Low voltage differential signaling
SATA technology transmits signals in a single stream rather than in multiple parallel streams. SATA
incorporates an low voltage differential (LVD) signaling scheme that uses two pairs of data lines to
transmit and receive low-voltage signals (250 mV). The data is represented by the voltage potential
between the two wires in each pair (Figure 3). Because it takes less time to apply low voltages to the
wires, LVD signaling can occur at a much greater speed than in parallel ATA. The low voltage
reduces the effects of capacitance, inductance, and noise. Noise sources tend to add the same
amount of voltage to both wires, so the voltage difference between the wires remains the same.
Figure 3. LVD signaling
Serial architectures encode (embed) the clock signals into the data stream, thus eliminating the skew
problem with aligning data and clock signals. Serial architectures require significantly fewer data
lines to switch simultaneously, which reduces the introduction of electrical noise. As a result, serial
signaling rates can be increased well beyond those attainable with a parallel bus. Serial
communication requires a device to convert parallel data into a serial bit stream and vice versa. This
device, called a serializer/deserializer (SerDes), contains a parallel digital interface, First-In-First-Out
(FIFO) caches, 8 bit/10 bit (8b/10b) encoder and decoder, a serializer, and a deserializer (see
Figure 4). The 8b/10b encoder converts each 8-bit data byte to a 10-bit transmission character,
which enables clocking information to be encoded into the data stream. Although this adds about
20 percent embedded overhead to the data stream, it eliminates the clock skew problem experienced
by parallel ATA.
Figure 4. The SerDes core integrates 8b/10b coding and decoding logic.