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Discussion in 'Electronic Basics' started by [email protected], Oct 9, 2006.

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  1. Guest

    I'm having a hard time finding the logic voltage levels for RS422.

    Anyone know if a high voltage in RS422 is a one or zero?
  2. I don't know the exact details, but it is a differential method, not
    single-ended one like RS-232. You can't look at it quite the same

  3. Chris

    Chris Guest

    Differential output -- A is non-inverting, B is inverting. The nominal
    signal levels are +5V and 0V.

  4. Guest

    What exactly does that mean to say the nominal signal levels are +5V
    and 0V?

    Isn't one signal always the negative of the other? So, if one is at
    3v, the other is at -3v?
  5. No, when one is +5 the other should ideally be zero.
    When it is at zero, the other should ideally be at +5.

    They should, ideally cross each other at 2.5 volts, though
    this happens so fast, it is not really important. When one
    signal is more positive than the other, that is one logic
    state. When it is more negative than the other, that is the
    zero state. The exact voltage of either is unimportant,
    since all that matters is one relative to the other.
  6. Dana

    Dana Guest

    The output will go from 0v to 5v.
    To determine your value of high and low, you would have to compare your A
    and B outputs
    The relationship of the levels on the A and B output determines your logic
    If A is negative of B you have a mark(off), or binary low.
    If A is positive of B you have a space(on), or binary high
  7. Eeyore

    Eeyore Guest

    No they are the 'logical inverse' of each other like any digital signal. Not
    'inverted' !

  8. Dana

    Dana Guest

    And if your logic states stay stable for a long period of time a voltmeter
    works great. If on the other hand your logic is a train of pulses, a 2
    channel oscope may be better.
  9. Guest

    Thanks for the help.

    So, if A > B, it's a one, and if A < B, it's a zero?

    Also, I remember reading that the noise cancellation properties of
    differential signaling were due to the currents being opposite. Hence,
    I thought one was the exact negative of the other. If the voltage is
    always positive though, then the current is always going in the same
    direction. So, I guess currents being opposite isn't that important
    for noise cancellation?
  10. Guest

    Depends on which line is A and which is B. ;-)
    With conductors taking a similar path through electric and magnetic
    fields, similar noise voltage tend to get added to both signals.
    Taking the difference of the two resultant voltages subtracts these
    similat noise voltages with an approxmately zero effective error.
    Regardless of the amount of common voltage added to each side, the
    difference between them remains essentially + or - 5 volts, so the
    decoded state is unaffected.

    This method has a big advantage over single ended systems (like RS 232)
    that subtract the signal from the local ground voltage to determine
    whether the signal is positive or negative.
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