On Wed, 23 Apr 2014 20:55:22 +0300, Levas wrote:

> o ka sako specifikacijos apie ilgi? Galima ir kilometro padaryti, ar
> veiks jis?

Pasyvus neveiks, kiek zinau taimingai leidzia max 5m.

Seip http://www.usb.org/developers/usbfaq#cab4

Cables and Long-Haul Solutions

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.


3.   What about using USB signal repeaters to make a cable longer than 5 
meters?

A:   Don't bother. The best solution is self-powered hub with a fixed 10m 
cable that had a one-port bus powered hub in the middle. The maximum 
range will still have to deal with the timeout, so any out of spec 
tweaking of the terminations between the two hubs and the timing budget 
still won't yield more than 5cm of extra distance. A better solution is 
described in the following question.


4.   I really need to put a USB device more than 30 meters away from my 
PC. What should I do?

A:   Build a USB bridge that acts as a USB device on one side and has a 
USB host controller at the other end. Use a long-haul signaling protocol 
like Ethernet or RS-485 in the middle. Using cables or short-haul fiber, 
you can get ranges upwards of a kilometer, though there's no reason why 
the long-haul link in the middle of the bridge couldn't be a pair of 
radio transceivers or satellite modems.
Embedded host solutions capable of doing this already exist. Also, two 
PCs connected via USB Ethernet adapters are essentially a slave/slave 
version of this master/slave bridge.


-- 
kthxbai.