SCSI 101 Information

SCSI (Small Computer System Interface) is a standard based on parallel technology that currently provides the fastest data input/output interface for small computers. In additional to super fast speed, SCSI also offers the ability to daisy chain multiple devices for easy access.

SCSI standards were established in 1985 that specified 5MBS speed on a 8bit bus, and allowed up to 7 devices to be daisy chained. The initial application for SCSI-1 was data intensive hard drives. When 16Bit bus was introduced, the term Wide SCSI was used, and when the speed was doubled to 10MBPS, the term FAST SCSI was used. SCSI-2 standards came out in 1993 that specified 10MBPS speed in response to a growing demand of matching faster microprocessors. SCSI-3 soon followed that and increased I/O speed to 20MBPS. In 1996 the UltraSCSI standard was established which increased I/O speed to 40MBPS. The new Ultra-SCSI standard can double I/O speed to 80MBPS.

As the SCSI standard evolved over time from SCSI-1 to Ultra, various SCSI connectors were introduced. The number of pins per connector was increased from 25, 50, 68 to 80. The distance between pin, or the pitch was changed from 2.54mm for regular DB25 (D-Sub) connector to 1.27mm for Half-Pitch (HP) connectors, then reduced again to 0.8mm for Very High Density (VHD) centronics connectors. The radical change in connector size has reduced the size of devices and can better fit space limitations of notebook computers.

Two terminators are needed to terminate a SCSI chain, one at each end. SCSI adapter cards have built-in terminators leaving the last peripheral on the cahin to be terminated. For Non-Differential SCSI which covers the majority of cases, the following are recommended: Use Passive SCSI terminators if the total chain distance is less than 6 feet, and I/O speed is 10MBPS or less. Use Active SCSI termination if SCSI-2 and 3 devices are used, and when the total length of the chain is 20feet or less. Use Force Perfect Terminator (FPT) if total distance is more than 20 feet, more than 5 devices are chained, and when passive or active terminators cannot do the job. Use Differential Termination only when the SCSI card and devices are designed for differential transmission.

The SCSI standard was designed to be backward compatible. This means a SCSI chain can be a mixture of SCSI-1 to Ultra SCSI Devices with I/O speed varying from 5MBPS to 40MBPS. Up to 15 devices can make up a chain that can stretch up to 80 feet using 16 cables of various lengths. The point is, unless the quality of cables are maintained, you are inviting trouble. The higher speed of SCSI devices require more consistency in impedance. To reduce interference, the cable must be double shielded with foil and braided, wires must be twisted pairs, and shielding must be end to end with the metal shiedling of the hood solderd 100% to the connector.

Internal SCSI cable must be made with the same impedance. Shielding is difficult to provide for Flat Ribbon cable, but luckily, the metal computer enclosure provides excellent shielding for the cable.

Hope this has cleared up some mystery behind SCSI.

SCSI Cables Information Profile

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