Introduction

The Serial ATA standard (S-ATA or SATA) is a standard bus allowing high-speed storage peripherals to be connected to PC computers.

The Serial ATA standard was introduced in February 2003 in order to compensate for limitations of the ATA standard (better known by the name "IDE" and retroactively called Parallel ATA), which uses a parallel transmission mode. Indeed, the parallel transmission mode is not designed to work with high frequencies due to problems related to electro-magnetic disturbances between the different wires.

The S-ATA standard cables and peripherals can be identified by the presence of the following logo:

Principle of the Serial ATA

The Serial ATA standard is based on serial communication. A data path is used to transmit the data and another path is used to transmit acknowledgements of receipt. On each of these data paths, data are transmitted via the LVDS (Low Voltage Differential Signalling) transmission mode which involves transferring a signal to a wire and its opposite to a second wire to allow the receiver to recreate the signal by difference. The control data are transmitted on the same path as the data using a specific sequence of bits to distinguish them.

Thus the communication requires two transmission paths, each one comprising two wires, with a total of four wires used for the transmission.

Serial-ATA connectors

The cable used by the Serial ATA is a round cable containing 7 wires and with an 8mm connector on the end:

Three wires are grounded and two pairs are used to transmit data.

The supply connector is also different: it comprises 15 pins which supply the peripheral with 3.3V, 5V or 12V power and looks similar to the data connector:

Technical characteristics

The Serial ATA offers speeds of 187.5 Mo/s (1.5 Gb/s), and each octet is transmitted with a start bit and a stop bit, with a theoretical effective speed of 150 Mo/s (1.2 Gb/s). The Serial ATA II standard should help achieve 375 Mo/s (3 Gb/s), i.e. theoretical effective speed of 300 Mo/s, then finally 750 Mo/s (6 Gb/s), i.e. theoretical effective speed of 600 Mo/s.

Serial ATA cables can measure up to 1 metre in length (compared with 45cm for IDE cables). Furthermore, the low number of wires in a round casing allows greater flexibility and better circulation of air in the casing than with IDE cables (even if round IDE cables exist). Contrary to the ATA standard, Serial ATA peripherals are alone on each cable and "master peripherals" and "slave peripherals" no longer need to be defined.

Moreover, the Serial ATA standard allows for Hot Plugging).

More Information

You can find all technical specifications on the website of the Serial ATA Working Group, the body responsible for maintaining the Serial ATA standard:

• http://www.serialata.org