Categories: PCs and Hardware

Continued from Introduction

Although it's considered a single standard, SCSI actually comes in a variety of types. Among these are SCSI, SCSI-2, SCSI-3, Fast SCSI, Fast/Wide SCSI, Ultra SCSI, Wide Ultra SCSI, Ultra2 SCSI, Wide Ultra2 SCSI, and more. The latest specification is SCSI-3. I'll explain the differences among the different versions very briefly, and I'll spend the rest of this column detailing the process by which the SCSI interface transfers data among devices.

The original SCSI specification, completed in 1986, established a hardware interface for connecting disk drives to computers. The SCSI bus transfer rate in its standard asynchronous (or handshake) mode was roughly 3MB per second over an 8-bit bus. In synchronous (or streaming) mode, the SCSI bus was able to perform at more than 5 MBps. The basis of the interface was and still is the set of commands that control data transfer and communication among devices. The commands were the strength of SCSI, because they made the interface intelligent; but they were also its initial weakness, because there wasn't enough of a standard for the command set to be truly useful to device manufacturers. So the Common Command Set (CCS) was developed to standardize SCSI commands and was accepted as an extension to SCSI.

With the SCSI-2 specification (completed in 1990 and adopted in 1992), the standard moved into its role as a multiple-device interface. Instructions were added for new equipment classes, including CD-ROM drives, optical drives, media changer devices, printers, communications devices, and more. SCSI-2 also introduced two important performance options, Wide SCSI and Fast SCSI. Wide SCSI offered 32-bit transfer by adding a second cable (called a B-cable) between device and adapter. Fast SCSI increased the synchronous-mode clock frequency to 10 MHz, thereby doubling the data-transfer rate to 10 MBps. Combining these options produced Fast/Wide SCSI, whose transfer rate jumped to as high as 40 MBps. SCSI-2 also incorporated CCS instructions and added numerous others to allow effective control of new device types. SCSI-3, the most recent, predicts speeds of more than double that of SCSI-2.

The SCSI-3 specification (drafted in 1996) splits SCSI into a number of standards. These include the Parallel Interface specification, which governs the workings of SCSI cables; the Architectural Model, defining the instructions needed to implement a data transfer; the Primary Commands specification, which sets out the commands for all SCSI devices; and specifications for device types. SCSI-3 also eliminates the need for a second cable for Fast SCSI or Wide SCSI, adds support for fiber-optic cable, and adds new instructions to the command set. SCSI-3 is less of a dramatic departure from SCSI-2 than SCSI-2 was from the original SCSI, but the new standard signals a further maturing of the technology.

Next: Targets and LUNs

Published as Tutor in the 3/10/98 issue of PC Magazine.