PC Hardware A Beginner’s Guide

(^184) PC Hardware: A Beginner’s Guide

Bezel

Many older hard disk drives included a faceplate, orbezel, with the drive. Older form

factor cases, such as the early AT cases, did not have LEDs on the front of the case for the

hard disk. On these cases, the hard disk bezel was allowed to show. The bezel included

LEDs for activity and power. As the front panel of the system case now provides this

function, hard disk drives typically do not offer a bezel as a standard feature, but one can

be obtained as an optional item.

Interfaces

The mechanism that controls the transmission of data between the CPU and other devices

on the PC is an interface. Disk storage devices, such as hard disk drives, floppy disk

drives, tape drives, CD-ROM drives, DVD-ROM drives, all use a transfer interface to

move data to and from themselves and the rest of the PC. The form and function of an

interface is defined in the device controller and other drive electronics. Because hard

disks and other storage devices are manufactured to work with a wide range of PC sys-

tems,avarietyofinterfaceprotocolstandardshavebeenadoptedtoensurecompatibility.

The interface standards that have been used with hard disk drives are:

 ST506

 ESDI

 IDE

 SCSI

 FC-AL

The first two are largely obsolete now, along with the PC AT, the PC on which they

were used. Most of the hard disks in use today use either an IDE or a SCSI hard disk

drive interface. FC-AL is found on very high-end disk array products associated with

large network servers.

ST506/412 Interface

Seagate Technology developed the ST506/412 drive interface for its 5MB (ST506) and

10MB (ST412) disk drives in the early 1980s. Nearly all manufacturers making hard disk

drives for PCs used the ST506/412 standard, which made it virtually the standard for PC

hard disk interfaces at the time. It was essentially universal in that no custom cables were

needed to connect ST506/412 drives from any manufacturer to any ST506/412 controller.

This interface is obsolete for all new systems. It lacks the capacity, speed, and expandability

needed to survive in today’s market.

ESDI

TheEnhanced Small Disk Interface (ESDI), pronounced “ez-dee,” was the hard disk interface

standard that replaced the ST506/412 standard. It introduced a number of innovations,