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superceded by SATA.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.
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