SCSI vs. IDE

SCSI dates from a time when a small computer was roughly the size of your father's Oldsmobile. Its adaptation to the personal computer market has been slowed by complexity, independent standards development, and relatively low demand for its special features. Only on the Mac has SCSI been a great success, mainly because Apple rigorously controls every aspect of the development of peripherals for the Mac. The SCSI experience on PCs has been quite the opposite, with different vendors implementing their own version of SCSI communication.

SCSI is a separate bus structure rather than a connector like IDE, so it offers many features that IDE cannot. First, SCSI is extremely flexible in the type and timing of the devices attached. Everything from disks and other storage media to scanners and printers can be attached to the same SCSI host adapter. Up to seven devices can be connected to each adapter.

In addition, the SCSI connector cable can be up to 6 meters long, which means that peripherals can reside outside the host computer system. In fact, SCSI is the only interface used with large drive arrays or optical jukeboxes that support network servers. In contrast, enhanced IDE offers a maximum cable length of 18 inches, which means that it is a strictly internal solution. SCSI also offers speed that standard IDE can't match, and Fast/Wide SCSI offers sustained data rates (20 MBps) that even enhanced IDE can't achieve.

But there is a cost, literally. From host adapters to peripherals, SCSI is more expensive than IDE. Part of the reason for this is the amount of logic required on both ends of the cable. All devices on the SCSI bus are intelligent; each has its own ROM that contains the basic operating parameters of the device. The host adapter is also an intelligent device.

But the bigger cost is that incurred by the development of software to support all the possible combinations of hardware, software, and operating systems that a peripheral is likely to encounter in the diverse and, at times, chaotic world of the PC. Rich Rutledge, marketing manager of storage products for Western Digital, explained it this way: "SCSI has a single command set (per peripheral), but at the chip level, each product has a different register set, requiring a different hardware device driver. The result is that everyone has to produce different drivers for each and every operating-system type and release."

Western Digital believes that with the emergence of enhanced IDE, SCSI will be largely constrained to the server market and external systems. Of course, SCSI proponents see things differently. Adaptec, which developed the EZ-SCSI software that has taken much of the anguish out of implementing SCSI on a PC, believes that ATAPI (AT Attachment Packet Interface) and SCSI combined will squeeze proprietary interfaces out of the CD-ROM market. The company also sees SCSI's perfor mance advantages and flexibility becoming increasingly important as multimedia applications come to the fore.

Both viewpoints have merit. Enhanced IDE will be the interface of choice where ease of use and value are imperative and will take a good deal of midrange business away from SCSI. Where performance is the determinant, however--and performance is a big determinant in multimedia--SCSI retains advantages that should ensure its position as the interface of choice for high-end desktops and workstations.