Family pressures were brought to bear on me yesterday and I had to go to
Costco. So, to do my part to keep America and China strong I bought a
SanDisk 2 gigabyte flash drive.

The specs on this gizmo claim to need a "high power USB hub port". So I
looked briefly at Dell dot com but couldn't learn if my Precision M-60 or my
Inspiron 1705 has such a port.

Can you please help me? Point me to a reference on the Dell site or on the
USB.org site? Or explain that the wording "hub port" means I need an
external hub to run this gizmo? (I'll go look at sandisk.com as well).

Thanks in advance!

Al

Al Gillis wrote:
http://support.dell.com/support/syst...2&~ subcat=87

Both machines are new enough that you should have USB 2.0

The terminology is so non-standard that I don't think it's possible to
determine what it means. The most likely meaning is that it's going to
want close to 500ma, which is officially the "limit" for a USB port.

Frankly, I'd just try it where you intend to use it (try it on a port
that is directly on the motherboard if at all possible). If it doesn't
work, and you need to use it with that computer, then try it with a
powered hub.

Al Gillis wrote:

Al Gillis wrote:
It needs a port that is capable of providing the full 500mA. Any USB
port on the computer will be fine as each USB port can provide a minimum
of 500mA. What you probably can't do is plug it into an unpowered hub
when there are other devices on the hub that also require quite a bit of
power. There are also some PDAs which can be a USB host that probably
can't source 500mA.

Some HP machines have a special USB connector that can provide 1.5A and
it's used for external CD-ROM/DVD drives. You can see a photo of one at
"http://nordicgroup.us/chargers/" click on "Explanation of USB Current
Limits".

While the USB specification guarantees only 500mA per USB port, this is
a minimum, the port doesn't shut down at 501ma! In most cases a USB port
will supply 750-1000mA before the over-current protection circuit shuts
it down, especially if not all USB ports on the system are used (there
are no guarantees, so don't run out and design a device that draws
1000mA). For example, the National Semiconductor LM3544, a widely used
over-current protector, limits the current per port to 1.0A (typical)
even though the total current limit for four ports is 2.0 amps (to
protect the power supply). What this means is that if you don't use all
the USB ports, the ones you do use will be able to supply more than
500mA of current. This is why external notebook hard disk drives will
usually work without using the external power adapter, or Y adapter (two
USB ports power the device), even though they draw more than 500mA.
Obviously you shouldn't try to connect four external DVD±R/W drives to
one computer, without using the external power supplies for the drives.
While you shouldn't design a USB peripheral that exceeds the guaranteed
500mA, the reality is that the limit is often exceeded without any
consequences.

Re: "It needs a port that is capable of providing the full 500mA. Any
USB port on the computer will be fine as each USB port can provide a
minimum of 500mA."

That's not always true on laptops, unfortunately.

SMS wrote:

"SMS" <scharf.steven@geemail.com> wrote in message
news:46d61b1b$0$27165$742ec2ed@news.sonic.net...
That is not actually quite correct. The USB spec requires a port to supply
a minimum current of 100 mA. Anything larger is entirely by negotiation
with the host and will only be available if the power supply can support it
once the other loads are accounted for. In general terms, this usually
means that USB ports on desktop machines can deliver 500 mA. Also ports on
self powered hubs can generally deliver 500 mA. However, ports on non
powered hubs are usually limited to 100 mA. Some laptops also limit the
current on their ports to 100 mA.

Also note that according to the USB spec., 500 mA is specified as a maximum
not a minimum. Although most ports will, in practice, deliver a bit more
than 500 mA, they shouldn't deliver much more before the overcurrent
protection kicks in. Any port designed to deliver more than 500 mA (and I
know that do exist) are strictly non-compliant.

In news:46d66e4e$1_1@glkas0286.greenlnk.net,
M.I.5¾ typed:
I've seen some USB powered 2½ inch external hard drives that actually
come with two USB plugs. The extra one taps into extra 5VDC so it
doesn't overload using a single USB port.

--
Bill

BillW50 wrote:
Yes, this is quite common. They tell you to use two if necessary.

However if you look at how the USB power circuit is designed on
notebooks, you'll see why this is rarely necessary. Four USB ports will
share a single power controller. The power controller will limit total
current through the four USB ports to 2.0A (500mA per port). However
each individual port won't cut off with over-current protection at
501mA, in reality they'll cut off somewhere closer to 1.0A. Of course
you can't use all four of them at 1.0A because the total current is
limited to 2.0A. Since most USB devices are low-current devices or are
devices that are not powered by the bus, you're usually just fine with a
single power plug on the higher current drives.

SMS <scharf.steven@geemail.com> writes:

However, this probably won't work with desktop USB ports. The USB2
PCI cards I've looked at have one "fuse" per port. That's probably
cheaper than a power controller, since regulated +5V is already
available from the bus connector. But it means if you want to draw more
current than the fuse rating, you'll need to use 2 ports. (They aren't
really fuses; they are self-resetting polyfuse things).

I'd expect motherboard USB ports to be similar, but I haven't looked.

Dave