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Introduction to SCSI technology.

Introduction to SCSI technology


SCSI (Small Computer Standard Interface), pronouced "scuzzy", enable you to connect your computer to wide range of peripherals such as hard disks, CD-ROMS, printers and scanners. SCSI is the way peripherals communicate with the main processor of your computer.
Three advantages of SCSI are its speed, compatibility, and expansibility. In terms of speed, the fastest part of a computer is the processor, and SCSI gets its information in and out of the processor fast. And it keeps getting faster.


Highly Compatible

SCSI is highly compatible. All Macintosh® computers come with a SCSI port, and you can order many PCs with a SCSI port. If your computer has a SCSI port, you can get another device with the same type of SCSI, plug it in, and the new device should work. SCSI also makes it easy to expand. If you want to add an additional hard drive to your computer, just connect via the SCSI port. If you want to add another CD-ROM tower or a printer, just daisychain the additional devices up to seven or more.


UltraSCSI

The history of computers has been shaped by getting information in and out of the processor faster, and the history of SCSI has been shaped by trying to open a wider path in and out of the processor. The path, or interface, can become an obstruction that slows down your data traffic. As microprocessors get faster, hard drives get larger and high-speed becomes a necessity. The solution? UltraSCSI.


How to UltraSCSI work?

Until recently, Narrow and Wide SCSI set the standards for speed at 10 and 20-MBps respectively. UltraSCSI doubles those speeds - Narrow SCSI goes from 10 to 20-MBps and Wide SCSI goes from 20 to 40-MBps. Surprisingly, the new SCSI speed limit isn't reached by introducing radical new technology but by making subtle improvements in the delays and hold times of the chip, adding up to great gains in speed. The improved times are the result of new semiconductor technology in the SCSI chip sets.


UltraSCSI is backward compatible

Because the technology isn't radically altered, UltraSCSI remains compatible with all previous SCSI standards, so it's backward compatible. And since the internal drivers, firmware, and most of the drive circuitry remain the same as your existing SCSI's, you don't have to change your current network topology. In fact, when you switch to UltraSCSI, your peripherals won't be affected at all - all you'll see is better performance.


Some background on SCSI

UltraSCSI is the latest step in a long line of SCSI adaptations and improvements. And although you could fill an encyclopedia documenting the improvements and revisions between the different levels of SCSI, the average SCSI user only cares about speed. SCSI-1 supports transfer rates of up to 5 MBps. SCSI-2, approved in 1994, usually supports 10 MBps but can go up to 20-MBps when combined with Fast and Wide SCSI. SCSI-3 is found in many high-end systems and has transfer rates of 40-MBps.
At 20 and 40-MBps, UltraSCSI leaves all previous standards behind. You don't need to combine other forms of SCSI to get top speeds anymore. Just imagine a two-lane bridge on a four-lane highway: If there's very little traffic, things flow smoothly. When traffic increases, you start to get a backup at the bridge. But if you install a wider bridge with the same number of lanes as the highway, there shouldn't be any obstruction at all. On the SCSI bus, UltraSCSI opens up more lanes for traffic.


What's new? SCSI-5

SCSI-5, a new type of connector interfac, is also called VHDCI (Very High-Density Connector Interface) or a 0.8mm connector. It's similar to the SCSI-3 MD68 connector in that it has 68 pins and a much smaller footprint.
SCSI-5 is designed for next-generation SCSI connections where high-performance is a key requirement. It's expected to become the connector of choice for advanced SCSI applications, such as UltraSCSI Fast-40 and the new Low-Voltage Differential Signal (LVDS) technology.

SCSI Terminology
SCSI Bus
Width (bits)
SCSI Bus
Speed (MBps)
SCSI-1
8
5
Fast SCSI
8
10
Fast Wide SCSI
16
20
UltraSCSI
8
20
Wide UltraSCSI
16
40
Ultra2 SCSI
8
40
Wide Ultra2 SCSI
16
80


SCSI Name
Max. Bus Length, Metres
Single-Ended
Differential
LVD
Max. Devices
SCSI-1
6 m
25 m
12 m
8
Fast SCSI
3 m
25 m
12 m
8
Fast Wide SCSI
3 m
25 m
12 m
16
UltraSCSI
1.5 m
25 m
12 m
8
UltraSCSI
3 m
   
4
Wide UltraSCSI  
25 m
12 m
16
Wide UltraSCSI
1.5 m
   
8
Wide UltraSCSI
3 m
   
4
Ultra2 SCSI    
12 m
8
Wide Ultra2 SCSI    
12 m
16
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