SATA-SERIAL ATA

8/9/2003

SATA -- Serial ATA cabling -- is a technology that replaces the conventional 40 (80) wire ATA/IDE cabling with cable including only two signals on four wires. This is intended to support future increases in burst rates over the ATA data channel. It is projected that without SATA, disk cable geometry and configuration issues would become major problems in future years.

Actual and anticipated ATA cable issues include user installed 40/80 wire cabling making case airflow design difficult; eventual projected problems in generating the 5 volt signal levels on parallel ATA cabling; and the projected increasing difficulty of holding ever faster parallel data and clock signals in synchronization over lengthy data paths.

SATA is designed to be software compatible with parallel ATA. All ATA (IDE) and ATAPI devices that work with current software should work identically if built with SATA instead of parallel ATA. SATA is intended as an internal storage device connection technology for PCs. It is not intended to compete with USB or Firewire (IEEE1394) which are designed for the connection of external devices.

The serial cables and SATA controllers will initially permit data rates comparable with the fastest parallel ATA and will allow higher transfer speeds when the current PC PCI data bus is replaced with buses (e.g. PCI Express) that permit higher data rates. SATA will also allow somewhat longer data cables than parallel ATA. The smaller cables should reduce airflow issues within the case and should alleviate cable reach problems in multidrive servers.

Serial ATA cables are four wire cables that include one low voltage (250mv) differential pair of data lines in each direction. They use a seven pin connector. They appear actually to consist of two pairs of 100ohm coax in each direction. Initial data rates as of 2003 are about 150mByte/sec using cables to 1 meter in length. SATA cable construction may be somewhat more critical than parallel data construction as it is important that grounds at the ends be reliable and that the two wires in each pair be the same length. The parallel ATA midwire tap to allow a master and slave drive on one cable is not present in SATA.

Converters are available to allow normal parallel ATA disks to be connected to SATA cables. Serial ATA disks include a new 15 pin power connector that permits 3.3 volts be provided from the motherboard.

The only problem reported in testing of production prototypes by independent labs was with SATA connectors so loose that they made erratic contact and or fell off the drives. It is probable that this problem will be cleared up in production equipment.

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