Marcelo Puhl <PICLIST@mitvma.mit.edu> wrote:

> Is there a way to DIY [an active USB extension] cable?

    Not easily, no.  The active extension cables actually
    incorporate single-port USB hub chips; they're not just
    electrical amplifiers.  This means, by the way, that active
    cables with full-speed-only (12 Mbps) hub chips will limit the
    communication between a high-speed (480 Mbps) host and a
    high-speed (480 Mbps) device to only 12 Mbps... So if you buy one
    of those active cables, make sure it's fast enough for your
    needs.

    The 5-meter maximum USB cable length was chosen to allow
    reflections from the far end of the cable to settle between
    bits; if a longer cable is used, reflections won't be damped as
    well, and the line voltage could rise high enough to cause
    physical damage to the drivers.

    The 5-hub limit on serially-connected hubs is a consequence of
    the USB spec for maximum turnaround delay between an outgoing
    packet and the incoming response.  That spec allows for 70 ns of
    propagation delay through each cable/hub combination, and 30 ns
    through each cable alone; 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.

    There ARE very long-range USB links available, but they're more
    complicated than what we're discussing here; they consist of two
    USB chips -- one that looks like a device (peripheral) and one
    that looks like a host -- connected by ethernet or whatever.

    -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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