Hardware Reference
In-Depth Information
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 is fairly balanced, with
a more regular and steady stream of pulses. This presents a steadier load on the circuits, increasing
reliability. The conversion from 8-bit data to 10-bit encoded characters for transmission leaves
several 10-bit patterns unused. Many of these additional patterns provide flow control, delimit
packets of data, perform error checking, or perform other special functions.
The physical transmission scheme for SATA uses differential NRZ (Non Return to Zero). This uses a
balanced pair of wires, each carrying +0.25V (one-quarter volt). The signals are sent differentially: If
one wire in the pair carries +0.25V, the other wire carries -0.25V, where the differential voltage
between the two wires is always 0.5V (one-half volt). So, 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 is 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 3 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 plenty for even the most power-hungry drives. For compatibility
with existing power supplies, SATA drives can be made with 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 need, adapters are available to convert from one type to the other.
Figure 7.10 shows what the SATA signal and power connectors look like, and Figure 7.11 shows
SATA and PATA host adapters on a typical motherboard.
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