Lowering USB Transfer Rate?

[M. Warner Losh]

In message: <44B2FB8A.7010104 at rcn.com>
            Gary Corcoran <gcorcoran at rcn.com> writes:
: M. Warner Losh wrote:
: > In message: <200607101326.17709.frank at barda.agala.net>
: >             "Frank J. Beckmann" <frank at barda.agala.net> writes:
: > : there is no difference in cables between USB 1.1 and 2.0. Only special low 
: > : speed cable for low speed devices may be different.
: > 
: > Actually, there *IS* a difference between 1.1 and 2.0 cables.  The
: > plugs on the ends are the same, however.  The problem is that you can
: > get data dropouts using 1.1 cables to get 2.0 speeds.
:
: Ummm - I thought one of the design goals of USB 2.0, which Intel
: *knew* they were going to do while USB 1.x was coming out, was to be
: able to use _exactly_ the same cables.  Which is why they specified
: such good, braided full shielding cables, were required for USB 1.x.
: And why they didn't change the connector - because the (unknowing)
: consumer then wouldn't have to worry about whether they had the
: "right" cable.  If they really were intended to be different,
: wouldn't they have changed the connector?

The cables are different.  2.0 certified cables definitely are
required to work at 2.0 speeds.  I've had problems with certain
designs where 2.0 cables worked, and 1.1 didn't.  It is an absolute
fact.  I don't have my copies of usb 2.0 standard handy, but there's a
section in it that addresses this issue.  That's why cables for a
while came out with 2.0 certified on them.

: Of course as Frank alluded to, there may have been some unscrupulous
: manufacturers making el-cheapo cables, which they knew they could
: get away with for low speed USB 1.x devices.  Is that what you were
: referring to, Warner?  Or what (specifically) are you talking about?

When 1.0 came out, the specs were very loose. In anticipation of 2.0,
the standards were tightened (maybe in time for 1.1), and new cables
started to appear on the market.  But since 1.1 cables were cheaper to
make, they were cheaper in the marketplace and persisted until 2.0
devices became ubiquitous.  I have several of these cables, and they
are utterly useless with 2.0 devices.

: But as far as I know, general, well shielded USB cables, from reputable
: manufacturers, should be usable for USB 2.0, even if they were manufactured
: during the USB 1.x days.  It's easiest to tell with the clear jacket cables
: showing you the full braided shielding...  ;-)

Well shielded usb cables are likely certified 2.0.  The older cables
aren't shielded, and are the ones to which I'm referring to.  They
definitely do not work at usb 2.0 speeds.

Warner