USB portable drive weirdness

Kevin Cozens kevin-4dS5u2o1hCn3fQ9qLvQP4Q at public.gmane.org
Tue Aug 14 02:55:08 UTC 2007


phiscock-g851W1bGYuGnS0EtXVNi6w at public.gmane.org wrote:
>> I am not sure, but I think USB2 ports are supposed to handle more power
>> than USB1 ports.
>>
> As far as I know, all USB ports can supply 500mA. IBM were proposing a
> version that could supply more current, but it doesn't seem to be in use
> yet.

The way I interpreted the information about USB (from wikipedia at 
http://en.wikipedia.org/wiki/Usb#Power) a port will start out supplying a low 
current (no more than 100mA) and the USB device has to ask for more. It is up 
to the computer whether it wants to (or can) honour the request for higher 
current. If the USB device doesn't have the smarts in it to make a request for 
higher current, it will only be guaranteed 100mA. Getting more than that 
without negotiation would be a crap shoot.

The proposed USB PlusPower spec would make it a lot easier to be sure of 
getting higher currents and/or voltages.

-- 
Cheers!

Kevin.

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