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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
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- 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.
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- 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.
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- Connectors and Cables
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- 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!
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
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