Robert Rolf <PICLIST@mitvma.mit.edu> wrote:

> the absurd thing is that the LOW SPEED USB devices have a much
> SHORTER cable limit than the high speed ones? Any rational
> explanation for that would be appreciated.

    Reading the USB spec (available for free at http://www.usb.org)
    would explain it.  See Chapter 7.

    Paraphrased from the spec:

    Low-speed USB signalling has slow rise and fall times (no less
    than 75 ns and no more than 300 ns under specific test
    conditions).  The 3-meter maximum cable length is to avoid
    transmission-line effects:  Limiting the cable length to 3
    meters (18 ns propogation delay) ensures that reflections will
    appear during the first half of the rise or fall, which allows
    the cable to be approximated by a lumped capacitance.

> It take it that the USB designers never heard of 'terminations' to
> prevent reflections? Or 'transient clamp diodes'?

    Yeah, I'm sure they never heard of any of that.  Sounds too
    complicated for those dumb USB guys to understand.

> > The sum of the propagation delays through 5 hubs and 6 cables
> > (and back), plus the delay allowed in the device itself, beats
> > the full-speed timeout spec by less than half a nanosecond.
> > Adding even a few inches of cable to a full-length serial string
> > of USB cables and hubs would violate that spec.
>
> Would it not help to use a 'low capacitance' cable, which has a
> higher propagation velocity?

    The USB spec is already written and allows 30 ns for the cable
    propagation delay.  If you have a cable that meets all the other
    requirements and has a propagation velocity closer to the speed
    of light than what the USB spec assumes, you can make a cable
    longer than 5 meters.  If you can exceed the speed of light,
    that'd be even better.

> I've read of a videomaker in Montreal who runs firewire links to
> his camera that are WAY over spec (150' he claims) by using Cat5
> wire. I've been meaning to do this with my firewire camera, but
> have not been able to find a Male-Female firewire extension cable I
> could hack up.

    Firewire's significantly different from USB; there's no
    necessary correlation between what works for one and what'll
    work for the other.

> > There are more-exotic USB extenders that can give you miles of
> > range, but they're expensive... [they communicate] using
> > something other than USB.
>
> So what's the point of using USB then?

    The point is mostly contained in the phrase "but they're
    expensive".  There's a LOT more, but cost is a big-enough deal
    that further explanation is usually unnecessary.

> How long before we see ethernet mice/keyboards and cameras?

    Ethernet cameras?  There are already some; they have an
    absolutely insignificant share of the webcam market.

    You'll never see ethernet mice and keyboards.

> Looks to me like a market opportunity, particularly for cameras. If
> I can get a 100BT card for under $10, putting the chip IN the
> camera can't be THAT expensive.

    Keep studying the problem; you'll see why USB is better.

    -Andy

=== Andrew Warren -- aiw@cypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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