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Aside from the neater design of serial cables, what is the new technology that will conquer the mass storage world? As the name indicates, the new interface will be serial and capable of 150 MB throughput. Since we are looking at a point to point connectivity, the required operating frequency will be 150 MHz x 8 bit and if we add a 20 % overhead for cyclic redundancy check and other error correction mechanisms, we'll end up with 1.5 Gbit /sec or 1.5 GHz operating frequency. How is this high frequency possible?

1. Embedded Clocking Scheme

Instead of relying on an external clock, with an elaborate clock forwarding scheme and synchronization of data and strobe signals, Serial ATA is using an "embedded" clock meaning that the data themselves act as clock signal or rather as synchronizers for the internal receivers at both ends of the transmission. One potential problem with this scheme is that during periods of no data transfer, the system could get out of sync. This issue can be completely avoided by sending a dummy signal consisting of a regular IOIOIOI pattern. The result is a high speed interface with internal clocks at both ends that are synchronized by the data flow. Because there are no parallel paths, clock skew is simply impossible.

2. Low Voltage Differential Signaling

As we outlined earlier, many of the Parallel ATA problems stem from the 15 years of legacy of a standard 5V-tolerant 3.3V signal protocol. More modern signaling methods are using LVDS, that is, a pair of wires is used to send two voltage signals and the voltage potential between the two wires is the actual data. Since we are no longer talking about reaching a cross-over or trip point with the necessary overhead to ensure signal integrity, ultra-low voltages can be applied which allow to greatly speed up the signaling scheme, for the simple reason that the number of electrons that are pumped into / received from the wires is much lower and therefore the transaction is faster.

What is needed, though, for LVDS is a DC Bias, that is a reference voltage. The two differential signals are swinging 0.125V above and below. In detail, the reference bias is around 500 mV with +0.125V amplitudes and nominal Vmin and Vmax points of 400 and 600 mV. I mentioned it earlier that any kind of differential signaling is prone to bias shifts that can occur in periods of no-transfers or else if the majority of signals are either high or low. To compensate, the earlier mentioned dummy signals (IOIOIOIO ..... ) are used to re-adjust the DC bias.

A "drab" red cable next to a metallic Cerise shade, flat vs. dual pairs, when will we see glow in the dark SATA cables? Note the L-shaped connector opening and the additional key on the right of the connector which make it impossible to reverse the orientation.

3. Cabling

If only four transmission lines are needed for a full duplex (independent unidirectional upstream and downstream) signaling, and the additional ground pins can be used for shielding of the four data wires, we are looking at a reduction of total wires by a factor of 20 compared to the 80 wires in the UATA cables. Realistically, this is not entirely true since instead of parallel ground wires, the shielding is accomplished by coaxial wrapping of grounds around the data wires. Nonetheless, there are no imminent space constraints and, therefore, the diameter of the individual leads can be increased to reduce the resistance and impedance of the cables and, by extension, speed up signal propagation. The latter is the really important factor, since signaling frequency increases by a factor of 30 even in the first implementation of Serial ATA.

Another important aspect of the SATA cabling is that there are hardly any length constraints. Officially, SATA cable length is limited to 1m, however we have seen cables as long as 2.5m running in demo systems without any measurable performance loss. Last not least, it is no longer necessary to destroy signal integrity with rounded IDE cables for esthetic or air flow reasons. Serial ATA cables are sleek, flexible and offer a whole new wealth of cool designs.

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