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PC Hardware in a Nutshell, Second Edition by Barbara Fritchman Thompson, Robert Bruce Thompson

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Chapter 24. USB Communications

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:

Low bandwidth

Standard serial ports top out at 115 Kb/s, and parallel ports at 500 Kb/s to 2 Mb/s. Although these speeds are adequate for low-speed peripherals, they are unacceptably slow for high-speed peripherals.

Point-to-point connections

Standard serial and parallel ports dedicate a port to each device. Because there is a practical limit to the number of serial ports and parallel ports that can be installed ...

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