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Serial ATA

Over the last decade and a half, engineers have used several tricks to extend the life of the ATA interface, such as altering timing and developing new wiring systems. And they've succeeded beyond anything that could have been expected at the introduction of the interface. Today's ATA is potentially 25 times faster than the original. Hard disk drives have yet to exploit the full potential of its latest improvements. But disk developers know they won't be able to stretch ATA out another 25 times. Nor do they want to. The design has several weaknesses (not all of which are technical).

Looking to the future, on the 15th of February, 2000, Intel, along with several drive-makers including APT, Dell, IBM, Maxtor, and Seagate Technology, announced a radically new conception of ATA that they called Serial ATA. Through an industry association called the Serial ATA Working Group, they published a final specification on August 29, 2001. Although the group expects getting the official imprimatur of a standards organization is still a year off, several companies are already manufacturing to the standard. Seagate demonstrated the first Serial ATA drive in June 2002.

The name indicates the basic concept. Today's ATA is a parallel interface that uses 16 separate data connections. Serial ATA will use a single data channel operating at a much higher speed—at least 20 times the clock rate of current parallel ATA systems.

The initial specification for Serial ATA calls for an initial peak transfer rate of 150MBps, a little more than a 10-percent improvement over the current top speed for conventional parallel ATA, 133MBps. But parallel ATA has no future, according to the Serial ATA Working Group. It's unlikely that the speed of the parallel interface could be doubled. The Serial ATA specification, on the other hand, provides a growth path to double and quadruple that initial rate to peak rates of 300MBps and 600MBps. According to the Serial ATA Working Group, that speed potential should be enough to satisfy speed needs for the next decade.

To serialize data on the Serial ATA channel, the system uses conventional 8b/10b encoding (that is, it encodes bytes in ten-bit data groups). To reduce interference levels, it also uses a combination of spread spectrum clock and data scrambling to minimize repetitive bit-patterns in the data stream. The group-coding overhead (ten bits to the byte) results in a 1.5GHz signaling rate on the Serial ATA channel. The higher-speed versions of Serial ATA will push the signaling rate to 3.0 and then 6.0GHz.

The wiring uses four pairs of wires for two balanced signaling pairs—one to the device and one from the device—for full-duplex communications. The total voltage swing between the wires in each pair is only 250 millivolts.

Serial ATA uses a point-to-point connections scheme. That is, only one device connects to each Serial ATA port. Drives will not daisy-chain as they do with today's parallel ATA systems. To accommodate multiple devices, a host computer needs multiple ports. The typical installation will follow current ATA practice and deliver four ports per computer system—two primary and two secondary ports.

For computer-makers, one of the chief allures of the Serial ATA system is the smaller cable that it used to connect the drive to the computer. Instead of a wide 40- or 80-conductor ribbon cable, Serial ATA will likely use a four-conductor cable with compact connectors, similar to those of USB and FireWire. Such a design will eliminate some of the clutter inside computers. Beside aesthetics, the compact cable will also improve the cooling of the host computer and will likely reduce interference.

Serial ATA retains the ATA designation despite the radical wiring change because it retains full software compatibility with the ATA system. Drives will use the same commands as parallel ATA. Consequently, Serial ATA will work with today's operating systems and software.

On the other hand, Serial ATA will be incompatible with today's parallel ATA drives and controllers at the hardware level. You simply won't be able to plug one directly into the other, although manufacturers may produce adapters to mate one to the other.

The Serial ATA Working Group maintains a Web site at www.serialata.org.

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