R

Rolavine

Jan 1, 1970
0
Subject: Re: Simple data bus over DC Power Wires
From: [email protected]
Date: 6/23/2004 7:19 PM Pacific Daylight Time
Message-id: <[email protected]>

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
You could do it simple with two wires. For example if you were using 24 volts
for the valves, from an adjustable regulator, you could adjust it down to say
20 volts for a logic low, then send simple serail address - data, pick it up
with a voltage divider - comparator circuit and send it to a processor at the
switching nodes.

Rocky
 
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