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 both of the following hard disk interfaces:

Integrated Drive Electronics (IDE)

Pronounced as individual letters, IDE is the overwhelmingly dominant type of hard disk found in modern computers. Modern IDE hard disks are large, fast, standardized, well supported by PCs, and inexpensive.

Small Computer System Interface (SCSI)

Usually pronounced scuzzy (but sometimes sexy), SCSI hard disks are usually used in servers and high-end workstations, where they provide two major advantages: improved performance relative to IDE 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 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.

SCSI disks are seldom used in desktop PCs because they cost more than IDE disks with similar capacity and performance. For example, if an IDE hard disk costs $90, a similar model with a SCSI interface may cost $175. In addition to the higher cost of the drives themselves, using a SCSI disk 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. In our experience, even relatively slow SCSI hard disks outperform fast IDE disks under load, particularly under multitasking operating systems like Windows NT/2000/XP and Linux.