Serial ATA Hard Drive Interface

The Serial ATA bus [SATA] is the serial version of the IDE [ATA] spec. SATA uses a 4 conductor cable with two differential pairs [Tx/Rx], plus an additional three grounds pins and a separate power connector. Data runs at 150MBps [1.5GHz] using 8B/10B encoding and 250mV signal swings, with a maximum bus length of 1 meter . Later SATA enhancements move the data transfer speed to; 300MBps [3.0Gbps], and then 600MBps [6.0Gbps]. The current speed for SATA is 300Mbps [3Gbps]. Shielded external SATA [eSATA] data cable runs out to a maximum of between 3 feet and 6 feet. eSATA cables are used external to the chassis or case.

SATA Protocol
The SATA Frame structure used between Host and Device is shown in the graphic below. The frame is made up of multi Dwords, which are in turn encapsulated by flow control and CRC information. The SATA frame begins with a Start-of-frame [SOF]. The SOF is followed by the Frame Information Structure [FIS]. Then the Cyclic Redundancy Code [CRC] is placed in the frame. The final block in the message is an End-of-Frame [EOF].

SATA uses a 32bit CRC [calculated over the contents of a (FIS) Frame Information Structure], stored as the 'Dword'. The 32-bit CRC polynomial is X³²+ X²⁶+ X²³+ X²²+ X¹⁶+ X¹²+ X¹¹+ X¹⁰+ X⁸+ X⁷+ X⁵+ X⁴+ X²+ X + 1.

SATA Electrical
Serial ATA uses LVDS [EIA/TIA-644] with voltages of 250mV; while the obsolete parallel ATA interface is based on TTL signaling levels and rates. Serial ATA is a point-to-point interface where each device is directly connected to the host via a dedicated link. Because Serial ATA uses a dedicated link, adding another drive to the computer will have no impact on bandwidth. With Serial ATA the additional hard drive uses a separate SATA link, while the older IDE parallel standard [PATA] would see a degradation in speed because the drives would share the same link band width.

The Bit Encoding used is: Non Return to Zero (NRZ) encoding for data communication on a differential two wire bus. The use of NRZ encoding ensures compact messages with a minimum number of transitions and high resilience to external disturbance. The termination resistor is 100 Ohms [+/- 5 Ohms] differential.

SATA Physical
Serial ATA uses only 4 signal pins, improving pin efficiency over the parallel ATA interface which uses 26 signal pins going between devices [over an 80 conductor ribbon cable onto a 40 pin header connector].
Serial ATA also provides the opportunity for devices to be 'hot-plugged', devices may be inserted or removed while the system is powered on. The pinout tables for Serial ATA are listed below.

The primary function of Serial ATA bus is to form an interface between the Motherboard and the Hard Disk Drive [HDD]. The Hard Drive may have a SATA connector and a legacy PATA data connector, with a legacy PATA power connector, so the device may function in either a legacy [older] motherboard or a currently produced motherboard. In this case the mother board S-ATA interface would be developed from a peripheral add-on board and not the motherboard. Power connectors on a HDD are header pins for a P-ATA interface and card-edge finger blades in the case of S-ATA. Some drive connectors shield the S-ATA power connector preventing their use, so you must use the P-ATA power pins to supply power to the drive. Terms used to describe the obsolete Harddrive interface which preceded the SATA interface include; IDE, Parallel ATA, PATA, and P-ATA.