The first PCs shipped in 1981 used serial ports and parallel ports to connect external peripherals. Although the RS-232 serial and Centronics parallel technologies had improved gradually over the years, by the mid-90s those technologies had reached their limits. In terms of connectivity to external devices, the PC of 1995 differed very little from the PC of 1981; the ports were a bit faster, perhaps, but they were fundamentally similar.
In the interim, the bandwidth needs of external peripherals had increased greatly. Character-mode dot-matrix and daisy-wheel printers had given way to graphic-mode page printers. Modems were pushing the throughput limitations of RS-232. Also, it was obvious that emerging categories of external peripherals such as digital cameras, CD writers, tape drives, and other external storage devices would require much more bandwidth than standard serial or parallel connections could provide. Neither was bandwidth the only limitation. Serial and parallel ports have the following drawbacks for connecting external peripherals:
What PCs really needed was a fast bus-based scheme that allowed multiple devices to be daisy-chained together from a single port on the PC. SCSI had the potential to fulfill this need, but its high cost and complexity made it a non-starter for that purpose. The PC industry had long been aware of this need, but it was not until 1996 that they finally began to address it. Their solution was called Universal Serial Bus (USB).
USB is aptly named. It is universal because every modern PC or motherboard includes USB and because USB allows you to connect almost any type of peripheral, including modems, printers, speakers, keyboards, scanners, mice, joysticks, external drives, and digital cameras. It is serial in that it uses serial communication protocols on a single data pair. It is a logical bus (although the physical topology is a tiered star) that allows up to 127 devices to be daisy-chained on a single pair of conductors.
One convenient way to think about USB is as an outside-the-box Plug-N-Play bus. All connected USB devices are managed by the USB Host Controller Interface (HCI) in the PC, and all devices share the IRQ assigned to that HCI. Devices can (in theory, at least) be plugged or unplugged without rebooting the computer.
Although nearly all PCs and motherboards made since 1997 have USB ports, for a long time those ports were nearly useless, for three reasons:
Despite these problems, by mid-2000 USB had achieved critical mass. With Windows 98/SE/Me and Windows 2000 available and USB peripherals shipping in volume, USB transitioned from a developing standard with great potential into a real-world solution, albeit a flawed one. USB is now in the process of replacing most of the legacy connectors that clutter the back of recent PCs.
Legacy-reduced motherboards that began shipping in 2000 replaced or supplemented serial and parallel ports with additional USB ports usually four rather than the previously standard two. Legacy-free motherboards provide nothing but USB ports for connecting external peripherals (other than perhaps video), and are usually equipped with six USB ports four at the rear and two on the front panel. Most inexpensive printers and scanners now have a USB interface many have only a USB interface as do most digital cameras.
Despite its slow start and the nagging problems that continue to plague it, USB really is the wave of the future. The following sections tell you what you need to know about USB.