serial data with power - 2 wires only

Discussion in 'General Electronics Chat' started by casdsys, Sep 5, 2012.

  1. casdsys

    Thread Starter Member

    Mar 15, 2009
    Does anyone know how to, or have data, links on how to send data ( both ways ) as well as power along 2 wires such as RS-485???
    We need to set up a master controller hooked up to multiple boxes with only 2 wires.
    Many thanks.
    Or direct me to an electronic engineer that can design a schematic for me plus software.
    This would be paid work after quotation.
  2. crutschow


    Mar 14, 2008
    In general you can send DC power down the wires and use AC coupling to send the signal. The AC is isolated from the DC by having inductors in series with the DC at both the source and receiving ends.

    Depending upon the signal characteristics, you may need a DC-restore type clamp at the receive end to eliminate offsets due to AC coupling of the signal.

    If the signal is a low frequency then you could also consider a 4-20mA current-loop connection which can send both signal and power.
  3. BMorse

    AAC Fanatic!

    Sep 26, 2009
    or you can use the same scheme as lego sensors do..... they use the TX/RX lines for their sensor data as well as for powering the sensor, they have a memory capacitor on the sensor side that will hold a charge long enough for the sensor data to be transferred, then they switch it back to "power mode" to power the sensor/charge the cap again....
  4. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    or consider a 4-20ma current loop.
  5. Robin Mitchell

    Distinguished Member

    Oct 25, 2009
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Thanks for mentioning my page MitchElectronics, but I think you may have meant a different page of mine;
    which is a system where many devices can communicate on the same pair of wires. It's not optimal for providing power AND communications, but if the devices are very low powered (like a few mA) then it can be workable by using caps in the devices so they have power when the main line is switching on and off (during the communications).

    If the OP needs two wires with larger power needs for the devices it's going to get a bit more complex than a couple of transistors. He probably needs a high power DC wire pair, then modulate with an AC component which probably needs some ICs for modulation and demodulation, like modem ICs. It's not a beginner project if larger power is needed.