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USB Cables

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The maximum length of a standard USB cable is 5.0 meters (16.4 ft). The primary reason for this limit is the maximum allowed round-trip delay of about 1500 ns. If a USB device does not answer to host commands within the allowed time, the host considers the command to be lost. When adding up the USB device response time, delays from using the maximum number of hubs, and delays from the connecting cables, the maximum acceptable delay per cable turns out to be 26 ns. The USB 2.0 specification requires cable delay to be less than 5.2 ns per meter (which is close to the maximum achievable speed for standard copper cable).This allows for a 5 meter cable.
                                    
            
Maximum useful distance
USB1.1 maximum cable length is 3m. USB2.0 maximum cable length is 5m. Maximum hubs connected in series is 5. Maximum devices connected in series is 127.
Although a single cable is limited to 5 meters, the USB2.0 specification permits up to five USB hubs in a long chain of cables and hubs. This allows for a maximum distance of 30 meters between host and device, using six 5-meter cables and five hubs. In actual use, since some USB devices have built-in cables for connecting to the hub, the maximum achievable distance is 25 meters + the length of the device's cable.
USB 3.0 does not define cable assembly lengths, except that it can be of any length as long as it meets all the requirements defined in the specification. However, electronicdesign.com estimated cables will be limited to 3 m at top speed.
Since USB provides power for devices connected to the bus, a special type of USB extender cable was created, consisting of a miniature one-port USB hub molded onto one end of a 5-meter cable. These mini-hubs are fully self-contained within the cable, requiring no separate bulky hub device and no external power. They are as simple to use as plugging cables together, with each hub drawing power through all the previous single-port hubs in the chain. Because bus power is limited, the most practical arrangement consists of four single-port hub extender cables, one plain 5 meter cable and, at the very end, a powered multiport hub to support multiple USB devices.

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