[mythtv-users] OT: USB inductance (or other) problems with external peripherals

Raymond Wagner raymond at wagnerrp.com
Tue Oct 15 20:04:21 UTC 2013


On 10/15/2013 3:29 PM, Stephen P. Villano wrote:
> On 10/15/13 10:45 AM, Raymond Wagner wrote:
>> On 10/15/2013 10:25 AM, Stephen P. Villano wrote:
>>> It isn't always impedance causing a problem, especially with USB. USB
>>> uses a lower frequency than gigabit ethernet.
>>> But, one can suffer from ringing on long runs, secondary to
>>> inductance in the line.
>>> That is why many long length USB cables come with built in Schmitt
>>> triggers as buffers.
>> Define "long runs", as you're not allowed to have a USB cable longer
>> than 16'.  The only ethernet runs I have that short are the ones in my
>> basement, to devices mounted on the same wall as my switch.
> I have 16 footers, they're all active cables with buffer circuits
> (pretty much Schmitt triggers) to eliminate ringing caused by the
> inductance of the long cable.
> Needless to say, 16 feet is a long run for USB.
> That said, there are longer active cables available.
> http://www.amazon.com/PTC-Meters-Extension-Repeater-Supports/dp/B0040IASMS
> is one example. 20 meters worth of example.
> If it didn't have the buffer circuit, it'd be useless.

In that case, though, it's not really a 20 meter cable.  It's 
effectively four independent cables, with attached hubs, daisy-chained 
together.  The signal is terminated at each repeater.


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