USB portable drive weirdness

[Kevin Cozens]

William Park wrote:
> On Thu, Aug 09, 2007 at 04:40:01PM -0400, Peter King wrote:
> Even with Y-cable, you have to plug into "powered" USB ports.  500mA is
> the max, but only for initial surge.  I believe steady-state max is
> about half of that (can't remember).

Wikipedia has some information about the available power from USB ports at 
http://en.wikipedia.org/wiki/Usb#Power. Quoting from that page:

"Initially, a device is only allowed to draw 100 mA. It may request more 
current from the upstream device in units of 100 mA up to a maximum of 500 mA.

If a bus-powered hub is used, the devices downstream may only use a total of 
four units — 400 mA — of current. This limits compliant bus-powered hubs to 4 
ports. The host operating system typically keeps track of the power 
requirements of the USB network and may warn the computer's operator when a 
given segment requires more power than is available."

Bottom line is you can be certain to have 100mA available for a device. More 
than that is likely to be available. 500mA is likely to be pushing the limits 
of what some computers will let you have.

The number of USB ports might have something to do with how much power each 
port is allowed to have. My old PII computer which had only two USB 1 ports 
could charge my PDA via its sync cable after a few hours. My new "state of the 
art" Core 2 Duo machine with its 6 (or is it 8?) ports takes longer to charge 
the PDA when using just the sync cable.

-- 
Cheers!

Kevin.

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Owner of Elecraft K2 #2172      |"Same thing we always do, Pinkutus:
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