usb voltage and usb hubs

Posted on 2005-04-10
Last Modified: 2013-11-09
I am wondering about the technical difference of usb hubs and motherboard ports.

When a usb hub is powered is it able to handle more 'un powered' devices ?
What is noticed between non powered usb hubs and powered ones ?

I have a kick ass fluke meter and I want to know what I should be looking for power wise
on the usb hub. ( I cut into an unused usb cable I had laying around and use alagator clips)

What is the most kick but usb hub you can get ?

How do you detect usb errors ?

Where can I go to learn about how usb works ?

Bla bla bla...
Stuff like that.

Thanks for your help.
Question by:joncolby
    LVL 55

    Assisted Solution

    by:Jaime Olivares
    >Where can I go to learn about how usb works ?
    All your answers are in this book:
    LVL 23

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    USB is like a very fast serial port -- it is different, of course, but the idea of USB was to get serialized communication concurrently on different leads that was a much faster and more reliable communication means than where standard serial ports at the time.  The initial 1.0 specification was fast, but largely a "prototype" for testing the technology.  The initial tests discovered 2 major problems --
    1. it wasn't fast enough for heavy throughput devices like scanners and hard disks, and
    2.  There was a significant power problem if too many devices were chained together.

    The first is why firewire became a viable contender with USB -- it was a lot faster than USB1.0.  So the USB group took on the task of improving the bus performance, and through several stages of upgrades, finally got to USB 2.0 -- which is generally faster than firewire (which is now going out of fashion and has largely vanished from a lot of devices).

    Since USB draws its power from the motherboard, it is a serious power issue to attach too many devices  on one serial line.  Hence USB hubs were born -- the idea being to not only multiply the ports, but through the USB hub adapter, add extra DC voltage along the line to power more devices.

    It probably isn't that good an idea to use clips on a long term basis -- better to use the standard connectors if you can.  here is a good technical summary of the power specifications --

    Some of this is worth posting here, for general information sake --

    Full speed:       12 Mbps. (Shielded cable)
    Low speed:       1.5 Mbps. (Non-Shielded)

    Power Usage
    USB Device       Power Requirements
    Bus-Powered Hub       Draw 100 mA (max) at power up & 500 mA normal operation.
    Self-powered Hub       Draw 100 mA (max). Be able to supply 500 mA to each port.
    Low power, bus-powered function       Draw 100 mA (max)
    Self-powered function       Draw 100 mA (max)
    Suspended device       Max 0.5 mA

    Operational Voltages

        * Voltage supplied by a host or powered hub must be in the range of 4.75V - 5.25V.
        * Max. voltage drop for bus-powered hub is 0.35V from its host or hub to the downstream port(s)
        * All USB devices must be able to send data at 4.4V, but only low power functions need to be working at this voltage.
        * Operational voltage for functions is 4.75V.


    The rest of what I think you want is also in that and other articles.
    LVL 91

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