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Serial ATA technology promises to replace today's parallel technology for connecting internal computer drives. Summarily, This equates to the following improvements:

  • More bandwidth (exceeding 3GB/s).
  • Longer and thinner connection cables.
  • Lower power consumption.
  • Backwards compatibility with parallel technology.

The following is a general overview of the Serial ATA standard obtained at serialata.org.

Serial ATA: The Next Generation Internal Storage Interconnect

Provides Long-Term Solution for Higher Performance and Easier, More Flexible System Design

A group of computer industry leaders have developed a next generation ATA specification that is intended to provide scalable performance for the next decade. Serial ATA is designed to be 100% software compatible with today’s ATA, but has a much lower pin count, enabling thinner, more flexible cables. APT, Dell, IBM, Intel, Maxtor, and Seagate are jointly leading this initiative, with broad industry support from the 74 companies that make up the Serial ATA Working Group.

Last year at the Intel Developer Forum (IDF) Spring 2000, the formation of the Serial ATA Working Group was announced. Since then, tremendous progress has been made in the development of the technology. The final Serial ATA 1.0 specification was released on August 29, 2001 at the IDF Fall 2001, with an initial speed of 1.5Gb/s (Gigabits per second). Product development is in full swing, with several industry vendors currently targeting their products to be available in the first half of 2002. Also, several public Serial ATA demonstrations have been shown at recent events such as the Intel Developer Forum. It is anticipated that Serial ATA will allow the performance of internal storage devices to continue to increase for generations to come.

Overview of the ATA Interconnect

Originally introduced in the 1980s, the parallel ATA interconnect has been the dominant internal storage interconnect for desktop and mobile computers. It has been used to connect storage devices such as hard drives, DVD and CD drives, and others to the motherboard. Parallel ATA’s relative simplicity, high performance, and low cost has enabled it to attain the cost/performance ratio that is essential in the mainstream desktop and portable PC market.

Parallel ATA’s longevity can be attributed to frequent improvements in the interconnect’s speed and overall performance, allowing it to stay ahead of the performance needs of the industry’s fastest hard drives. For example, ATA’s data transfer rate has increased steadily from less than 3 Megabytes per second (MB/s) to its current maximum burst data transfer rate of up to 100MB/s. Other evolutionary improvements have helped it keep up with overall system requirements, including enhanced data modes such as Ultra DMA, and ATAPI for DVD and CD drive support.

However, parallel ATA has a number of limitations that are exhausting its ability to continue increasing performance. Some of these limitations are as follows:

  • 5-volt signaling requirement: In the near future, integrated circuits manufactured on the leading manufacturing processes will not be able to efficiently support 5-volt signaling voltages.
  • High pin count: Parallel ATA, with its 26 signals, requires a 40-pin connector and uses an unwieldy 80-pin ribbon cable to route inside the chassis. This high pin count is problematic for chip design and makes it difficult to route traces on a motherboard. The wide ribbon cable impedes airflow in the chassis, making thermal design more difficult. These issues become especially acute in notebooks and small form factor desktops, servers, and networked storage.
Serial ATA: The Long-Term Solution

Serial ATA is intended to replace today’s parallel ATA. It is designed to address many of the limitations of parallel ATA, while maintaining 100% software compatibility. This will significantly ease the transition to Serial ATA, as no changes in today’s drivers and operating systems should be required. In fact, demonstrations have already shown prototype hardware running on several Windows and Linux operating systems.

Serial ATA enables the industry to move to the lower voltage and lower pin count required for efficient integration in future chipsets and other integrated silicon components. The lower pin count also benefits the system design by making it easier to route traces around a motherboard. It enables a thinner, more flexible cable that improves airflow and therefore facilitates the development of better thermal design and smaller form factor systems. In addition, the new connector is more reliable, which should improve the end user upgrade experience.

Parallel ATA cannot scale to support several more speed doublings, and is nearing its performance capacity. By contrast, Serial ATA’s roadmap starts at 1.5 gigabits per second (equivalent to a data rate of 150 MB/s) and migrates to 3.0 gigabits per second (300 MB/s), then to 6.0 gigabits per second (600 MB/s). This roadmap supports up to 10 years of storage evolution, based on historical trends.

Parallel ATA disk drives are limited by their signal and power connectors to cable-attached applications. The Serial ATA connector, which can be blind-mated and hot-plugged, enables Serial ATA disk drives to be used in both cable and backplane attached applications. This capability is expected to accelerate the acceptance in enterprise market segments.

As with parallel ATA, Serial ATA is expected to be integrated into industry chipsets. Once these chipsets reach high volume, it is expected that Serial ATA will be about the same cost as parallel ATA is today at a system level. Because of this, Serial ATA is expected to eventually completely replace parallel ATA, which is already ubiquitous in PCs today. Also like today’s ATA, Serial ATA will satisfy the storage interface needs of desktop and mobile PCs, as well as entry servers and networked storage solutions. Over time, Serial ATA is expected to be deployed in enterprise segments that parallel ATA has not reached before.

Other Alternatives?

Serial ATA is the most suitable technology to continue the success of parallel ATA into the future. Just like today’s ATA, it is dedicated to internal storage and its protocol has been optimized to provide the highest performance at the lowest cost. With integration into industry chipsets, it can achieve a similar cost structure as today’s ATA, which is an absolute requirement especially for low-cost desktop PCs. Also, its 100% software compatibility with today’s ATA will ease the industry transition to Serial ATA. Other high-speed serial interconnects such as USB 2.0 and 1394 are useful for external storage connections, but as more general-purpose interconnects they do not achieve the performance levels nor relative simplicity that Serial ATA can provide.

Summary

The Serial ATA specification now provides a stable platform for product development and deployment. Serial ATA delivers the best long-term storage interface solution, addressing the shortcomings of parallel ATA while delivering scalable performance to support an interface roadmap spanning at least 10 years. It enables the industry to move to the lower signaling voltages and reduced pin counts required for efficient integration in future integrated silicon components. At the same time, it dramatically improves the cable and connector plant, improving both manufacturability and ease of use. The first products offering Serial ATA are expected in the marketplace in the first half of 2002.

For more information, see the Serial ATA Working Group web site at www.serialata.org.

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