Lengde på USB kabel...

[George]

Jeg har et webkamera av type USB som skal stå et stykke unna data'n... jeg ser at det selges "skjøteledning" til USB hos bl.a. Komplett, med lengder på opptil 5 meter.

 

Mitt spørsmål er da; Er det noen grenser for hvor lang en slik kabel kan være før signaltapet blir for stort ?? Eller er det bare å skjøte ivei ? :smile:

 

Har også sett noen butikker som selger USB med repeater forsterker (husker ikke i farta hvor det var)...

[tk-428]

Her er det bare å finne frem ordboka:

 

 

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.

 

Kopiert fra USB.org

[dudsi]

USB (har ikke sjekket for USB 2.0)

 

12Mbps (STP) max. 5m (uten forsterker)

1,5Mbps (UTP) max. 3m (uten forsterker)