RS485 and power on the same cable

- H
- Henrik [6650]
  
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posted
17 years ago

Tue, Jun 13, 2006 9:27 AM

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

- W
- William at MyBlueRoom
  
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posted
17 years ago

Tue, Jun 13, 2006 2:11 PM

- C
- Captain Dondo
  
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posted
17 years ago

Tue, Jun 13, 2006 2:12 PM

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

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- C
- cs_posting
  
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posted
17 years ago

Tue, Jun 13, 2006 4:43 PM

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
  
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posted
17 years ago

Wed, Jun 14, 2006 5:43 AM

"Henrik [6650]" a écrit dans le message de news:

448e83f8$0$172$ snipped-for-privacy@dread11.news.tele.dk...

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,

--
Robert Lacoste
ALCIOM - The mixed signal experts
www.alciom.com
France

- A
- Anthony C Smith
  
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posted
17 years ago

Wed, Jun 14, 2006 6:11 AM

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
  
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posted
17 years ago

Wed, Jun 14, 2006 6:31 AM

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