Categories: PCs and Hardware

By Neil Randall

Other hardware interfaces come and go, but SCSI (small computer system interface) seems to go on forever. Like the more familiar IDE (integrated drive electronics) and EIDE (Enhanced IDE), SCSI (pronounced "scuzzy") provides a standard means for computers to exchange data among hardware devices such as disk drives and CD-ROM drives.

Some PCs ship with SCSI interfaces built onto the motherboard. In most cases, however, you have to install a SCSI host adapter card. These devices are readily available from computer stores and are no more difficult to install than any other peripheral card. When you boot with a fully installed SCSI card, you'll see a set of new boot-up messages indicating that the BIOS for the SCSI interface is starting. The SCSI BIOS is separate from your computer's BIOS and allows the newly installed SCSI bus to exchange data with the CPU and other SCSI devices.

SCSI's most obvious strength is the number of devices it can control. Where IDE interfaces were restricted to two disk drives and today's EIDE interfaces are restricted to four devices (including hard disks and CD-ROM drives), a single SCSI controller can handle up to eight devices (including the host adapter card, which counts as a device). Furthermore, the devices can vary from hard disks and CD-ROM drives to compact disk-recordable units, scanners, printers, optical drives, and more. High-end servers frequently use a SCSI interface exclusively, in large part because of its easy expandability. But the exclusivity is unnecessary. IDE/EIDE and SCSI interfaces can be freely combined inside a PC, offering expanded possibilities for hardware configuration. In PCs, the SCSI host adapter takes up a hardware interrupt (IRQ), but the devices attached to the card do not, which significantly increases expandability. In fact, it's possible to add a second SCSI card for 7 additional devices. Better still, a "twin-channel" SCSI card takes up only one IRQ and handles 15 peripheral devices.

Next: SCSI Flavors

Published as Tutor in the 3/10/98 issue of PC Magazine.