Serial ATA is the next-generation internal storage interconnect, designed to replace parallel ATA technology
Serial ATA (SATA, is a computer bus technology designed for transfer of data to and from a hard disk. It is the successor to the legacy Advanced Technology Attachment standard (ATA, also known as IDE). This older technology was retroactively renamed Parallel ATA (PATA) to distinguish it from Serial ATA. This interface uses 7-pin cables for the data connection, and transmits the data serially rather than in parallel. In addition, Serial ATA should give users the ability to hot swap hard drives. This adds a capability that more expensive systems such as SCSI and Fibre Channel have had for a long time, though the future will tell how widely users exploit that aspect of the technology. Serial ATA also reduces the signalling voltage from the 5 volts used in P-ATA down to 0.5 volts, which reduces power consumption and electrical interference. Due to serial transfer and lower power the maximum allowable length of SATA cables exceeds that of ATA ribbon cables.
The Serial ATA (SATA) bus is defined over two separate connectors, one connector for the data lines and one for the power lines (SATA power pinout). A Serial ATA Hard drive may also have a third connector for legacy PATA power connections. The PATA power connector used instead of the SATA power in some early devices.
Physically, the SATA power and data cables are the most noticeable change from Parallel ATA. The SATA standard defines a data cable using seven pins to supply four conductors shielded with ground supplied by the other three pins.
Transmit pins are connected to Receive pins on the other side. The SATA connector is keyed at pin 7. Pin 1 may be used on Hot Plug arrays for signalling.
SATA uses a 4 conductor cable with two differential pairs [Tx/Rx], plus an additional 3 grounds pins and a separate power connector. SATA runs at approximately 150MBps(SATA/150), ~300MBps(SATA II), or ~600MBps transfer rates. Faster SATA implementations are backward compatible with older devices. 8B/10B encoding used for data transfers. Maximum unshielded cable length is about 1 meter. eSATA shielded cable may be up to 2 meters length.
SATA is 1.5GHz, which means it transfers 1.5 Gbits per second. GHz and Gbits is always measured as 1Gbit = 1000000000, not as 1024x1024x1024. Since there's 10 bits per byte, not 8, we'll end up with a calculation like this:
1500000000 bits /10 bits per byte / 1024 / 1024 = 143.051147 MB/sec (not 150MB).
SATA2 (3G SATA) can thus transfer 286.1MB/sec (not 300MB) and SATA3 (6G SATA) can transfer 572.2MB/sec (not 600MB).
Sadly, less than 1% of the population will ever utilize the speed that the high speed SATA interfaces make available. The interface is rarely 'saturated', because most drives only deliver between 100MB/sec - 150 MB/sec. However you can replace one internal 3.5" drive with a RAID0 enclosure and insert two WD RED 2.5" drives (137MB/sec each) to achieve a transfer rate of 274MB/sec.
Related info: On harddisks, the manufacturers like to write that a harddisk that has 500000000 Bytes avaialble actually has 500MB available; this is the same thing; the actual number of MB avaiable is 500000000 / 1024 / 1024 = 476.837158 MB.
SATA interface revisions:
SATA/150 (1500Mbps) - first-generation of Serial ATA interfaces, also known as SATA/150, run at 1.5 Gigahertz (GHz). Actual data transfer rate is up to 1.2 Gigabits per second (Gb/s), or ~150 megabytes per second (MB/s). The simplicity of a serial link and the use of LVDS allow to use of longer drive cables.
SATA 3.0Gb/s (SATA II, 3.0Gbps) - second-generation of SATA interfaces. A 3Gb/s signalling rate was added to the PHY layer. SATA II devices are required to support the original 1.5Gb/s rate. In practice, some older SATA systems require the peripheral device's speed be manually limited to 150MB/s with the use of a jumper. SATA II uses same encoding as SATA I featuring an actual data transfer rate of 2.4 Gb/s, or ~300 MB/s.
SATA 6.0Gb/s (SATA III, 6.0Gbps) - SATA revision 3.0 doubles the speed of the current 3Gb/s version, reaching transfer speeds of 6Gb/s.
SATA drives may be plugged into Serial Attached SCSI (SAS) controllers and communicate on the same physical cable as native SAS disks. SAS disks, however, may not be plugged into a SATA controller.
External SATA (eSATA)
eSATA was standardized in 2004, with specifically defined cables, connectors, and signal requirements for external SATA drives. eSATA features full SATA speed for external disks, no protocol conversion from IDE/SATA to USB/Firewire, cable length up to 2m.
USB and Firewire hard disks requires conversion of all communication, so external USB/Firewire enclosures include an IDE or SATA bridge chip. Some drive features (like S.M.A.R.T.) cannot be exploited that way and transfer speed with USB/Firewire bridge is limited. However, SATA does not provide power, which means that external 2.5 disks which would otherwise be powered over the USB or Firewire cable need a separate power cable when connected over eSATA.