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Definition from Wikipedia

Universal Serial Bus (USB) is a serial bus standard to interface devices. USB was designed to allow peripherals to be connected using a single standardized interface socket and to improve plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.

USB is intended to help retire all legacy varieties of serial and parallel ports. USB can connect computer peripherals such as mouse devices, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives. For many of those devices USB has become the standard connection method. USB is also used extensively to connect non-networked printers; USB simplifies connecting several printers to one computer. The large volume of USB memory devices and their ease of use has created a security concern that is often overlooked. USB lock software can lock out memory devices and still allow other USB peripherals to function. The USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs, mobile phones and video game consoles.


The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries.

Classic USB

The Universal serial bus was developed as a specification for connecting computers to external peripherals. This requires no prerequisite knowledge of the hardware that is being used. The cabling used for USB consists of four wires, two for power (+5 volts and GND) and the two for data. The data lines (D+, D-) are differential pair and the data is encoded using NRZI which is bit stuffed to ensure good transitions in the data stream. A differential ‘1' is transmitted by pulling down D+ to 2.8 volts using a 15k resistor connected to ground and D- under 0.3 volts with a 15k resistor pulled up to 3.6 volts. A differential ‘0' is D- > 2.8 volts and D- < 0.3 with the same values for pull-ups and pull-downs. The Figure below illustrates this type of connection. The two types of terminating connectors used type ‘A' and type ‘B', type ‘A' connectors are generally used to connect upstream to the host, type ‘B' is used to connect downstream to the external hardware.
A major advantage when using USB is Power (Vbus) is bus powered devices, so called bus powered devices can draw their power from the usb bus without using any external power supplies or sockets. Careful considerations must be made according to the following criteria. The three classes of USB functions are:

• Low power bus powered funtions
• High power bus powered functions
• Self power functions

Low power bus powered fuctions cannot draw all of its power from Vbus and cannot sink more than 100mA. High power bus powered functions cannot draw more than 100mA until it has been configured and then is allowed to draw 500mA as long as it has asked for it in its descriptor. self power functions can draw 100mA but has to draw the rest of the power for its operation from an external power supply. The format used for data transfer in USB is asynchronous serial and the bus can support up to 127 devices connected to it. The maximum length of cable for a single device can be 16ft. The USB 2.0 protocol supports three different bit rates:

• low speed - (1.5 Mbps)
• full (12 Mbps)
• high speed (480 Mbps)

The four main methods of transferring data between a PC and the peripheral are Control transfer, Bulk-transfer, Interrupt-transfer, and Isochronous-transfer.

Control Transfer:

-Used to send high priority control signals to the target device.
-Transfers initialisation and low speed data with error correction.
-This type of transfer is supported by all USB devices.

Bulk Transfer:

-Used to transfer large amounts of data independently of time.
-Low priority.
-Applications include printers, disk drives.

Interrupt Transfer:

-This type of transfer is low speed for devices that need to send data quickly but periodically.

Isochronous Transfer:
-Large amounts of data at a defined rate and does not include error protection.

Today USB expands its coverage to USB-OTG, which has host/device dual role, and 2.4GHz Wireless USB, which has more flexibility, and USB3.0 which has 5.4Gbps fiber inside. Please check USB news for update information.