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prop to prop

pi159pi159 Posts: 2
edited 2012-10-04 07:20 in Propeller 1
I am working on a project where two props will talk back and forth. RX->TX...TX->RX. Each prop has a separate power source. Therefore, I have two power sources. Do I need to connect the gnds together...or is it okay to just connect the RX and TX lines?

Comments

  • LeonLeon Posts: 7,620
    edited 2012-10-03 07:12
    Grounds need to be connected.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-10-03 09:48
    Leon wrote: »
    Grounds need to be connected.

    I second the motion.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2012-10-03 11:14
    Unless the communication is fiber optic or some form of wireless media, the grounds should be connected.
  • SRLMSRLM Posts: 5,045
    edited 2012-10-03 13:39
    Leon wrote: »
    Grounds need to be connected.
    Beau Schwabe (Parallax) wrote: »
    Unless the communication is fiber optic or some form of wireless media, the grounds should be connected.

    Wouldn't it matter based on how the devices are powered? If the devices are both plugged in to AC outlets, then connecting the ground wires would create a ground loop in the system. This would create noise, and possibly spurious readings.

    Reference: http://siber-sonic.com/electronics/GLoopwhatis.html
  • LeonLeon Posts: 7,620
    edited 2012-10-03 14:08
    Ground loops aren't usually relevant with digital systems.
  • prof_brainoprof_braino Posts: 4,313
    edited 2012-10-03 15:49
    Leon wrote: »
    Ground loops aren't usually relevant with digital systems.
    I thought ground loops are always relevant, in digital you just don't notice the buzz you would hear if it were for example an audio system. But we still only ever want one ground point in a system.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2012-10-03 16:11
    If your having those kind of ground loop issues, then you need to consider magnetics on each end, or opto isolators on each end, etc. Differential pairs can get rid of common mode noise but the major point here is that you still need a reference that agrees on both sides. Usually this reference is ground, but it does not have to be. Magnetics, usually used or references with a differential connection such as Ethernet can provide a galvanic isolation at each end (essentially two coupled transformers with a long transmission line) and eliminate the need for anything outside of a local ground.

    Perhaps a more detailed explanation of your actual setup environment is necessary for us to give an appropriate response.
  • pi159pi159 Posts: 2
    edited 2012-10-04 06:52
    For the project one propeller is powered by 9v battery and the other propeller is going to be powered USB (quickstart board)
  • Mark_TMark_T Posts: 1,981
    edited 2012-10-04 07:20
    prof_braino wrote: »
    I thought ground loops are always relevant, in digital you just don't notice the buzz you would hear if it were for example an audio system. But we still only ever want one ground point in a system.

    Well the point is digital logic has a noise-margin, often around 1V - any noise below that level is irrelevant. For analog circuitry (read audio amplifier) 1mV of noise could be a show-stopper. So whether you need to worry about ground loops depends on the amount of noise you can tolerate and the rate of magnetic flux change through the loop and the resistance round the loop.

    Large loops such as mains-completely loops are usually a problem for everything (mainly because of switching or lightning transients on the mains wiring coupling to the earth wire). Thus for digital electronic devices is it normally the case that circuit ground is isolated from mains earth. (Or lightly coupled via RFC and resistor).

    Where this cannot be achieved for whatever reason opto-isolated, pulse-transformer-isolated or differential logic signalling is employed (RS485, ethernet, MIDI etc)
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