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Now that the basic components of the modern hard drive have been examined, it is time to take a look at the two main standards that exist for hard drives. Two different sets of interface standards have been developed for hard drives: advanced technology attachment (ATA) storage interface and small computer system interface (SCSI). (ATA is also known as IDE: integrated drive electronics.) ATA drives are generally less expensive and more common, while SCSI devices cost more and tend to be utilized for higher performance applications. However, within each camp is a move to change this situation. SCSI drives are finding uses in less high-end applications as their price comes down. Similarly, advancements such as serial ATA have allowed the ATA to move into high-performance disk drive applications.
"ATA" stands for AT Attachment, as it was first used for the IBM PC/AT computer ("AT" was short for "Advanced Technology" and was first used by IBM when it released the PC/AT in 1984). Other variants of the AT name include IDE (Integrated Disk Electronics), EIDE (Enhanced IDE), and DMA (Direct Memory Access), as well as Ultra ATA, Ultra DMA, ATA-66, and numerous other permutations of these terms. Such ATA drives were the first to incorporate a controller into the drive itself, rather than just having one on the motherboard, hence the name IDE. Originally, the drives sat on a card that was then plugged into an expansion slot, but due to various problems they were later put into a drive bay. ATA drives use a 16-bit interface, and each channel can support two drives. ATA standards are developed by Technical Committee T13 (www.t13.org), which is part of the InterNational Committee on Information Technology Standards (INCITS) and are published by the American National Standards Institute (ANSI). Ultra ATA provides for data transfer rates of 33, 66, or 100 megabytes (MB) per second. In 2001, a serial ATA interface was announced, and devices using this type of connection will be available before the end of 2003 according to the latest estimates.
While ATA drives are primarily used for PCs, Small Computer Systems Interface (SCSI) is used more broadly for a wide variety of devices, not just hard drives. It is possible to use more than one SCSI device on a single interface. Like ATA, SCSI is published by ANSI, but it has been developed by INCITS' Technical Committee T10, rather than T13. An abundance of SCSI standards currently exists (see Exhibit 4), and new developments in the SCSI field show great promise. An industry association known as the Serial Attached SCSI Working Group is working on a specification for serial attached SCSI, which will combine the serial ATA interface with some of the features of SCSI. Devices meeting this standard are expected by 2004 or thereabouts. The bottom line is that for workstations an ATA drive is preferable due to its lower cost, but for servers and mission-critical applications a SCSI drive is recommended. Even if the specs on ATA and SCSI drives seem similar, they are built for different types of usages.
Exhibit 4: SCSI Standards
Name | Maximum Bus Speed (megabytes per second) | Bus Width | Maximum Devices per Connection |
---|---|---|---|
SCSI-1 | 5 | 8 | 8 |
Fast SCSI | 10 | 8 | 8 |
Fast Wide SCSI | 20 | 16 | 16 |
Ultra SCSI | 20 | 8 | 8 |
Wide Ultra SCSI | 40 | 16 | 8 |
Ultra2 SCSI | 40 | 8 | 8 |
Wide Ultra2 SCSI | 80 | 16 | 16 |
Ultra3 SCSI | 160 | 16 | 16 |
Ultra320 SCSI | 320 | 16 | 16 |
Source: SCSI Trade Association (STA), San Francisco, California. |
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