Serial ATA - It's Time to Get in Line Page 4


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The Serial ATA Solution

As previously explained, Serial ATA has eliminated the limitations of the Parallel ATA interface. Serial ATA maintains register compatibility and software compatibility with Parallel ATA because its architecture changes the physical interface layer only. No device driver changes are necessary, and the Serial ATA architecture is transparent to the BIOS and the operating system.

Actual Benefits of Serial ATA

Serial ATA offers a number of benefits over Parallel ATA, including:

  • Improved performance — Serial ATA is faster than parallel ATA
  • Reductions in voltage and pin count
  • Smaller, easier-to-route cables; elimination of the cable-length limitation
  • Improved data robustness
  • Backward compatibility
  • Increased disc drive data rates
  • Serial ATA integration
  • Bundled costs

Serial ATA Is Faster

A few years ago, if someone would have said that "Serial ATA is fast," or that it was faster than a "parallel port," he or she would have gotten some strange looks. The COM port was never known for its speed. Let's not forget, however, that today's most important standards (USB 2.0, Firewire, Ethernet, V-Link, MuTIOL, HyperTransport, RapidIO) are all serial-based, yet they are fast and provide high performance.

Thanks to serial transfer, Serial ATA needs only two data channels — one for sending and one for receiving. These are supplied with a more modern 250 mV, in contrast to the 5 V typically used with IDE. With differential signaling, interference on one signal affects the other signal by the same amount. Because the signals run phase reversed, interference is self-canceling. Twisting the wires is no longer necessary.

Reduction in Voltage

Serial ATA's low-voltage requirement (500 millivolts [mV] peak-to-peak) effectively alleviates the increasingly difficult-to-accommodate 5-volt signaling requirement. This requirement hampers the current Parallel ATA interface.


The Serial ATA architecture replaces the wide Parallel ATA ribbon cable with a thin, flexible cable that can be up to 1 meter in length. The serial cable is smaller and easier to route inside the chassis. The small-diameter cable can help improve air flow inside the PC system chassis and facilitates future designs of smaller PC systems. The lower pin count of the smaller Serial ATA connector eliminates the need for the large and cumbersome 40-pin connectors required by Parallel ATA.

Improved Data Robustness

Serial ATA offers more thorough error checking and error correcting capabilities than was available with Parallel ATA. The end-to-end integrity of transferred commands and data can be guaranteed across the serial bus.

Backward Compatibility

Serial ATA provides backward compatibility for legacy Parallel ATA and ATAPI devices. This can be accomplished by two methods. First, you can use chip sets that support Parallel ATA devices in conjunction with discrete components that support Serial ATA storage devices. These discrete components are now available. An integrated chip set, which supports a mix of serial and parallel channels, is also available. Second, you can use serial and parallel dongles, which adapt parallel devices to a serial controller or adapt serial devices to a parallel controller.

Increased Disc Drive Data Rates

Since disc drive data rates have not yet exceeded ATA100 limits, why should you switch to Serial ATA? The maximum internal data rate on an IDE disc drive today is ~72MB/sec. The ATA/100 data transfer rate has not been reached. But one of the reasons IDE performance is where it is today is due to the expandable data path PATA has allowed.

That data path in PATA has reached its limit. Serial ATA allows disc drives to continue to offer performance and reliability at cost parity to Parallel ATA. In addition, the Serial ATA interface requires less voltage, meaning better power consumption and management in both desktop and mobile applications. The thinner cable allows for flexible designs and improved airflow in smaller form-factors.

Page 5: Serial ATA Integration

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