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Power over Ethernet Vs RS422

Discussion in 'Electronic Design' started by martin griffith, Aug 17, 2005.

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  1. Any suggestions on protecting RS422/485 when using CAT5 cable?
    explanation here

    Most 422 line drivers seem to have a max common mode Voltage of +-12V.

    These days everbody seems to be using CAT5 for almost every
    conceivable signal.

    Just wondering what you do for protection (insert condom jokes if

  2. Void the warranty of your device when connected to an improper
    cable or source.

  3. LOL, I agree


  4. Richard H.

    Richard H. Guest

    Does RS-422 have the signature 25K resistance? (IIRC there is also a
    capacitance check.) i.e., the -48v is not applied to the cable unless
    the signature is present, to prevent damaging non-PD devices. Early
    non-standard implementations were hard-wired on, and they quickly
    learned of the incompatibilities.

    The discovery process applies a low current-controlled voltage to probe
    for the "signature" before the power is enabled. I don't know the
    characteristics of this signal, but I'd expect it to be closer to 5v
    than 48v for exactly this reason.

  5. Richard H.

    Richard H. Guest

    Practical, but I doubt customers will see it that way.

    Corporate cable plant is RJ-45 jacks for everything, and the telephony
    gear is less friendly than PoE. A very real hazard is a digital phone
    jack or a modem line, which will have up to 48v on the center pair and
    possibly the others, depending on the system.

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