Chapter 13. Hard Disk Interfaces

The hard disk interface defines the physical and logical means by which the hard disk connects to the PC. In the 1980s, the most popular disk interfaces were ST506/412 and ESDI, which are now obsolete. These old drives use two ribbon cables (a 20-pin data cable and a 34-pin control cable) versus the single-ribbon cable used by modern drives. Finding one of these old dual-cable drives in a PC by itself establishes that that computer is too old to be upgraded economically. A modern PC uses one or more of the following hard disk interfaces:

Integrated Drive Electronics (IDE)

IDE (pronounced as individual letters) was by far the most common hard disk interface used in PCs from the early 1990s through 2003. IDE is officially designated ATA, but is now often informally called Parallel ATA or PATA, to differentiate it from the new Serial ATA (SATA) interface. IDE is still used in new systems, although it is being superceded by SATA.

Serial ATA (SATA)

Serial ATA (SATA) is a new technology that will ultimately replace parallel IDE/ATA. SATA has several advantages over PATA, including superior cabling and connectors, higher bandwidth, and greater reliability. Although SATA and PATA are incompatible at the physical and electrical levels, adapters are readily available that allow SATA drives to be connected to PATA interfaces and vice versa. SATA is fully compatible with PATA at the software level, which means that the ATA drivers supplied with current operating systems work equally well with either SATA or PATA interfaces and drives.

SATA motherboards and hard drives began shipping in volume in late 2002 and had become readily available by July 2003. During the 2003/2004 transition period, most PCs and motherboards will include both SATA and PATA interfaces to ease the changeover. Manufacturers, loath to complicate inventory management and concerned about distribution issues, have been slow to introduce SATA versions of ATAPI devices such as optical drives, knowing that a PATA interface will almost certainly be available for connecting ATAPI devices. As SATA PCs and motherboards become more common, we expect most ATA/ATAPI devices will become available in SATA form.

Small Computer System Interface (SCSI)

Usually pronounced scuzzy (but sometimes sexy), SCSI hard disks are generally used in servers and high-end workstations, where they provide two major advantages: improved performance relative to IDE and SATA in multitasking, multiuser environments, and the ability to daisy-chain many drives on one computer. SCSI interfaces are available in various subtypes, which have different physical and electrical interfaces and transfer rates. Modern SCSI hard disks are the largest, fastest disks available, although recently IDE and SATA hard disks have begun to approach SCSI in size and speed. Within the different SCSI flavors, interfaces are well defined and standardized, but configuring SCSI to work on a standard PC can be complicated.

Modern ATA hard disks?whether PATA or SATA?are inexpensive, large, fast, standardized, and well-supported by PCs. SCSI disks are seldom used in desktop PCs because they cost more than ATA disks with similar capacity and performance. For example, if an ATA hard disk costs $90, a similar SCSI model may cost $175. In addition to the higher cost of SCSI drives, using a SCSI drive requires installing a SCSI host adapter, which may add $50 to $300 to system cost.

However, spending extra money on SCSI may increase overall system performance more than spending the same sum on a faster processor or a high-end video card, so don't rule SCSI out. The fastest hard drives, those that run at 15,000 RPM, are available only in SCSI versions. Also, in our experience, even midrange SCSI hard disks outperform fast ATA disks under heavy load, particularly under multitasking operating systems such as Windows NT/2000/XP and Linux.