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Isolating serial noise

Discussion in 'Electronic Basics' started by Chris, Jan 19, 2005.

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  1. Chris

    Chris Guest

    Can i isolate my pic from my pc's noise by using 2 optocouplers and
    powering the serial comms from the serial port as in the schematic?

    Thanks for any input / pointers
  2. Andrew Holme

    Andrew Holme Guest

    You can't draw enough current from a PC serial port to power a MAX232 -
    typical MAX232 power consumption is 8mA.

    MAX232 datasheet:

    Good article about taking power from the serial port:

    Why don't you forget about the opto-isolators and just power the MAX232
    from the same supply as the PIC?

    The PIC should not be disturbed by noise from the PC; and any
    noise-sensitive circuitry will likely pickup noise from the PIC too!
  3. Chris

    Chris Guest

    yep thats the article i followd, the pic a/d does pickup noise when
    plugged into the serial port, verified by readings and a scope, all ic's
    have filtering caps ad is also filtered, if i unplug the serial gnd from
    the pic board the readings are clean as soon as i plug it back in i see
    a wave......

    unless is there a way to split off my pic power supply for that circuit
    instead of the serail bu that would not cause noise??
  4. Andrew Holme

    Andrew Holme Guest

    How many milli-volts are we talking about? I wonder if you have a
    ground-loop problem? Check your earthing.
    Could you post a schematic and/or photo of your layout?

    You might be able to do this without the MAX232: drive the opto LED,
    via a series current-limiting resistor, directly from the PC serial
    port. Going the other way, if you can independently control DTR and
    RTS, you could do this:

    DTR (+12V)------[1K]------[opto-transistor]-----RTS (-12V)
    Connect RxD to the junction of the 1k resistor and the transistor.
  5. Chris

    Chris Guest

    yep i do have a ground loop problem, this was the solution i was
    thinking about to solve it.....

    the max setup is currently like

    is there another way to get rid of the ground loop?


  6. Andrew Holme

    Andrew Holme Guest

    Google for "ground loop."

    You could possibly place a "star point earth" at the PIC GND terminal.

    Do you get noise if you only connect the GND wire and leave the other
    RS232 signals disconnected?

    You mentioned them in your original post, but you don't show any power
    supply decoupling caps in that circuit.

    A photo might be more helpful since a schematic doesn't show if you
    have a layout problem.
    What is the amplitude of the noise at the PIC A/D input?
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