The Universal Serial Bus, or USB, is an interfacing standard used for connecting peripheral devices to computers through a serial bus.  Initially designed for use on personal computers, it soon became a common feature in video gaming consoles, multimedia players, cellphones, portable memory devices, etc. because of its ease of use and reliable performance.

The development of the USB was aimed at minimizing, if not eradicating, the lack of adequate standardization among the various serial and parallel interfacing standards used in older PC's.

A USB system has a host controller that can control a group of devices that are daisy-chained together.  A USB system can be expanded by adding more USB hubs to the chain, which in turn can also accommodate additional hubs.  A USB system with a tree structure can thus be formed, although only up to five levels of branching per USB controller is allowed.

The daisy-chaining of devices in a single USB system did not become popular among users, since the connection of additional hubs to the daisy chain to accommodate more devices is an economic and technical obstacle for the user.  Instead, what became common practice is to equip PC's with many USB ports so that the user can attach a device directly to each USB port without having to worry about daisy chaining.

The ready-to-use nature of USB ports in PC's is one reason why it became popular.  Another reason is the ability of a USB port to handle 'hot-swapping', which is the plugging or unplugging of a device from the port while the computer is running.  Thus, there is no need to power down a PC to connect or disconnect a device.  Furthermore, the host immediately recognizes a newly plugged device and loads the device driver that it needs so that it can be used by the PC.  USB devices are therefore true 'plug and play' devices.

    

Figure 1.  USB ports and a USB plug

            

When a device or hub is connected to a USB system, the host controller gives it a unique 7-bit address on the bus.  The controller orchestrates the functioning of the devices on its serial bus by polling them in round-robin fashion.  A device on the bus is not allowed to transfer data unless the host controller requests for a transfer.

  

USB is now so widely used that it is not just seen in mice, keyboards, gamepads, printers, scanners, cameras, and networking peripherals, but in table lamps and electric fans as well!  Unfortunately, USB still can't be used in computer monitors since they can not handle the large data rates required by computer display systems.

        

   

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