Why Serial?

The world is moving from parallel interfaces to serial interfaces. PCI and PCI-X are moving to PCI-Express. SCSI is moving to Serially Attached SCSI (SAS). IDE and ATA are moving towards Serial ATA (SATA).

The reason is two fold. The first is speed. With parallel interfaces, there is always a bit skew between the bits due to differences in capacitance, inductance, and resistance inherent in the circuits for each data bit or wire in the interface. As a result, there is a timing difference between when the first and last bits arrive at the other side of the cable.

This difference forces a slower interface. A parallel interface has a strobe that falls when the data is stable and can be sampled. Due to bit skew of the data as well as the strobe, there must be a limited lower bound on how soon the strobe can fall after the data is presented. This limits the overall bandwidth to the circuit.

In a serial interface, the data is shifted out over a single wire like water coming out of a garden hose. Each data bit or drop of water flows along the path. There are no forces causing one data bit to try and move in front of or behind the neighboring data bits. All of the impeding forces are acting equally on all of the data bits.

The other reason for the moves towards serial interfaces is cost. With a serial interface, there are effectively two circuits, receive and transmit, carried over only four wires. Instead of eight, sixteen, or thirty-two bi-directional circuits, there are only two uni-directional circuits.

It is possible to have highly matched data circuits in a parallel style interface but this requires tighter tolerances in manufacturing. Tight manufacturing tolerances drive up the cost for each circuit. A serial interface, while higher speed, does not need to be matched since it is operating independently. These two factors combine to give a serial interface not only fewer circuits but also lower cost circuits.

This also creates smaller cables and smaller connectors. The smaller connectors take up less real estate. The smaller cables are eaiser to route. Since there are fewer circuits, the cables can be made more robust and better shielded from outside interference.

There is also an implied benefit that serial circuits allow for a growth path for higher and higher speeds. As the base electronic circuits gain speed, that speed can be incorporated more easily. With a parallel interface, the bulk of the timing issues are caused by the time of the data traveling across the cable – not with the electronic circuits. The time for the data to travel across the cable, as well as the skewing problems, is not reduced with faster electronics.