Inside a USB cable: There are two wires for power -- +5 volts (red) and ground (brown) -- and a twisted pair (yellow and blue) of wires to carry the data. The cable is also shielded.

USB Features

The Universal Serial Bus has the following features:

  • The computer acts as the host.
  • Up to 127 devices can connect to the host, either directly or by way of USB hubs.
  • Individual USB cables can run as long as 5 meters; with hubs, devices can be up to 30 meters (six cables' worth) away from the host.
  • With USB 2.0,the bus has a maximum data rate of 480 megabits per second (10 times the speed of USB 1.0).
  • A USB 2.0 cable has two wires for power (+5 volts and ground) and a twisted pair of wires to carry the data. The USB 3.0 standard adds four more wires for data transmission. While USB 2.0 can only send data in one direction at a time (downstream or upstream), USB 3.0 can transmit data in both directions simultaneously.
  • On the power wires, the computer can supply up to 500 milliamps of power at 5 volts. A USB 3.0 cable can supply up to 900 milliamps of power.
  • Low-power devices (such as mice) can draw their power directly from the bus. High-power devices (such as printers) have their own power supplies and draw minimal power from the bus. Hubs can have their own power supplies to provide power to devices connected to the hub.
  • USB devices are hot-swappable, meaning you can plug them into the bus and unplug them any time. A USB 3.0 cable is compatible with USB 2.0 ports -- you won't get the same data transfer speed as with a USB 3.0 port but data and power will still transfer through the cable.
  • Many USB devices can be put to sleep by the host computer when the computer enters a power-saving mode.

The devices connected to a USB port rely on the cable to carry power and data.