Small Computer Systems Interface

SCSI stands for Small Computer Systems Interface, and was a parallel interface standard used mostly 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, the iMac and Later, come with a SCSI port for attaching devices such as disk drives and printers.
SCSI interfaces provide for faster data transmission rates (up to 80MB per second) than standard Serialand 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 Macintoshes, the iMac comes with IDE, a less expensive interface, in which the controller is integrated into the disk or CD-ROM drive, freeing up space on the Motherboard. Other interfaces supported by PCs include Enhanced IDE (EIDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots (Typically a PCI Slot, although ISA was used as well).
SCSI has more than one standard, and because of this, there is a small chance that the SCSI device you've purchased may not be compatible with your SCSI Controller.
==Current SCSI Versions==
* 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.