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

Sam Couter sam at couter.id.au
Thu Oct 21 14:20:27 MDT 2010

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
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