Osobne jsem se setkal s pasivnimi prodluzkami max. 5 m. Cokoli nad uz byly jen aktivni za pomerne dost penez (1000+ Kc). S aktivni prodluzkou lze dosahnout az 25 m. Nikdy jsem ale nezkousel treba spojit dve pasivni prodluzky co by to udelalo.

Norma pripousti max. 5m pro USB 1.1 a max. 3m pro USB 2.0 (resp. pro jeho max. rychlost). Pomoci aktivniho prodlouzeni lze dosahnout az 25 m.

Vice treba na oficialni strance USB.ORG...

Q3: Why can't I use a cable longer than 3 or 5m?
A3: USB's electrical design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment, the range limitations were deemed acceptable. If you're familiar with transmission line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ.