Winchester Interface Controversy Heats Up
MINI-MICRO SYSTEMS/June 1984 Pages 46, 49, 50 physical

Chris Bailey, Western Editor

Amid the clamor of claims and counterclaims, system OEMs and integrators are facing a difficult choice in determining which disk drive and controller manufacturers to believe when choosing a high performance, 51/4 -inch Winchester disk interface.

Seagate Technology, Scotts Valley, Calif., is touting its ST-412HP interface that ups the current ST-412's 5M-bit-per-second (bps) data-transfer rate to between 5M and 15M bps. Meanwhile, proponents of the l0M- to 25M-bps Enhanced Small Disk Interface (ESDI) are attracted by its on-drive data separator and disk/controller intelligence.

Most observers agree that OEMs and system integrators want a low-cost standard with a higher than 5M-bps data-transfer rate, but there is no consensus yet on which proposed interface will become the de-facto standard. In the meantime, system builders are moving ahead with planned new systems while waiting for a definitive standard to emerge.

Some support both
Seagate is proclaiming the ST-412HP, essentially a higher-speed ST-506/-412 interface, to be the most logical, economical and easily manufactured solution. But ESDI proponents believe ESDI is the only reliable l0M-bps interface suitable for 50M-, l00M- and more-than l00Mbyte drives.

For Skip Kilsdonk, director of marketing at ESDI proponent Maxtor Corp., San Jose, Calif., there is no controversy. "We did not define the ESDI; we merely sponsored the meetings that led to its creation. The ESDI in its present form is a result of the input of 35 to 40 companies including drive manufacturers, controller manufacturers, system integrators and even tape drive manufacturers. It was derived after looking at the system level problems and determining which features best met the needs of the ultimate end customer.'' Kilsdonk feels that the ST-412HP could find a place in low-end, medium-capacity drives with capacities of 50M bytes or less in the high-capacity, high-performance end of the market. But he believes the ESDI will be the choice because of its high data-transfer rate, intelligent drive/controller interface and superior data reliability.

Many drive and controller manufacturers claim they will eventually support both interfaces. Jim Adkisson, vice president of Vertex Peripherals Inc., San Jose, Calif., says Vertex will support the ST-412HP in the company's upcoming 84-byte drives [typo from article. Maybe 84 MB?] in the third quarter of 1984 and the ESDI in the first quarter of next year.

Taking a different point of view is Donald Redmond, vice president of engineering at Xebec Systems Inc., Sunnyvale, Calif. "The ST-412HP has not solved any of the problems of the ST-506 interface," he declares. "Primarily, the data reliability problems will intensify at the doubled data rate, and the specified driver/receivers at the data-bus level do not yet exist."



Integration causes problems
Redmond says the major problem of any interface is system integration, which caused Xebec customers the most problems with the ST-506. Integrating ST-506/-412 drives into small systems resulted in excessive soft-error rates. Redmond believes this problem is compounded by the fact that the data separator is not located on the drive itself. Since the ST-412HP calls for the data separator to be on the controller interface board, this• problem remains.

In contrast, the ESDI, first touted by Maxtor in early 1983 and now supported by Control Data Corp. (CDC), Micropolis Corp., Xebec, Memorex Corp., Fujitsu America Inc., OMTI and others, calls for placing the data separator electronics in the drive. Proponents contend that this architecture solves the soft-error rate problem by eliminating the major source of noise and timing problems. They also bill the ESDI as the higher performance interface because it supports data-transfer rates as high as 25M bps.

Xebec's Redmond disputes that figure: "The ST-506/-412 interface is already pushing transistor-to-transistor logic limits. No one will support the HP at more than 7M bps this year, and l0M bps and beyond is unlikely."

Seagate's Don Manneman, product manager for high-performance drives, points out that the ST-412HP is a logical evolution of the older interfaces. "The similarity of the 412HP to the existing ST-506/-412 standard should result in a lower cost as compared to the other alternatives," he states. He does not forsee problems with parts availability for the 412HP. Manneman expects the interface to first appear in high volumes in mid-1984 in the l00M-byte, half-height, 8-inch ST-8100 drive.

Manneman suggests that the 412HP can reduce overall system costs in multidrive systems due to the placement of the data separator. ''When you place the data separator logic in each drive, the end user or OEM pays that cost in each drive of a multidrive system. With the 412HP, that cost is paid only once- in the controller board."

Xebec's Philip Devin, director of marketing, disagrees. He says the cost of the data-separator circuitry is low and should not impact multidrive configurations. Further, the relaxation of phase-margin problems on the heads and media due to on-board data separation will help reduce the cost of the interface.

Xebec's Redmond disputes the lower-cost notion. "There has been so much finger-pointing at the 5M-bps data rate between drive, controller and system manufacturers that from a customer-support point of view it would be cost prohibitive for us to even consider manufacturing a 412HP product."

According to John Worden, manager of disk systems engineering at CDC, the• increase in the data window margins can provide drive manufacturers with more design leverage in the rest of the drive. "With today's ST-412 and the proposed ST-412HP, 30 percent to 50 percent of the theoretical data-detection window is allocated to the controller subsystem. This means that drive manufacturers must produce a drive to tighter tolerances in the head, media and positioning system. With the ESDI, drive manufacturers have control over the data separator, write precompensation circuitry and data recording code and thus 100 percent of the data-detection window. This gives them more options in designing the more costly portions of the drive. Reduced costs in the heads, media and positioning system should result in a less expensive drive."

CDC announced its ESDI support with the introduction of the Wren-2 family of 86M-, 174M- and 213Mbyte, 51/4-inch drives, which are expected to be in production in early 1985. Experience from using parallel production lines for modified frequency-modulation and nonreturn-to-zero (NRZ) models of the 36M-byte Wren-1 drives has led CDC to favor the NRZ, or ESDI, interface.

Larry Boucher, president of Adaptec Inc., Milpitas, Calif., takes a softer view. "Technical superiority is by no means the determining factor in winning the acceptance battle. Almost everyone agrees that the American National Standards Institute (ANSI) standard is the most well-thought-out proposal from a technical point of view, but it has failed in the marketplace." Early supporters of the ANSI standard have now adopted more mainstream interfaces.

A recent ANSI supporter to switch allegiances is Micropolis, Chatsworth, Calif. Vice president of marketing Chet Baffs cites a lack of demand from OEMs and system integrators and says, "ESDI meets the requirements for a higher performance interface. It's an economical and relatively easy migration from our ST-506/-412 interface.

One of the early members of the ESDI committee, Western Digital Corp., Irvine, Calif., is now supporting the ST-412HP. Explains Kathy Braun, director of marketing of storage-management products, "As the proposed ESDI standard became more complex and expensive, we began to lose interest. When Seagate and Tandon [Corp.] announced its support of the ST-412HP, we felt that the company's market presence would result in wider use of this interface." Adds Joseph Jaworski, director of strategic planning, '”We are driven by our ability to produce [very-large-scale integration (vlsi)] components, and to do this we need volume markets. With Seagate and Tandon behind the ST-412HP, we feel comfortable that there will be the volumes we need to support VLSI development." In addition, Western Digital's interface components are more adaptable to the ST-412HP than to the ESDI.