Scope

The ATA and SCSI interfaces used to connect hard drives to your PC each has a long history. Early storage interfaces were essentially serial connections with clock speeds limited by the relatively slow controller chips. Although it was technically feasible to build fast serial interfaces, it wasn't practical because the high-speed controller would cost more than the 8MHz PC it would go into.

Engineers seeking a cost-effective alternative to serial began to use parallel connections. Hewlett-Packard and IBM each developed proprietary parallel interfaces, but the two that were to survive were ATA and SCSI.

ATA (AT Attachment) is an interface specification which incorporates the IDE (Integrated Drive Electronics) concept originally proposed by Western Digital and Compaq in 1986 to overcome the performance limitations of earlier subsystem standards.

ATA is extremely inexpensive to manufacture, and has become the most common storage interface on personal computers. The standard has evolved considerably over the years. The first version, introduced in 1981 had a throughput of 4 MBps. By the time ATA/133 (also known as UltraDMA 133) - expected to be the final generation of Parallel ATA interface before the industry completes its transition to Serial ATA - was announced in mid-2001, throughput had been increased to 133 MBps.

SCSI evolved from the Shugart Associates Standard Interface (SASI), which Shugart Associates, an early hard disk manufacturer, developed in 1981 in conjunction with NCR.

Like ATA, SCSI is a bus which controls the flow of data (I/O) between the computer's processor and it's peripherals, the most common of which is the hard drive. Unlike ATA, SCSI requires an interface to connect it to a PC's PCI bus. This isn't a controller: it's correctly called a "host adapter". The actual controllers are built into each SCSI device, an arrangement which gives SCSI its most obvious advantage over ATA, the number of devices it can control. Whereas the EIDE interface is restricted to four devices - which can include hard disks and CD-ROM drives - a SCSI controller can handle a "chain" of up to eight devices (including the host adapter card, which counts as a device).

SCSI has also come a long way since the original, 5 MBps 1986 SCSI-1 standard, with data transfer rates of up to 320 MBps being supported by the 2002 Ultra 320 SCSI incarnation. Moreover, each successive generation maintains backwards compatibility, allowing a slower SCSI device to connect to a newer implementation.

However, by that time it's true to say that SCSI had lost ground to the inexpensive ATA interface in the mainstream PC market, being viewed as the preferred high-performance storage interface, and the interface of choice for servers, RAIDs, high-performance tape drives and other high-end storage systems.