[ale] USB A -> USB A Cable

[Mike Panetta]

Nope.  The hardware does not support it.  To start with there is a very specific bit of handshaking that goes on when a device is plugged in (at the hardware level) that allows the hub to know that a device was plugged in.  Then after that the USB driver software "enumerates" the newly installed device (basicly gives it an address of sorts).  You can't enumerate a host, because it does not need an address, as its supposed to be (by protocol design) the only host on the wire.  Also, USB devices only speek when spoken to (with one specific exception IIRC).   Basicly the USB protocoll is a unidirectional one in that the host controlls everything, so it does not lead itself to multi-master communication at all.

Oh, and at the hardware level there is really nothing to "cross over" as there are a differential data pair, power, and ground.  The data lines are both transmit and recieve (half duplex) so you do not need to connect a TX line to an RX line etc.  

Mike

-----Original Message-----
From: synco gibraldter <synco at xodarap.net>
Sent: Sep 21, 2003 3:41 PM
To: Atlanta Linux Enthusiasts <ale at ale.org>
Subject: Re: [ale] USB A -> USB A Cable

On 21 Sep 2003 at 15:37, Mike Panetta wrote:

> A hub won't do what he wants.  Assuming what he really wants is a USB
> A->A cable (which does not or may, but should not exist). The USB A
> port is the host port.  A female USB A port is on every computer and
> hub.  Hubs also have a USB B port (to go from the computer to the hub
> using a USB A->B cable), as hubs are devices.  USB only supports host
> to device connections (ie its not multi master like firewire) thats
> why there are 2 different types of connectors, its to force you to
> only be able to connect devices to hosts, and not devices to devices
> or hosts to hosts.  Now there are things that will allow you to
> connect USB hosts to other USB hosts, they are a kind of bridge (the
> "networking" thingie I mentioned) that shows up to each computer as a
> USB device with special properties.
> 
> Either way the USB spec does not allow USB host to host connections
> without some imtermediary device (bridge) in between.  That is why
> there are no USB A->A cables (or there should not be because they
> would serve no purpose other then to frustrate the user and possibly
> damage the hardware).
> 

would it be possible to create a makeshift cross-connect cable?  what you said holds 
true for ethernet as well but switching the position of two wires results in hub <--> 
hub or client <--> client connections.  i've never seen the specs for usb cabling... i 
don't even know how many wires there are, but with proper software, it seems like 
you could do that.

--    synco gibraldter
--    atlanta, ga
--    synco at xodarap.net
--    key id: 0xC5117E0A

_______________________________________________
Ale mailing list
Ale at ale.org
http://www.ale.org/mailman/listinfo/ale