[Linuxtrent] Re: Prolunga USB e chiavetta wifi
- From: Piro <lachocalad@xxxxxxxxx>
- To: linuxtrent@xxxxxxxxxxxxx
- Date: Tue, 26 Jan 2010 14:56:36 +0100
Sono sicuramente fuori specifiche e usarlo è anche rischioso (fonte
http://www.usb.org/developers/usbfaq/#cab1) :
1. Why are there cable length limits, and what are they?
A: The cable length was limited by a cable delay spec of 26ns to allow
for reflections to settle at the transmitter before the next bit was sent.
Since USB uses source termination and voltage-mode drivers, this has to be
the case, otherwise reflections can pile up and* blow the driver*. This does
not mean the line voltage has fully settled by the end of the bit; with
worst-case undertermination. However, there's been enough damping by the end
of the bit that the reflection amplitude has been reduced to manageable
levels. The low speed cable length was limited to 18ns to keep transmission
line effects from impacting low speed signals.
2. I want to build a cable longer than 5 meters, why won't this work?
A: Even if you violated the spec, it literally wouldn't get you very far.
Assuming worst-case delay times, a full speed device at the bottom of 5 hubs
and cables has a timeout margin of 280ps. Reducing this margin to 0ps would
only give you an extra 5cm, which is hardly worth the trouble.
Eccoti la soluzione finale:
http://rawlab.mindcreations.com/imgs/laptop_cantenna.jpg
:p
Il giorno 26 gennaio 2010 14.27, Emanuele Olivetti
<emanuele@xxxxxxxxxxxxxx>ha scritto:
>
>
> Puo' essere quindi che i produttori di hw non rispettino gli standard?
> Questo arriva a 12m:
> http://www.usbfirewire.com/Parts/rr-usb2-ext-40ft-wht.html
> Wikipedia accenna a "low resistance usb cable" che possono raggiungere
> distanze maggiori delle specifiche.
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