rs485 over power

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I'm trying to communicate between multiple devices and I only want to
run 2 wires between devices. I want the 2 wires to be used for both
power and communications.
I've tried capacitive coupling the output from the RS485 chip onto the
lines. I have inductors between the power supply and where I'm tying
the caps into.
When I have the power disconnected, I get a fairly normal looking
signal on the lines (a square wave with an initial spike). When I
apply power to the lines, I only see an initial spike and then an
almost immediate return to 0 (I'm only looking at the AC signal).
Would different size caps or inductors help?

I've seen a simular system work that used bi-directional RS485 over
power and the polarity of the wires didn't matter (I didn't get to
spend a lot of time looking at the system, only enough to verify that
they are using RS485).

Any help would be appreciated.

Thanks
-jeff

Re: rs485 over power
On 16 Jul 2003 10:58:09 -0700, snipped-for-privacy@engineer.com (Jeff Hendrix)

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You don't say what kind of "power" is in the lines.  AC 120v? DC 5 v?
In any case, you will have a hard time unless you abandon the baseband
and use some sort of modulation.  The impedance of any type of power
line is going to be very low at mid frequencies.  Bigger caps would
just ask more of your "RS485" drivers - probably more than they can
deliver.  But if you can get all your signaling up above 10kHz, then
you can separate power from signaling using caps and inductors.


-Robert Scott
 Ypsilanti, Michigan
(Reply through newsgroups, not by direct e-mail, as automatic reply address is
fake.)


Re: rs485 over power

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Definitely. You'll have to use FSK or Manchester encoding, or
some other encoding to make sure that there's no significant DC
component to your data stream.

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Re: rs485 over power

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The nodes on that system have low power consumption and significant energy
storage within the nodes.

The simplest solution for this type of system would be to disconnect power
from the wires while they are used for communication.



Re: rs485 over power

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Just goes to show that there's lots of clever engineers out there (:

I honestly don't know how this could be made to work unless there
was some kind of carrier.  That the polarity didn't matter is
really confusing.

Think about a current-based system instead of voltage.  I once did
a system that would power and communicate bidirectionally over a
twisted pair for something like 4 miles.  What I did was force
100 ma into the pair from one end.  At the other end, the pair
connected to VCC and ground of the board in parallel with a
5 volt zener.  The board drew about 50ma and the zener burned
the rest.  Now at each end I had 600 ohm transformers in series
with the loop and I used the secondaries to send Bell 103 modem
signals for bidirectional communications at 300 baud.  There
was other trickery involved, but the end result worked very
well for the application.








Re: rs485 over power

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That's true, but there are other options too, depending on your interpretation.

We run comms over power in our new multi-drop industrial products, and each drop
can be connected either way around. That works because the comms are coupled
AFTER the bridge rectifier, so the polarity gets re-aligned to the power
polarity.

Also, while Manchester and many other coding systems fulfil the need for zero
DC, it's also possible to do it on the cheap using a simple UART as long as you
send only characters with equal 1s and 0s,  and use a look-up table (sacrifice a
couple of bits per byte, eg send 4 characters for every 3 bytes - more efficient
and cheaper than Manchester).

Regards,
Tony (remove "_" from email address to reply)

Re: rs485 over power
Jeff,

If you look carefully, you probably notice that the signal is a positive
going spike that coincides with the leading edge of the data bit square
wave, and a negative going spike that coincides with the trailing edge.
Manchester encoders and decoders are typically used in this type of
application since manchester code is concerned with transition logic rather
than logic levels.

We used to run upto several megabit/sec data rates over high voltage AC
power circuits for 10's of kilometers.

Regards
Larry


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Re: rs485 over power
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This really doesn't seem that hard with a totempole driver for each side of
the pair so you can drive some current onto the line.

At the receiver end, you rectify  the 'data' and use a cap for filtering (of
course with some regulation)

So long as you don't draw more current than your driver can supply you
should be able to happily read the data using a standard RS485 Transceiver.

If you need By directional communications, you clearly need enough power
stored at the slave to drive the bus for the whole transmission, however the
slave only needs to drive voltage so can be low power.

Ralph



Re: rs485 over power
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Sorry to be pedantic, but you're talking about serial comms over power -
which is not RS-485, which is a physical hardware spec. While you may well
be able to use RS-485 drivers/receivers in some way, it won't conform to the
RS-485 spec.

I'm pointing this out in the hope that it helps you to think outside the
envelope, i.e. forget about RS-485 and just think in terms of
data-over-power.

Steve
http://www.sfdesign.co.uk
http://www.fivetrees.com



Re: rs485 over power
snipped-for-privacy@engineer.com mentioned...
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This is total insanity.  The first high voltage transient that comes
down the line will wipe out your Txes and Rxes.

You need to modulate the data onto a carrier, maybe a hundred or so
kHz, and couple it to the power line with a transformer and caps to
isolate your stuff from the AC line.

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Re: rs485 over power

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There's no requirement for modulating a carrier.  You can do Manchester
encoding and isolate the drivers/receivers appropriately just as you would
isolate the modem.  Poor isolation will result in a blown-up modem just as
it would a blown-up line driver/receiver.

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Re: rs485 over power
snipped-for-privacy@visi.com mentioned...
'Watt Sun' wrote:
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There's no requirement for receiving uncorrupted data, either.  By
putting the data onto a carrier, the receiver can then bandpass filter
the incoming signal and eliminate a whole lot of the garbage and 60 Hz
that's already on the AC line.  

Another helpful procedure is to detect the AC zero crossing and
transmit during that time.  That also saves a lot of corrupted data.

You just have to realize that the AC line is a very hostile, noisy
medium, and not using some of these methods opens up your data to
total, unrelenting corruption when the first vacuum cleaner is turned
on and those nasty brushes and commutators send a ton of hash and
noise down the line.  So you don't use these methods, and you wait for
the data to start to pass again after m'lady is finished vacuuming in
a half hour.  Your choice.  :-)

And this doesn't include all the times when you use the electric
drill, the blender, the portable saw, the weed whacker, the light
dimmers and even the newer fluorescent lights that have the electronic
ballasts that put out a lot of interference at 20 to 50 kHz.

Or you can just skip trying in vain to use the AC line as an error-
free transmission medium and instead use RF at 900 MHz or 2450 MHz,
like cordless phones and such do.  It will probably result in a lot
less complaints from users about not being able to receive any data
while their significant other is vacuuming!

Oh, did I mention the arc welder?  But then, that probably doesn't
happen very often.  Unless, of course, one of your hobbies is
welding...

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Re: rs485 over power
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This is on a DC power rail, not the AC mains.  But regardless, an error
correcting protocol should be used even if it is just as simple as
adding a CRC to each message and retransmitting ones that are
corrupted.  

--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
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Re: rs485 over power

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Looking at stuff like ProfiNet, DeviceNet, and Foudation
Fieldbus would be a lot more applicable.  They all do
power/comm over twisted pair for microprocessor based devices
in industrial environments (baud rates in the 32K range).

Some of them are even intrinsically safe.

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Re: rs485 over power
Hi,
What you need to do is modulate the power! 2 wire fire alarm systems have
been doing this for years. There are various schemes depending on the
required power and voltage. Give us more info.
Keith Buck
wrote:
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the



Re: rs485 over power
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--
Are you talking about the old windup motor driven code wheels that
interrupted the monitored current loop to tell which building was on
fire?

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Re: rs485 over power
Hey Mike, I've actually got one of those panels bolted on my 'shlock room'
wall.
Still works too !!

Jay in Greensville,Ont.

Got a 25 cycle time clock/ break bell system too. Runs a perforated plastic
tape as the 'program' for the time of bells.





Re: rs485 over power
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I worked for an old man in Ohio that serviced Standard Electric,
Edwards, and several other brands of school clocks and fire alarms.
There were odds and ends in the back room he had salvaged when an old
school building was torn down to keep some real antique systems in
service, in cities that still used controlled loop alarm systems.
--


Michael A. Terrell
Central Florida

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