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Thread: USB Hub question

  1. #1
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    USB Hub question

    Hope this off-topic question is OK here....

    I have a 7 port USB 2 hub with one power on/off switch for the ports.
    It has an external power wall-wart.

    When the external power is applied, 5V at considerable current capacity appears on the hub's input cable. I'd think this is a no-no.
    I think it does no harm but seems like the port to which the hub's main input USB cable connects is sourcing 5V.
    By what rationale would the hub do this?

  2. #2
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    I've read on the Minnowboard Max forum, the Parallella forum, and others, that some USB hubs are not standards-compliant in "providing" power back to the host system. Your hub may be one those.

    This issue of USB hub power backflow has been a source of some consternation for developers and devices that are not prepared for it.

    There's no way to address "rationale" of non-standards compliance... :-\

  3. #3
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    Quote Originally Posted by LenSamuelson View Post
    I've read on the Minnowboard Max forum, the Parallella forum, and others, that some USB hubs are not standards-compliant in "providing" power back to the host system. Your hub may be one those.

    This issue of USB hub power backflow has been a source of some consternation for developers and devices that are not prepared for it.

    There's no way to address "rationale" of non-standards compliance... :-\
    The one of which I speak is branded "GearHead". But I suppose the brand names are just buying on the spot market without much regard to the innards.

    I hope this can't damage the upstream hub! Esp. the root hub on the mainboard of the PC.

  4. #4
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    (Indistinctly recalled) The issue generally breaks initialization and enumeration state machines but not hardware. I had forgotten about backflow causing Raspberry Pi issues, again causing difficulty without damaging hardware.

    http://elinux.org/RPi_Powered_USB_Hubs

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