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Introduction to SCSI Part 1| Category | | | Summary | | | Body | ::-=Introduction to SCSI Part 1=-::
SCSI stands for Small Computer Systems Interface. Most people pronounce the acronym as skuzzy although some people will say the initials separately. Technically, SCSI is not simply an interface, but rather a system bus. Controllers on each device attached to the SCSI bus cooperate making data transfer very efficient. Today, SCSI is used almost exclusively in high-end systems (or on the desktops and dream-desktops of geeks every where :) ). SCSI draws computer nuts and those requiring speed, reliability, and power. What keeps SCSI from being embraced by the average user is cost. SCSI hard drives often carry hefty price tags (because of their cutting edge technology that these souped-up drives contain) compared with their cheaper IDE counterparts.
SCSI has had a long and interesting history. The ball got rolling on SCSI way back in 1979 (Kozierok). A company called Shugart Associates (which was led by the man who would later found Seagate Technology) developed a system called Shugart Associates Systems Interface the acronym for this system was obviously SASI (Kozierok). In order to make SASI more widely used, Shugart decided to make it into an ANSI standard. In 1981, the process that led to the creation of the first SCSI standard was begun (Kozierok). (Because it was a standard, SASI was obviously not an appropriate acronym.) From the earliest days SCSI was developed around hard disks, however SCSI was a system interface, therefore other devices were built into the standards.
Despite SCSI's respectable old age, very few hard drives have built-in support for the interface. This fact combined with the expensive nature of SCSI devices prevents widespread usage of the SCSI interface. Another reason for the lack of acceptance for SCSI is the widespread confusion over the various flavors of SCSI. In this tutorial, I will try and make the differences in these types of SCSI clear.
SCSI-1
SCSI-1 is the first SCSI standard that came out of SASI. In 1986, ANSI approved this standard as X3.131-1986 (Kozierok). Even though the SCSI-1 as a standard is archaic, it did lay the groundwork for what SCSI would become. Many of the basic elements of SCSI are defined in SCSI-1: cable length, signaling characteristics, commands and transfer modes (Kozierok). SCSI-1 was implemented using an 8-bit bus, and had a 5 MB/s maximum transfer rate (Kozierok). This standard allowed a maximum of eight devices on the bus because each device is addressed using one of the 8 data lines (Westergren). The original SCSI used single-ended signaling (Octane). This method allowed the bus (cables and internal wiring inside of devices) to be up to six meters long (Octane). Despite the many good parts of SCSI-1, there were problems. Most of these problems were compatibility issues caused by various manufactures. Problems would be ironed out in future SCSI versions (Kozierok).
SCSI-2
The second installment in the SCSI saga was started in 1985 (Kozierok), a year before the SCSI-1 standard was finalized. In 1990 SCSI-2 was released, but was later retracted for further work (Kozierok). In 1994, the SCSI-2 standard was released in its final form and was designed to fix many of the problems with SCSI-1 and is the first kind where there are flavors, or sub-types defined as part of the SCSI standard (Kozierok). SCSI-2 brought numerous changes to the computing world. Cable and connector definitions were improved adding more detail to the past definitions. A useful feature added in SCSI-2 was command queuing. This allows messages to wait in line before they are sent across the bus. Multiple commands can be stored in RAM rather than having to be sent in order only when they can be used. Another improvement offered in SCSI 2 was the addition of more device types that could be attached to the SCSI bus.
To keep signals from bouncing back and forth endlessly along the bus and wrecking havoc, active termination was defined. Another performance improvement came with the addition of differential signaling (high-voltage differential). Basically, this method for sending signals improved their strength and allowed for longer physical bus lengths.
The first of the SCSI-2 flavors is known as Fast SCSI. This is essentially SCSI on steroids. With a bus speed of 10 MHz (2x regular SCSI), transfer rates reached 10 MB/s (up from 5MB/s) using 8-bit (regular/narrow) SCSI cables
The second of the three flavors defined in SCSI-2 is Wide SCSI. The width of the SCSI bus was increased to 16 or 32 bits. With this improvement, more data could be sent across the bus at the same time. The original 8-bit SCSI is often referred to as Narrow SCSI. Wide SCSI also can support up to 16 individual devices on a single bus.
A combination of Fast SCSI and Wide SCSI is known by the highly original name of Fast-Wide SCSI Taking advantage of Fast SCSI's bus speed over Wide SCSI's bus width allows a transfer rate of 20 MB/s. Fast-Wide SCSI, as with Wide SCSI can support up to 16 devices.
Sources
Kozierok, Charles M. Small Computer Systems Interface (SCSI). The PC Guide. 17 Apr. 2001. 28 July 2003 http://www.pcguide.com/ref/hdd/if/scsi/index.htm.
Octane. SCSI Technology Primer. ARS Technica 18 July 2003 http://arstechnica.com/paedia/s/scsi-1.html.
Westergren, Markus and Mattias Sandgren. SCSI and IDE: Overview and Comparison. Umeå University. 31 Jan. 2001. 18 July 2003 http://www.acc.umu.se/~sagge/scsi_ide/.
Other Sources of In-depth Useful Information Lohmeyer, John. SCSI Storage Interfaces. T10 Technical Committee. 27 June 2003. 18 July 2003 http://www.t10.org/
SCSI Trade Association. 18 July 2003 http://www.scsita.org/
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Written by Darkwater685 (Feb 21 2004)
Member of Knowledge-Bank Tutorial Writers
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This article was imported from the CyberArmy University site. (original author: PixieLuv)
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