Re: High Power USB hub port...

Al Gillis wrote:
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!

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.
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