[SI-LIST] USB full-speed (*not* high-speed) layout and stubs.

  • From: "Andrew Morley" <andrew@xxxxxxxxxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 03 Sep 2007 08:49:36 -0000

I can find plenty of information about PCB/system layout for
high-speed USB.  That's to be expected at 480Mb/s where things really
are critical.  However I haven't found much on USB full-speed layout
(i.e. 12mb/s).

What I'd like to do is have a standard USB 'B' receptacle
in parallel with a proprietary multi-way connector.  They will be
mechanically arranged so that it will be impossible to plug both in. 
(If the user did manage to plug into both at the same time, all bets
are off!)

The standard USB 'B' must, of course, be certifiable as a USB
full-speed device port, i.e. the effect of the stub running to the
proprietary connector must be small enough so that USB compliance is
not impacted.  

The proprietary connector will carry USB when the standard connector
isn't used.  As it is not "standard" it merely has to work, not to
necessarily pass compliance testing, although the latter is obviously
highly desirable.

Oddly, I haven't found a reason why this won't work, provided I don't
have the two connectors too far apart, and that I have separate series
termination resistors for each connector.

However, I'm still somewhat uneasy about this.  Has anyone ever tried
this an either made it work or found it not to work?

I am aware that there are devices to support switching of the ports,
but you know how BOM pressures are.  But a higher BOM is better than a
product that won't meet USB compliance.  Thoughts?


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