🚀 go-pugleaf

On 2008-06-15, BigAl.NZ@gmail.com <BigAl.NZ@gmail.com> wrote:
> Hi All,
>
> I have come across this once or twice and its never really bothered
> me, but I am curious as to why:
>
> Occasionally I fond certain devices wont work on front panel usb
> headers, yet when plugged into the rear USB connectors that are "hard
> wired" to the motherboard, they are fine.
>
> At the moment, I have two external HDDs (with there own power supply)
> that wont work on the front but do work on the rear, however various
> other devices such as mouse and USB thumb drives do work on the front.

This sounds suspicously like an unpowered hub on the front panel.
How many sockets do you have there?  If more than two you will have
a hub there of some sort - if only two they may well be directly
connected to the motherboard with no hub involved.

To explain it briefly USB is able to supply a limited amount of
power to devices so that low current devices do not need an external
power supply of their own.  Up to 500mA current may be drawn from
a single USB socket, provided the device has advance clearance
(negotiated by the USB protocol) to draw that much.  Power is
divvyed up in units of 100mA.

However, an unpowered hub is itself a USB device and is powered by
the USB connection, and all power for any devices plugged into it
comes ultimately from that connection.  The hub itself needs some
power, so you can have four devices plugged in each drawing 100mA
+ the power taken by the hub, and that is all that can be taken
without going over the 500mA.  Even if devices are not plugged in
to each socket power still needs to be reserved for them because
they have the first 100mA as of right the instant they are plugged
in - they don't have to negotiate for that.

To relate this to your observations, things like mice and flash
drives are by their very nature low current devices, and can work
with that initial 100mA with room to spare.  Hard drives OTOH need
a lot of current to work.  When plugged in they will request the
right to draw the additional power and will be politely told where
to go.  The result is they won't spin up and will appear not to
work.  In reality they have be told not to work simply because the
power isn't there to drive them.

IMHO whoever designed this hub wants stringing up.  Unpowered hubs
do have their place - they are good for laptop users who may need
a hub and don't want to carry around an additional power supply.
However, this is not the case inside a desktop PC where there is
power already available.  A self powered hub that has its own
power supply and can deliver the full 500mA to each socket is a
much better proposition here.  But to save a few pennies someone
has gone the unpowered route and you are basically out of luck
short of replacing the hub with a more capable unit.

-- 
Andrew Smallshaw
andrews@sdf.lonestar.org