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RS485 and power on the same cable

H

Henrik [6650]

Jan 1, 1970
0
Hello group,

I am trying to figure out how to communicate serially (eg.RS485) and power
the devices on the string via the same pair of cables.

I want to be able to transmit data at baudrates of 19200 to 57600 via this
link. The reason I would like to have data and power on the same cable is
that I need to use existing cable in a large installation.

Could somebody advice me on how to get ahead, how to mix power and data? Or
perhaps provide a link or reference design.

Product price is an issue.


Thanking you in advance
 
C

Captain Dondo

Jan 1, 1970
0
Hello group,

I am trying to figure out how to communicate serially (eg.RS485) and power
the devices on the string via the same pair of cables.

I want to be able to transmit data at baudrates of 19200 to 57600 via this
link. The reason I would like to have data and power on the same cable is
that I need to use existing cable in a large installation.

Could somebody advice me on how to get ahead, how to mix power and data? Or
perhaps provide a link or reference design.

Product price is an issue.


Thanking you in advance

I don't think you can do loop power off an RS485 circuit...

RS485 is voltage differential, and each node turns on its transmitter only
when it has something to say, so it is possible that there is 0 voltage on
the line at times.

OTOH, if anyone can help you, it Mr. RS485: rs485.com

--Yan

--
o__
,>/'_ o__
(_)\(_) ,>/'_ o__
Yan Seiner, PE (_)\(_) ,>/'_ o__
Certified Personal Trainer (_)\(_) ,>/'_ o__
Licensed Professional Engineer (_)\(_) ,>/'_
Who says engineers have to be pencil necked geeks? (_)\(_)
 
Henrik said:
I am trying to figure out how to communicate serially (eg.RS485) and power
the devices on the string via the same pair of cables.

I don't think you can do that with RS485, but there are some options if
you build something custom.

If you were willing to go really slow and build hefty drivers, you
could put a full-wave bridge rectifier at the receiver, as long as the
communication was one way (or reply periods were short enough that you
could run off a capacitor).

If you went to a non-return-zero coding with no DC component, you could
AC couple the signal on top of a DC offset, using a series inductor to
add and draw the power and a series capacitor (or a transformer as in
ethernet) to couple the signals. Your transmitter would be single
ended, but your receiver could still be differential.
 
R

Robert Lacoste

Jan 1, 1970
0
Henrik said:
Hello group,

I am trying to figure out how to communicate serially (eg.RS485) and power
the devices on the string via the same pair of cables.

I want to be able to transmit data at baudrates of 19200 to 57600 via this
link. The reason I would like to have data and power on the same cable is
that I need to use existing cable in a large installation.

Could somebody advice me on how to get ahead, how to mix power and data?
Or
perhaps provide a link or reference design.

Product price is an issue.

I don't think that you can do that with RS485. However have a look at
solutions from Echelon (Lonworks & Neuron chips) : data+power (either AC or
DC) on the same 2-wires cable.

Cheers,
 
A

Anthony C Smith

Jan 1, 1970
0
You can "phantom power" the remote the same as using a microphone, add and
take power using a centre tapped transformer, returning via the screen; make
sure the transformers can take the additional DC current-
if you can change the impedance to 600R then lots of telecoms transformers
will do the job.
regards
Anthony
 
T

Terry Given

Jan 1, 1970
0
Robert said:
I don't think that you can do that with RS485. However have a look at
solutions from Echelon (Lonworks & Neuron chips) : data+power (either AC or
DC) on the same 2-wires cable.

Cheers,

yuk! expensive, slow, proprietary....

I've done a lot with 10Mbps RS485 + 48Vdc in an STP cable (two pairs) -
perhaps 25,000 or so in the last 3 years. I spent some time coming up
with a way to do it over 2 wires, so I could use coax cable. its not
hard. In practice, we found cheaper multipole connectors and didnt
implement it, but I did get one up and running.

Cheers
Terry
 
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