SCSI (pronounced "scuzzy") stands for Small Computer Systems Interface. It’s a type of connection from a computer to a piece of accessory equipment, or peripheral, such as a scanner, disk drive, or tape drive.
IBM-compatible computers (PCs) come with two types of connection sockets, or ports, called serial and parallel, to connect external peripherals. In a serial connection data travels in a single-file stream, one bit at a time, while a parallel connection allows data to travel in a wider path 8 bits at a time, and is therefore faster. An external modem typically connects via a serial port, since it handles data in a sequential stream. Most printers connect to the parallel port, allowing a relatively fast transfer of data to be printed.
However, most PCs do not come with a built-in SCSI port, which transfers data much faster than either a serial or parallel connection. It’s the Apple Macintosh that has popularized the SCSI interface, and because of its superior transfer characteristics and flexibility, SCSI is finally arriving on the PC scene. An add-in adapter circuit board, or SCSI card, is necessary to allow your computer to connect to SCSI-based peripherals, which can be internal, like a disk drive, or external, like a scanner. Most internal hard drives are available in IDE or faster SCSI versions. And most flat-bed scanners use a SCSI connection and come with the necessary interface card for PC models.
External CD-ROM drives, which are popular for laptop and notebook computers, and removable cartridge drives, which use media with high capacities like hard drives but which can be removed and interchanged like floppy disks, often have SCSI connectors, and thus can be used with either Macs or properly-equipped PCs. Alternatively, if convenience is more important to you than maximum performance, some of these drives are available with parallel connectors for PCs without SCSI ports, saving you from the expense and hassle of buying and installing a SCSI card. You unplug your printer from the parallel port, plug in the drive, and replug the printer in an additional socket supplied by the peripheral or its connecting cable. Usually only one parallel-based peripheral can be added in this way.
Another advantage afforded by SCSI is the fact that up to seven SCSI peripherals can be daisy-chained together and hooked to the same SCSI port on a computer. In fact, in one of the places I worked my Mac was connected to an internal hard drive, an external hard drive, a scanner, two removable cartridge drives, and a tape backup drive -- all through one SCSI port!
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