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Before we get into the discussion of SCSI-2, let's understand what SCSI-1 is first.

SCSI-1 actually is the original standard which was known as SCSI. The SCSI-1 standard defined two modes of data transfers: asynchronous (handshaked) and synchronous (streamed mode). Synchronous SCSI-1supports transfer rates of up to 5MBytes per second. Asynchronous supports transfer rates of up to 3MBytes per second. SCSI-1 disks originally faced stiff competition from ESDI (Enhanced Small Device Architecture) drive which were of comparable capacity and were more broadly supported but the promise performance enhancements all but cemented the demise of ESDI.

SCSI-2
SCSI-2 inherited some significant improvements of SCSI-1, including: improved connectors, faster data transfer speed, availability of a wider data bus path, increased reliability via synchronous negotiation, and parity checking. SCSI-2 doubles the SCSI bus clock rate from 5 MHz to 10MHz, which increases the SCSI data transfer rate from 5 MB/second to 10 MB/second. This change was called Fast SCSI. SCSI -2 also provides the option to double the bandwidth of the SCSI bus via the use of Wide SCSI. The width of the bus is its number of data lines. By doubling the width of the bus from 8 bits to 16 bits, a Wide SCSI bus can support up to 15 devices and transfer twice as much data in the same amount of time. Combining Fast SCSI with Wide SCSI results in a maximum data transfer rate of 20 MB/secibd.

There are two other features of SCSI-2 support overall performance: command queuing and scatter/gather data transfer. Command queuing offers the ability to rearrange or reorder the execution of I/O commands so that overlapping is optimized and throughput maximized. Scatter/Gather is a method of providing multiple host addresses for data transfer in one command packet. This greatly increases performance in environments such as Unix, Novell NetWare, Windows NT, Windows 95 and OS/2.

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