Q: I'm just starting to learn about SCSI, but I find all the terms terribly confusing. What's the difference between SCSI-1 and SCSI-2? What are Fast, Wide, and Ultra SCSI?

A: I'm not surprised that you're confused. The SCSI "standard" is so versatile and diverse that it can and does exist in many different variations (see the table "The Flavors of SCSI" on page 99).

For starters, SCSI stands for small computer systems interface. It began life in 1979 when Shugart Associates, one of the first PC hard drive makers, released its Shugart Associates systems interface (SASI) standard. The idea was to create a fast interface that could support multiple devices, from hard disks to scanners, off of a single controller card. The ANSI standards-setting body picked up on SASI and dubbed it SCSI-1. SCSI-1 devices were designed to work on PCs with an eight-bit bus, such as the original PC and XT; operate up to eight devices off of a single controller; and transfer data at up to 5MB per second.

However, ANSI didn't release the official SCSI-1 standard until 1986. By then, eager vendors had already released many variations on SCSI-1, resulting in all sorts of compatibility problems. A SCSI card from Vendor A didn't work with SCSI drives from Vendor B. So much for the "standard."

In an attempt to overcome SCSI-1's compatibility and speed problems, the SCSI-2 standard was released in 1994. SCSI-2 was backward compatible, which meant a SCSI-2 controller could run most of the SCSI-1 devices on the market, and SCSI-1 and SCSI-2 devices could coexist in the same daisy chain. There is no vanilla SCSI-2 standard but rather, variations based on the specification. First came Fast SCSI (also called Fast SCSI-2), which moved data across an eight-bit bus at speeds of up to 10MB per second. Then came Wide SCSI (or Wide SCSI-2), which supported 10MB-per-second speeds, could run up to 16 devices, and worked on a 16-bit bus. To accommodate this bigger data bus, Wide SCSI devices used a 68-pin cable instead of a 50-pin cable. Engineers then combined both variants into something called Fast Wide SCSI (or Fast Wide SCSI-2), which allows data transfers of up to 20MB per second on a 16-bit bus. As you might guess, the various SCSI-2 permutations aren't generally compatible with one another. In an attempt to keep up with faster hard drives, SCSI advancement and confusion have continued.

Although the forthcoming SCSI-3 standard hasn't been finalized, vendors are already selling SCSI devices and controllers that use SCSI-3 technology. Here are the variants. Ultra SCSI (aka Ultra SCSI-3 and Fast-20 SCSI) can send 20MB per second over an eight-bit bus. Wide Ultra SCSI (aka Wide Ultra SCSI-3 and Fast-20 SCSI) handles 40MB-per-second transfers via a 16-bit connection. As you can see from the accompanying table, the maximum cable length between adapter and device shrinks as throughput goes up.

In the near future, we'll see Ultra2 SCSI (aka Ultra2 SCSI-3 and Fast-40 SCSI), which can move data at 40MB per second on an eight-bit bus. A 16-bit version called Wide Ultra2 SCSI (aka Wide Ultra2 SCSI-3 and Wide Fast-40 SCSI) supposedly supports 80MB-per-second data transfers.

Keep in mind that SCSI has traditionally been a parallel bus. The eight or 16 bits of data are transferred at the same time across parallel data lines. SCSI-3 is proposing three new serial connection schemes that promise blazingly fast data transfers: Serial Storage Architecture (SSA), Fibre Channel, and IEEE P1394/FireWire.

You can learn more about SCSI from the SCSI FAQ page at www.cis.ohio-state.edu/hypertext/faq/usenet/scsi-faq/top.html.

CONNECTING CD-ROM AND DVD-ROM DRIVES

Q: I've been told that CD-ROM drives should be connected to the slower IDE port in your PC, and fast hard drives should be attached to the faster EIDE port. But what do I connect my 24x CD-ROM drive to? And where do DVD drives hook up?

A: The "x" speed rating for a CD-ROM drive refers to the speed at which the drive can grab data off the disc. A 24x CD-ROM drive retrieves data about 24 times faster than a floppy drive or at about 3.6MB per second. This speed isn't related to the type of interface the CD-ROM drive uses. If the CD-ROM drive has a standard ATA (also called IDE) interface, it should be connected to the slower, secondary IDE controller channel. If the CD-ROM specifically uses an ATA-2 (also called EIDE) interface, then it's capable of much faster data transfers and should be able to coexist with hard drives connected to the primary EIDE controller channel. Check the CD-ROM drive's spec sheet to determine which interface it uses. By comparison, DVD-ROM drives use the ATA-2 interface and can easily be connected as "slave" devices to a master EIDE hard drive on the primary EIDE controller channel.

© 1997 Stephen J. Bigelow. All rights reserved.
Stephen J. Bigelow is the author of "Troubleshooting, Maintaining, and Repairing Personal Computers: A Technician's Guide" and 12 other PC-related books from McGraw-Hill. Bigelow is also publisher of The PC Toolbox troubleshooting newsletter. Visit the newsletter site at www.dlspubs.com. Email questions to him directly at sbigelow@cerfnet.com.