Re: USB device just moves
- From: phil-news-nospam@xxxxxxxx
- Date: 11 Feb 2008 22:53:21 GMT
On Mon, 11 Feb 2008 10:38:13 -0800 (PST) Mike <michael.h.williamson@xxxxxxxxx> wrote:
| On Feb 10, 1:59 pm, phil-news-nos...@xxxxxxxx wrote:
|
|> This is a good example of how that legacy unix principle of assigning device
|> names in probe order is still a bad idea.
|
| But, if I understand correctly, is it not supposed to be possible to
| chain USB devices together somehow, using only one computer USB port?
USB hubs exist. Plug a hub into a computer USB port. You get multiple USB
ports. I have one that has 3 USB ports plus 4 memory card slots in the same
device. I've plugged external drives and USB keys in all three at the same
time. Of course that is now sharing the bandwidth of the one connection to
the computer. Otherwise I can get full speed on each USB port my computer
has (4 in back, 2 in front) concurrently (tested with the 4 in back at the
same time).
When this kind of "fan out" is done, there should be a _physical_ address
that also fans out, with a new range of numbers appended to the address of
the hub itself (maybe starting at .1 so the hub itself can be accessed at
the .0 subaddress). Maybe USB is doing this kind of thing behind the scenes.
But there is no indication of that at the OS level. It seems USB addresses
are dyanmically assigned. So I would guess the computer has no idea where
a USB device is plugged in, beyond the ports it has on itself, and either
the messages are broadcast over hubs, or the hub is smart enough to track
the dynamic addresses and send things the right way.
I'm sure someone will say I should read the USB standards docs. But I do
not want the mental pollution. I'd rather design my own.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-11-1645@xxxxxxxx |
|------------------------------------/-------------------------------------|
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