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An acronym for Small Computer Systems Interface. This is a particular protocol for the host motherboard, CPU and related devices to communicate with I/O devices such as disk drives, tape drives, removable storage drives, CD-ROMS, CD-RWs, DVDs, scanners and the like. This is pronounced “scuzzy”. The technology has been around for several generations of advancement. All SCSI systems are still current technology, even the 8-bit systems; however the advancements are in multi-user DMA ability and speed of data transfer from device to actual user memory pool. SCSI was introduced in small scale in 1976 on CP/M systems that were considered high performance and has been the default drive interface for most apple systems since then. SCSI systems are more efficient than IDE or EIDE devices in how fast data gets from one place to another and how many devices can be addressed and utilized from the same controller. Virtually all SCSI controllers allow up to eight SCSI devices; some allow up to sixteen. While there have been in the past and there are at present numerous SCSI controller manufacturers, Adaptec is the technology leader and controls the market share of SCSI device controllers. Although SCSI systems can be and are often used as high performance work stations, they are most often used as network and Internet servers. SCSI is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers. Nearly all Apple Macintosh computers, excluding only the earliest Macs and the recent iMac, come with a SCSI port for attaching devices such as disk drives and printers. SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface. Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors. While SCSI has been the standard interface for Macintosh, the iMac comes with IDE, a less expensive interface, in which the controller is integrated into the disk or CD-ROM drive. Other interfaces supported by PCs include enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI controller board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards. The following varieties of SCSI are currently implemented: SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI. Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers. Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10 MBps. Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps. Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps. SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI. Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps. Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.

scsi.txt · Last modified: 2006/10/15 09:35 (external edit)