Universal Serial Bus, USB, is a serial bus standard to interface devices to a host computer. The host has one or more USB master connectors.
USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug-and-play capabilities by allowing hot swapping, that is, by allowing devices to be connected and disconnected without rebooting the computer or turning off the device. 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 replace many legacy varieties of serial ports and parallel ports. USB can connect computer peripherals such as eBook Readers, mice, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives (memory). For many of those devices USB has become the standard connection method. USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs and video game consoles, and as a bridging power cord between a device and an AC adapter plugged into a wall plug for charging purposes. As of 2008, there are about 2 billion USB devices in the world.
 Integrated USB
Some devices are small enough that they simply have a USB connector on one side and the device itself seems to be an extension of the connector. These are usually such items as a Memory flash drive, or even a small memory adapter that will hold an SD card or micro-SD card. It could also be a wireless adapter such as Wi-Fi, Bluetooth, or even a wireless usb adapter to communicate with a second wireless USB adapter.
Some memory USB flash drives are intended as portable application carriers and may have built in operating system support such as U-3 devices. While these USB flash drives are often identified as memory they are really a solid state disk drive with a file system. The file system is the same as is found on an SD card.
 Master Slave
Unlike earlier serial ports available on computers the USB serial port is implemented as a master slave. What this means is that the implementation of a USB port on your device is specific to one type of connection. A given port will either be a master port or a slave port. This has ramifications as to what you can talk via a USB port. A master port only talks to slave ports and slave ports can only talk to master ports but not to each other. This prevents, for example, a PDA having a slave port to sync to a computer being able to add a keyboard using the same port.
The master slave limitation that has caused problems for many people and so the designers of USB have developed a new port called a USB-On the GO or USB-OTG. An on to go port is special in that it can be switched to be either a maser or a slave as needed to talk to another device. However there are limitations in this ability. For example a typical master port can talk to a wide range of devices and the device manufacturers are very interested in providing drivers for master devices so that their devices can talk to the master device. An USB-OTG master typically has a very small subset of drivers that are included and a user should not expect that this specific list will be expanded.
Two USB-OTG devices talking to each other can actually switch roles if need be. The master always initiates the connection so this can be important. USB-OTG devices a usually small portable units like PDAs and do not have sufficient power to provide much power to run external devices. Typically no more that 100 mA will be available.
Some Android devices have support for USB-OTG.
There have been a few hardware adapters that are designed to allow a slave to talk to another slave device, for example a GPS to a PDA. The idea is the hardware in the adapter is a master that can translate the data from the GPS to the PDA. One problem is that a slave device supplies no power so the adapter has to have external power ability.
There is also an adapter that can plug two masters together. It includes drivers that must be installed on both master computers and is designed to permit any disk transfers between the devices.
Hubs are designed to provide expansion capability for Computers with a limited number of USB ports. Hubs require specific implementation support and all devices may not work through a hub. In addition a USB-OTG device will lose the ability to switch device types. A hub might still be useful to provide external power to a device that needs it but there are no guarantees that this will work.
The initial implementation of USB provided a "full speed" capability of 12 Megabits/second. They also provided a lower speed mode (1.5 Mbits/s) for very lower power devices like a keyboard or mouse. Version 1.0 was quickly replaced with version 1.1.
Version 2 of the USB specification added the ability for a "Hi Speed" mode that could achieve 480 Megabits/second. This permitted disk drives and other high speed devices to be attached via USB. This change was backwards compatible and devices could still attach to USB 1.1 ports with slower performance.
Version 3 of the USB specification added a "SuperSpeed" mode that can achieve 5 Gigabits/second. The connector was changed such that an older device can use the new USB port (running at slower speeds) but a device designed for USB 3.0 cannot be plugged into an older USB port. Some devices, in particular, disk drives have been designed to use a USB 3.0 to USB 2.0 connector to provide connectivity to both standards. These are typically used to provide backups or exchange data from different computers.
Version 3.1 has doubled the speed of 3.0 to 10 Gigabits/second and is called "SuperSpeed+". A new connector is also available called USB-C. It is a smaller reversible 24 pin connector that can be used on both the host and device side. Adapters will be required for legacy devices. USB-C will provide additional power output of 1.5A and 3.0A at 5V to power remote devices. The thin connector will allow even thinner devices. This cable and protocol will support MHL as well.
 Cable connectors
The Computer (Master) end of the USB connector has been standardize to one size (Type A). However, there are several connectors available for the device end of the USB capable device. There are devices with custom connects (mostly PDAs) but most devices use one of the defined USB connectors. As devices continue to get smaller and thinner the connectors have gotten smaller as well.
- Mini-USB plugs are 6.8mm (+0.04/-0.05) wide by 3mm (+0.04/-0.05) high.
- Micro-USB plugs are 6.85mm (+0.02/-0.06) wide by 1.8mm (+0.02/-0.06) high.
- USB-C connectors are 8.25mm wide by 2.4mm high.
The USB-C 3.1 standard also supports analog audio and a cable adapter can be used to enable standard 3.5mm connectors on headphones to plug in. Cables that include microphones can be used. There is even a passive cable arrangement that can supply 500 mA power charge through to appropriate devices. The Type-C connector can be rotated, solving an age old problem. The same connector can also be at both ends of the cable.
USB-C also supports Thunderbolt 3 which is a new hardware transfer standard that can support 40 Gbps for short (1/2 meter) cables and 20 Gbps for longer cables. Thunderbolt capable devices can use USB-C connectors but USB devices will not work as Thunderbolt devices without the Thunderbolt hardware.
 USB Power
The USB master connector is often used to provide power to a device or can also be used to charge the battery on a device. Generally speaking a USB master port can supply up to 500 mA to an attached device. There are even 5V power bricks that use a USB connector to supply power to a device. Many of these power supplies can supply more that 500mA which can result in faster battery charging. Sometimes a high power charger can supply as much as 2A (10 watts). Recently the USB-IF group has defined a power specification for power use. This specification is available from USB.org.
The USB-C connector now has a standard power of up to 900mA plus a 1.5A and 3.0A capability. The USB Power Delivery (USB-PD) standard enables the devices to draw up to 100W power over USB Type-C port.
See also: Battery charging
 Wireless USB
The future of USB may be wireless USB (wikipedia). Today a search of this will turn up adapters to provide Wi-Fi service via USB. But a search of WUSB may turn up a few devices. The idea of WUSB is to replace a standard USB cable with a wireless connection. This would mean an eBook reader, for example, would not require a cable to connect to a computer. The technology of WUSB is UWB, a Ultra Wide Band radio transmission. Today this ability requires a unique driver on the computer end and AC power to the device on the receiving end. The end unit could be or could plug into a multi-port device that would allow multiple devices to connect. The hardware consists of a pair of unique devices to make the wireless connection. The range is similar to Bluetooth but the transfer rate is higher.