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data over dc power line using RS485(2 wire)

Intellectual 380 points

Replies: 12

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Can we use RS485, SN75176 to transmit data over DC power(2-wire)? any application note or reference design from TI?

- data rate 2400 Bd.

- distance 1000 mt.

NT

  • Dear Knighty, I am Valter from Italy. I have the same question. I need to transmit and receive data , based on RS-485 standard, on a 2 wire copper lien where there is a DC Voltage ( +175 and -175 Vdc) . Did you get any response ?

    Thanks

    valter.pileri@ascpoint.com

  • In reply to Valter Pileri:

    Dear Valter,

    Not yet, I waiting for TI whether they have suggestion or application note.

    NT

  • Sorry, there is no apps note or reference design. In principle you would couple the signal capacitively onto the power lines (see below). common-mode jokes could be used to provide high isolate the data signal from the 12V supply and from the voltage regulator. A customer experienced difficulties with inductor saturationwhen transmitting NRZ data, so basically the original data without coding.

    To avoid saturation, either Manchetsre coding or FSK-coding should be used to remove DC components. Usually power line communications (PLC) as well as low-volt power communication use commercially available power line modems for example for LinkSprite http://www.linksprite.com/

    we therefore stepped away from competing efforts against these powerline pros. The circuit below however, should give you a pretty good start. and as long as you keep the power lines busy with data transitions, that is during idle you should send some kind of frequency, no major saturation effects will occur.

    Regards, Thomas  

    Best Regards,

    Thomas

  • In reply to Thomas Kugelstadt:

    Has anybody tried this circuit, it's so simple it seems too good to be true.

    _______
    Rob

  • In reply to Rob Gray:

    yes, quite a few have. With some minor mods of course. After grasping the concept of tranformer saturation, most use an idle frequency instead of letting the bus drift on a DC potential. Good luck in your endeavour. - Tom

    Best Regards,

    Thomas

  • In reply to Thomas Kugelstadt:

    I'm replying to an old thread here, but nevertheless, this is a very useful circuit Thomas.  Thank you much.    That makes sense about using an idle frequency to avoid transformer saturation.  

    What would be the nature of the typical 'minor mods' that you mention?
     

    Thomas Kugelstadt

    yes, quite a few have. With some minor mods of course. 

  • In reply to Thomas Kugelstadt:

    Dear Thomas,

    Could you advice me transformers for using that schematic? Thanks in advance.

    Demet

  • In reply to demet aktaş:

    Thomas mentions using a common mode choke although the schematic shows differential mode chokes and I'm having trouble finding off the shelf parts, even less options for high current.  Anyone else?

    Aren't you also playing games with transmission line impedances?  Shouldn't the capacitance be tuned to the inductance the chokes put on the line?

  • In reply to oxobfe:

    True, the inductors for high currents are a problem. Many Chinese OEMs approach manufacturers of magnetics components for custom designs of their chokes. The only way I see is contacting companies like Wuerth-Electronics or Coilcraft and ask their support staff for help.

    With regards to the line impedance, the 12V DC across 120 ohms in parallel (so 60 ohms) causes some major currents to flow. Again some customers don't apply termination at all for that reason. Also quite a few have turned towards the SN65HVD96, which has a CAN-type signaling scheme. A logic Low at the driver input causes the A and B lines to assume Vcc/2, while a logic high at the input causes a differential voltage on the bus. Still the signal waveforms on the bus look bad and distorted.

    One Italien customer simply changed to using CAT-5 where he uses one pair for the 12V supply and another signal pair for data transmission.

    Unfortunately we never had time to conclude our efforts towards a successful solution.

    Sorry for that, Thomas

    Best Regards,

    Thomas

  • In reply to Thomas Kugelstadt:

    Hi Thomas,

    we're thinking about using SN65HVD1785 transceivers on an intra-PCB 24V bus. The overall length of that bus would be around 1.5m and we have one master and 4 slaves. The bus would carry 2A on those 24V. Would that be feasible with a design like you showed above?

    Regards

    Joern

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