A single USB host is capable of connecting 127 devices, either directly or through USB Hubs. Individual USB cables can run as long as 5 meters; with hubs and devices it can go up to 30 meters in length. A USB cable has two wires for power (+5 volts and ground) and a twisted pair of wires to carry the data. On the power wires, the computer can supply up to 500 milliamps of power at 5 volts.
Devices such as Mouse which need low-power can draw their power directly from the bus. High-power devices such as Printers have their own power supplies and hence they draw minimal power from the bus. Hubs too can have their own power supplies to provide power to devices connected to them. The devices that are connected to a USB port depend on the USB cable to carry power and data.
The USB standard uses “A” and “B” connectors to avoid confusion.
- “A” connectors head “upstream” toward the computer.
- “B” connectors head “downstream” and connect to individual devices.
By using different connectors on the upstream and downstream end, makes it impossible to ever get confused. Example, if you connect any USB cable's “B” connector into a device, you know that it will work. Similarly, you can plug any “A” connector into any “A” socket and be sure that it will work too.
USB is also faster than ports on older technologies such as serial and parallel ports.
Transfer Rates:
USB 1.1: Up to 12Mbps
USB 2.0: Up to 480 Mbps
In recent years USB has become a widely-used cross-platform interface for both PCs and Macs.
