On Thu, 11 Nov 2004, omar kfitz wrote:
>
> Hi everyone,
> I actually want to know why we need special USB cables for Host to Host
> transfer files using USB (I actually own one those special cables)? .
> What aspects of that protocole restrict its use to a host to device
> (master -slave) and restain transfer file from a computer to another
> using a simple cable A to A?
The design of USB is such that all transfers are initiated by the host.
Also it's not set up to handle the collisions that would occur if more
than one device tried to transmit at the same time. And there's the fact
that slave devices attached to a USB bus must be configured by a host
before they can function. These and many other similar considerations
mean that USB won't work unless the bus has exactly one host.
> I'm a newbie in USB technologie, and to
> tell the truth I've been through the USB specfication not without
> problems. Actually I thought that maybe we could access the usb source
> systemfiles under Linux and modify them to get a dummy USB root acting
> as a slave and not a master so that we could get a master slave system
> with two computers. I'm almost sure it s impossible, but I need to know
> why in that case. Has anyone ever tried this and can tell me why it
> wouldn't work?
Most host controllers are not designed to operate as slaves, and it's
impossible to make them do so. As just one example, a slave is not
supposed to supply power to the Vbus line but a master is.
No one has ever tried it because there's no way to do it.
On the other hand, there is the USB OTG ("On The Go") specification, which
provides a way for a device to act as either a host or a slave. Two OTG
devices can be connected to each other using a symmetrical cable and they
will be able to communicate (and transfer files, for example). Almost no
Linux computers have OTG hardware, however, and standard hardware does not
support OTG.
Alan Stern
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