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inverted voltages

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

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  1. Guest

    It seems that in RS-232, the voltages for data bits are inverted (a low
    voltage is 1 and a high voltage is 0). Anyone know the motive for
    this?

    Thanks.
     
  2. Just as a clarification, RS-232 has a 6V dead band around 0V (+ and -
    3V about 0), which isn't either a defined '1' or '0'. The MARK state
    uses the negative (relative to ground) voltage value and the SPACE
    state uses the positive (relative to ground) voltage value.

    Why the relative-negative voltage was specifically chosen for the MARK
    state, rather than the relative-positive, I don't know. It probably
    does have meaningful historical and electronic reasoning, though,
    appropriate for the time of the standard's development.

    Jon
     
  3. Archeology, chemistry, habit!
    The "ancient" IT of telephony found that it was more "technical" from
    corrosion standpoint to ground the positive terminal of batteries
    supplying the grid. So an agreed "standard" developed and the spider
    "web" of copper wires could connect different sectors to each other.
    On this backgroung the interpreting of what is "true" and "false" in
    binary terms was accepted and is with us till now.

    HTH

    Stanislaw
    Slack user from Ulladulla.
     
  4. Chris

    Chris Guest

    Hi, Mike. My understanding was that, when RS-232 was first developed
    as a standard, germanium ruled the earth. For various reasons, PNP
    transistors were more popular, so power supplies were typically
    negative, and PNP inputs to switching circuits required an active low
    voltage to turn on the transistor (pulling current out of the base of
    the transistor).
    |
    | GND GND
    | | |
    | .-. |
    | | | |
    | | | |
    | '-' |
    | ___ | |<
    | In o-|___|-o-| PNP Ge
    | |\
    | | Out
    | o------->
    | |
    | .-.
    | | |
    | | |
    | '-'
    | |
    | V
    | -12VDC
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    I can't find a reference on this (it's a bit before my time), but my
    hunch (or WAG, depending on your level of cynicism) might be a place
    for you to start looking.

    Good luck
    Chris
     
  5. jasen

    jasen Guest

    put a negative potential on a conductor an you get anodic protection
    same reaspon why negative voltages are used in phone lines as I understand it.
     
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