Scsi Technical Information, Scsi Basics

6 x 4 ELECTRONIC SCSI SWITCH

6.SCSI Technical Information

6.1SCSI Basics

6.1.1 SCSI-1

The original specification supports data transfers up to 5 MB per second on an

8-bit wide parallel data bus. SCSI-1 standards had some incompatibility problems between host adapters and peripheral devices. The need to improve compatibility, increase transfer rates, and add other features for better performance required a review of the specifications.

6.1.2 SCSI-2

Improved compatibility and higher transfer rates were provided in this enhancement. The addition of “Wide SCSI” permits 16 or 32 bits to be transferred in parallel, the latter requiring two cables. In combination with the “Fast SCSI” option, synchronous data transfers up to 10 MBps for 8-bit, 20 MBps for 16-bit, and 40 MBps for 32-bit buses were achieved.

6.1.3 SCSI-3

The most significant additions include the ability to address up to 32 devices, a 16-bit single-cable data bus, and serial SCSI protocol. The SCSI-3 standard has been split into several subdocuments, including the SCSI Parallel Interface (SPI), which is based on a layered protocol, and the SCSI Interlocked Protocol (SIP), a software link protocol.

6.1.4SIGNAL WIRING

The signal wiring used in a SCSI bus has an impact on bus performance. The two wiring techniques generally used for SCSI are single-ended and differential. With single-ended wiring, a single wire carries the signal from initiator to target. Single- ended circuitry is not noise-resistant and is generally limited to about 6 meters (20 ft.) at data-transfer speeds of 10 MB per second.

Differential wiring uses two wires for each signal and offers exceptional noise resistance because it does not rely on a common ground. This allows cables up to 25 meters (82 ft.) and reliable operation at 10 MB or more per second. Differential wiring and circuitry is more complex than single-ended and generally tends to be more expensive to implement.