insufficient power for Xcraft HD enclosure

[Bernd Walter]

On Sun, Jan 20, 2008 at 09:10:50PM +0100, Gary Jennejohn wrote:
> On Fri, 18 Jan 2008 21:21:52 -0800 (PST)
> KAYVEN  RIESE <kayve at sfsu.edu> wrote:
> 
> > it comes with a silly barebones manual that tells you to slide it in
> > and screw the screw or some such, not very helpful.  it also with a
> > USB connector that has one junction fitting the device coupled to
> > what i am calling a daisy chain of two USB connectors.
> > 
> 
> What you really mean is that you have a Y-cable. A Y-cable is used
> because USB only supports max. 500 mA per port.
> 
> What you're expected to do is plug both connectors of the Y-cable
> into your laptop and the other, single connector into the HD case.
> This should allow your laptop to provide enough juice to start the
> disk spinning without violating the USB specification.

It is still violating USB specification, because you can't draw
power from a port without telling the OS to do so.
You are limited to a few mA and if the port is enabled by the host
you can draw 100mA or 500mA depending what the host allows.
If the host didn't enable the port within a specified time you are
further limited to a few µA.
This means you can't even draw a single mA over a longer time without
beeing allowed by the host.
So your device officially needs to have two USB controllers for that
to be able to claim the power requirement on both ports, which they
likely didn't.
The Y-cable hack just works because most hardware don't enforce the
power usage, but a few do.

> If your laptop has only one USB port then you're SOL. The case seems
> to have no provision for attaching an external power supply.

And doesn't comply to USB specs.
But because of such Y-Cable violation being popular these days there are
power-supplies on the market with USB headers on output.
Anyway - it's better to get a cheap self powered USB hub, which
typically has 4 ports and can source enough current for your drives.
Normaly they don't do much about current limitation - most of them just
have a single fuse for all ports together.

-- 
B.Walter                http://www.bwct.de      http://www.fizon.de
bernd at bwct.de           info at bwct.de            support at fizon.de