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A Bus Tour of Computer Roadways

June 10, 1996|RICHARD O'REILLY

Q I would like to know what SCSI stands for. Also, what is a bus? (My notebook has an expandable bus, but I am not sure what it's for.)

--Albert Kim, via the Internet

A These issues are related, but let's take the second one first, because it's the more general of the two. A bus is an electrical (and electronic) pathway inside the computer. It's not like a mass-transit bus because it doesn't move. It's more like the roadway that a mass-transit bus travels on.

Just as with roadways, there are faster computer buses and slower ones. To carry the analogy a little further, most computers these days have at least two kinds of buses inside and maybe more. They are known by acronyms such as ISA, EISA, MCA, VESA, VL, PCI and SCSI.

SCSI (pronounced "scuzzy") is one kind of special-purpose bus that is installed in all Macintoshes and some PCs, usually higher-performance PCs. The acronym stands for small computer system interface. It began life as a proprietary way to connect a hard drive to a computer and eventually became a standard of sorts. It is used to connect various kinds of drives--hard, removable-media, tape, CD-ROM--and also scanners. Each of those devices has to be designed specifically for SCSI use. Today, most SCSI devices adhere to a standard called SCSI-2.

If your computer has a SCSI bus, also called an interface, it can be used to connect both internal devices, such as a hard drive and CD-ROM, and external devices, such as tape backup and removable-media drives--up to a total of seven devices on a single SCSI bus. It is even possible to have more than one SCSI bus, such as in a file server, with a multiple hard drive array on one SCSI bus and tape drive and CD-ROM drive on another.

A major competitor to SCSI is something called Enhanced IDE (integrated drive electronics), which can be built into the computer's system board for less money, can control up to four various kinds of drives and offer close to the same performance.

As for your notebook computer's expandable bus, it is a connector on the back of the computer into which an accessory device from your computer's manufacturer can be plugged. Typically, you can then plug other devices--such as tape drives, scanners, external hard drives, CD-ROM drives and, often, computer network cables--into that expansion device. It can give your portable virtually as much expandability as a desktop computer.

Richard O'Reilly. The Times' director of computer analysis, will answer questions of broad interest in this column. E-mail questions to, fax to (213) 237-4712, or mail to Answers c/o Richard O'Reilly, Business Editorial, Los Angeles Times, Times Mirror Square, Los Angeles, CA 90053.

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