[clug] A "mount" defeat - was Re: Persuading Debian to talk to an Android tablet via USB?

[David Cottrill]

The USB spec is 300mA without question, up to 1A on request.
If a device doesn't ask, or the port can't supply it (in the case of a hub
without an external power supply) then the USB port throws an error.
 On 22 Oct 2010 07:38, "Sam Couter" <sam at couter.id.au> wrote:
> Hal Ashburner <hal at ashburner.info> wrote:
>> On 21/Oct/10 10:37 PM, Alex Satrapa wrote:
>> >The theory is that a USB port controller is supposed to be able to
>> >negotiate with devices about whether to provide power, and if so,
>> >how much.
>
> There's really no negotiation going on. Devices demand power and don't
> always ask for it first. In the interests of consumers not complaining
> about USB stuff not working, hosts just supply whatever power they can
> without waiting for the device to ask.
>
>> Or more, eg the dual usb cable plugs that power 2.5" usb hard
>> drives, 3G modems and such. My understanding is modern usb ports
>> actually supply more than spec when requested meaning you only have
>> to plug into one port. My current laptop has no issue with one port,
>> my older IBM T42 required two ports for the same drive.
>
> Those dual plug cables don't even have the data wires connected to the
> second plug; they soak up as much power as they want and they're
> completely incapable of negotiating at all.
> --
> Sam Couter | mailto:sam at couter.id.au
> OpenPGP fingerprint: A46B 9BB5 3148 7BEA 1F05 5BD5 8530 03AE DE89 C75C