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Serial ATA

Serial ATA (S-ATA) is a computer bus primarily designed for transfer of data between a computer processor and hard disk. It has evolved from the legacy advanced technology attachment (ATA or IDE) standard. It has at least three main advantages over it predecessor, namely speed, cable management and it is hot swappable. It is probable that normal ATA will be renamed by back formation to parallel ATA so the two are not confused.

Initially Serial ATA was released at 150 megabytes per second but it is designed to scale up quite substantially from there. Serial ATA II will double throughput to 300MBps and then 600MBps is planned for around 2007. However at 150MBps it is still only 17MBps faster than the current (2003) fastest parallel ATA interface ATA/133. Parallel buses have difficulty in reaching ever higher speeds due to problems keeping all the data lines in sync. Serial ATA uses the newer LVDS for the signalling. Still, the need for such a high speed interface could be debated as hard disks are almost always a bandwidth bottleneck being mechanical devices.

Physically, the cables used are the largest change. The data is carried by a light flexible seven core wire with 8mm wide wafer connectors on each end. It can be anywhere up to one meter long. Compared to the short (18 inch, 45cm) ungainly 40 or 80 core ribbon cables of parallel ATA this will come as much relief to system builders. In addition, airflow and therefore cooling in equipment will be improved. The concept of a master/slave relationship between devices has been dropped. Serial ATA has only one device per cable. The connectors are keyed -- it should no longer be possible to install cable connectors upside down.

Native serial ATA hard disks also require a different power connector as part of the standard. It is wafer based but wider than the data cable so it should not be possible to confuse the two. Fifteen pins are used to supply three different voltages if necessary -- 3.3V, 5V and 12V. The same physical connections are used on 3.5 inch and 2.5 inch (notebook) hard disks.

In the transitional period between parallel and serial ATA various adapters are planned to convert one to the other. To perform the serial to parallel translation or vice versa a bridge is used. There is a noticeable performance penalty for such an arrangement however and tests conducted in early 2003 show throughput reduced around 30-50 percent. This restriction will only completely disappear when controllers and hard disks support Serial ATA natively.

A similar standard is planed for SCSI with Serial Attached SCSI[?] which is expected to be ratified in late 2003. It has a level of compatibility with Serial ATA.

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