Development for Serial ATA (SATA) started when the Serial ATA Working Group effort was announced at the Intel Developer Forum in February 2000. The initial members of the Serial ATA Working Group included APT Technologies, Dell, IBM, Intel, Maxtor, Quantum, and Seagate. The first Serial ATA 1.0 draft specification was released in November 2000 and officially published as a final specification in August 2001.
The performance of SATA is impressive, although current hard disk designs can’t fully take advantage of its bandwidth. Three proposed variations of the standard all use the same cables and connectors; they differ only in transfer rate performance. Initially, only the first version will be available, but the roadmap to doubling and quadrupling performance from there has been clearly established. Table below shows the specifications for the current and future proposed SATA versions; the next-generation 300MBps version was just introduced in 2005, whereas the 600MBps version is not expected until 2007.
|
SATA Standards Specifications |
|||||
| Serial ATA Type | Bus Width (Bits) | Bus Speed (MHz) | Data Cycles per Clock | (MBps) | Bandwidth |
| SATA-150 | 1 | 1500 | 1 | 150 | |
| SATA-300 | 1 | 3000 | 1 | 300 | |
| SATA-600 | 1 | 6000 | 1 | 600 | |
Serial ATA uses a special encoding scheme called 8B/10B to encode and decode data sent along the cable. The 8B/10B transmission code originally was developed (and patented) by IBM in the early 1980s for use in high-speed data communications. This encoding scheme is now used by many high-speed data-transmission standards, including Gigabit Ethernet, Fibre Channel, FireWire, and others. The main purpose of the 8B/10B encoding scheme is to guarantee that there are never more than four 0s (or 1s) transmitted consecutively. This is a form of run length limited (RLL) encoding (called RLL 0,4) in which the 0 represents the minimum and the 4 represents the maximum number of consecutive 0s in each encoded character.
The 8B/10B encoding also ensures that there are never more than six or fewer than four 0s (or 1s) in a single encoded 10-bit character. Because 1s and 0s are sent as voltage changes on a wire, this ensures that the spacing between the voltage transitions sent by the transmitter will be fairly balanced, with a more regular and steady stream of pulses. This presents a more steady load on the circuits, increasing reliability. The conversion from 8-bit data to 10-bit encoded characters for transmission leaves a number of 10-bit patterns unused. Several of these additional patterns are used to provide flow control, delimit packets of data, perform error checking, or perform other special needs.
The physical transmission scheme for SATA uses what is called differential NRZ (Non Return to Zero). This uses a balanced pair of wires, each carrying plus or minus 0.25V (one-quarter volt). The signals are sent differentially: If one wire in the pair is carrying +0.25V, the other wire is carrying0.25V, where the differential voltage between the two wires is always 0.5V (a half volt). This means that for a given voltage waveform, the opposite voltage waveform is sent along the adjacent wire. Differential transmission minimizes electromagnetic radiation and makes the signals easier to read on the receiving end.
A 15-pin power cable and power connector are optional with SATA, providing 3.3V power in addition to the 5V and 12V provided via the industry-standard 4-pin device power connectors. Although it has 15 pins, this new power connector design is only 24mm (0.945 inches). With three pins designated for each of the 3.3V, 5V, and 12V power levels, enough capacity exists for up to 4.5 amps of current at each voltage, which is ample for even the most power-hungry drives. For compatibility with existing power supplies, SATA drives can be made with either the original, standard 4-pin device power connector or the new 15-pin SATA power connector, or both. If the drive doesn’t have the type of connector you needed, adapters are available to convert from one type to the other.
Figure below shows what the new SATA signal and power connectors look like.
SATA (Serial ATA) signal and power connectors.
The configuration of Serial ATA devices is also much simpler because the master/slave or cable select jumper settings used with Parallel ATA are no longer necessary.
