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Universal Serial Bus (USB)
The conventional serial ports used on PCs have been around since the earliest PCs; they have changed a little, but not much. While functional, they have serious limitations in terms of expandability, software support and performance. In the mid-1990s, a consortium of PC and telecommunications industry giants--including Compaq, IBM, Intel, Microsoft, NEC and Nortel Networks (then Northern Telecom) developed a new interface standard for attaching relatively low-speed devices to the PC. This interface is called the Universal Serial Bus or USB. While USB support started being supported in many PCs many years ago, it has only recently started to gain widespread acceptance. All new PCs are now equipped with USB support, and USB ports can also be added to many older systems as well.
The USB interface is specifically designed to allow easy connection of a wide variety of devices; it is intended to be user-friendly and truly "plug and play". On a system equipped with USB, one can "hot swap" devices, meaning they can be plugged into the system or removed without needing to power the system down or doing anything to it before the change is made. Up to 127 devices are supported, and multiple devices can easily be added to a single PC by chaining them together using hubs. The USB connection is of course serial, and the current version (1.1) runs with a maximum throughput of 12 Mbits/second (1.5 Mbytes/second), which is shared by all devices. There is also a slower-speed, 187.5 kbytes/second mode available for very slow devices, such as keyboards.
USB's initial acceptance was relatively slow in coming, but once it got going, the variety of USB devices that appeared on the market surprised many people, probably including its developers. The flexibility and expandability of the interface, and the ease with which devices could be attached, made it very attractive to many users. Many of the devices that were made available in the USB interface were never intended for USB when it was first created. Most storage devices would certainly fall into this category.
Today, one can get a wide variety of storage units that use the USB interface. This includes floppy disk drives, hard disks and optical drives. They work on the USB interface, and certainly there is no issue with floppy disk drives for example. The problem is that the maximum throughput of the interface is 1.5 MB/second--and that's a theoretical maximum, meaning the reality is somewhat lower. This is fine for mice, modems and scanners, but it's very slow for hard disks, considering that modern drives can have throughput level 20 to 30 times above that number. Even something like a 40X CD-ROM drive has a maximum throughput far above 1.5 MB/second. Of course, USB was never envisioned as an interface for high-speed devices like hard disks, so its designers can't be blamed for these limitations.
USB remains popular for hard disks and other storage devices for those who don't have other options; however, in almost every case a higher-speed interface is a better idea if that's at all possible. Otherwise, you will seriously restrict the performance of the device. This is fine for some tasks, such as occasional backup, but far from ideal. Responding to the faster devices that many companies began developing for USB, USB 2.0 was developed, with similar attributes but much faster performance. IEEE-1394 is another high-speed serial interface that competes for the same areas of the market.
Note: For more information
on USB, see the USB Implementers Forum site.