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Appeared on: Saturday, January 26, 2002
Serial ATA


1. Introduction

Serial ATA - Page 1

Source: Maxtor

- Introduction

is the next generation personal computer (PC) storage interface. It will replace the Ultra ATA/100 interface used to connect most PCs to their primary storage, which is projected to become a bottleneck within two years. This paper describes the primary benefit of the Serial ATA interface, the increase in data rate. Other features and benefits are also outlined, together with a comparison to alternative storage interfaces.

- The Need for Change

The Advanced Technology Attachment (ATA) interface, previously called Integrated Drive Electronics (IDE), has existed in substantially the same form since 1989, and has become the highest-volume disk drive interface in production. Quantum has led continuous improvements to parallel ATA that extended its data transfer rate from 3.3 Megabytes per second (MB/s) to 100 MB/s, with only one cable change. As PC processor performance has increased, so have the read/write data rates of hard disk drive (HDD) heads and media. This disk rate is projected to exceed today's 100 MB/s interface bandwidth by 2003. Parallel ATA has kept pace in the past, but is nearing its limit, becoming a performance bottleneck. Serial ATA will eliminate this bottleneck by initially offering 150 MB/s and in the future it will provide significant headroom for future improvements.


2. Comparison

Serial ATA - Page 2

Source: Maxtor

- Desktop Storage Interfaces

- ATA Device Connectivity

Parallel ATA allows up to two devices to be connected to a single port using a master / slave communication technique. Both devices are daisy-chained together via one ribbon cable that is an unterminated multidrop bus.

The standard parallel ATA software and device driver access the Serial ATA subsystem in exactly the same manner as parallel ATA and will function correctly. For Serial ATA however, the software views the two devices as if they were masters on two separate ports. The drive interface section of the host adapter uses a new design that converts the normal operations of the software into a serial data / control stream. The Serial ATA structure connects each of the two drives with individual cables in a point-to-point fashion.


3. Benefits

Serial ATA - Page 3

Source: Maxtor

- Benefits of Serial ATA

Serial ATA is a high-speed serial link replacement for the parallel ATA attachment of primary internal storage devices. The table below lists some of the features and benefits of Serial ATA.


4. Connectors/Cables

Serial ATA - Page 4

Source: Maxtor

- Connectors and Cables



5. Developement

Serial ATA - Page 5

Source: Maxtor

- Development of Serial ATA

Seven key promoters have worked together to develop Serial ATA: APT, Dell, IBM, Intel, Maxtor, Quantum and Seagate. Of these, Intel has been the leading player on the host side, with Quantum taking a key role on the device side. All of the promoters are recognized leaders in their respective areas. They are collaborating to leverage their experience with previous interface implementations to ensure that Serial ATA is successfully adopted by the computer industry.

A major thrust of the development is to create a cost-effective solution for primary storage. An example of how Quantum has led this development occurred in October 2000, with Quantum?s proposed change in the physical layer communication protocol that will provide a projected overall cost-saving to end-users of $80 million in the year 2003.

The original specification called for a transmit and receive frequency tolerance of 150 ppm achievable only by using crystal oscillators costing around $0.50 each. The host already uses these oscillators. Some devices make use of cost-effective ceramic resonators costing around $0.10 each. The tolerance of these parts, however, is as large as 6,000 ppm.

The problem for the ceramic resonator is that the specification requires the device to be the first to transmit critical data at a precise clock frequency. The host uses that data to determine the transfer speed, which will enable the support of future generation products. Quantum recognized the opportunity to provide a lower-cost total solution by redesigning the protocol so that initially the host also provides a constant frequency clock signal, but without data content. The device could then use a ceramic resonator source and phase-lock-loop circuitry to synchronize with and track the host-generated signal. That more accurate source is used to generate the serial clock from the device back to the host.

This Quantum solution has been incorporated into the specification and will enable the Serial ATA system to be more cost effective. In place of the need for two crystal oscillators in a host/device system, only one is called for and the second clock source can be a ceramic resonator, saving $0.40 per system. At an estimate of more than 200 million systems in 2003, that adds up to over $80 million in industry savings!

- Future

Parallel ATA has succeeded as the primary storage interface for the past 10 years. A team of leaders in the PC and storage industries has recognized that this interface is now approaching its limit. This team has leveraged their experience to develop the storage interface for the next 10 years. The Serial ATA interface is optimized for internal primary storage and provides the capability for future enhancements. Serial ATA is designed for low cost, with ease of adoption in mind.

Projected next steps for the Serial ATA program are shown below. It is expected that drives and PC motherboards incorporating Serial ATA will be available in 2002.



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