Mailinglist Archive: opensuse (982 mails)

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Re: [opensuse] write several usb flash keys at once
On 09/06/2014 09:41 PM, jdebert wrote:
On Sun, 07 Sep 2014 00:37:02 +0200
"Carlos E. R." <robin.listas@xxxxxxxxxxxxxx> wrote:

On 2014-09-06 21:46, Cristian Rodríguez wrote:
But, considering that a flash stick writes much slower than what what
the USB can send, connecting a dozen sticks and writing to all of them
simultaneously is actually faster ;-)


Multiple devices can be written to/read from simultaneously on a serial
bus? Is that possible or even legal with the USB protocol?

Obviously not 'simultaneously' in the exact meaning of the word, but
simultaneously in the same sense that an un-switched Ethernet backbone
allows many devices to communicate 'simultaneously'; the same way a cell
phone tower allows many phone users to talk 'simultaneously', the same
way your wifi router allows many devices to communicate 'simultaneously'.


Reads & writes consistently
appear to be occurring one at a time. There's only one physical data
path and that happens to also be the command path.

Just like .... oh never mind.



How do you configure
a USB system to read/write multiple devices simultaneously?

Perhaps there is a broadcast mode just like with TCP/IP.
I don't know.

However, at
http://www.totalphase.com/support/articles/200349256-USB-Background
I read
<quote>
A USB 2.0 host broadcasts information to all the devices below it.
Low-speed and high-speed enabled devices will only see traffic at their
respective speeds. Full-speed devices can see both their speed and
low-speed traffic.

USB 2.0 works through a unidirectional broadcast system. When a host
sends a packet, all downstream devices will see that traffic. If the
host wishes to communicate with a specific device, it must include the
address of the device in the token packet.
</quote>




How do you
avoid collisions?

See above.
It seems to me that a host sending packets 'downstream' avoids
collisions in the same way that a machine gun sending bullets
'downstream' manages to avoid collisions between its bullets.
Sometimes, though, the packets go faster than bullets.
Sort of like Superman, eh?




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