Author: Marty Czekalski
Years ago, nearly all interfaces were serial. Communications, printers, and disks all used serial interfaces. Those were the days of discrete components, and reducing the component count was an important factor in keeping costs under control. As the level of integration increased, it became possible to put multiple drivers on the same piece of silicon as the logic. This made parallel interfaces both high-performance and cost-effective. As a result, parallel interfaces became the rule and serial interfaces the exception, except in long-distance communications, where the cost of the high-speed transmitter/receiver technology was offset by the savings in wire (or fibre) costs.
Interestingly, as technology has moved forward, the same advances in integration that caused parallel interfaces to gain favor are now working against them. The I/O cells are not scaling at the same rate as internal logic, and it is now easier and less costly to add sophisticated signal processing at lower cost than adding additional I/O cells needed by parallel interfaces. The result is that these high-performance serial interfaces are becoming cost effective at shorter and shorter transmission distances as the levels of integration increase. We now find ourselves switching back to serial interfaces such as USB for printers and PCI Express for PCI replacement. The next step will see this same migration in the interfaces we use to connect disk drives. The evolution of ATA and SCSI will move these interfaces to Serial ATA and Serial Attached SCSI. This article will explore some of the details driving this transition as well as the advantages of these serial topologies.