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SCSI Tutorial(Information compiled from internal and external sources. Wikipedia.org, etc.)
What is SCSI?SCSI stands for "Small Computer System Interface", and is a standard interface and command set for transferring data between devices on a computer bus. SCSI is pronounced "scuzzy" when spoken aloud.
What is SCSI used for?SCSI is most commonly used for hard disks and tape storage devices, but also connects a wide range of other devices, including scanners, CD-ROM drives, CD recorders, and DVD drives. In fact, the entire SCSI standard promotes device independence, which means that theoretically anything can be made SCSI.
Different SCSI Standards
- SCSI 1
- SCSI 2
- SCSI 3
- Ultra 640
Starting with SCSI-3, the SCSI standard has been maintained as a loose collection of standards, each defining a certain piece of the SCSI architecture, and bound together by the SCSI Architectural Model. This change divorces SCSI's various interfaces from the command set, allowing devices that support SCSI commands to use any interface (including ones not otherwise specified by T10), and also allowing the interfaces that are defined by T10 to develop on their own terms. This change is also why there is no "SCSI-4".
SCSI 1The original standard that was derived from SASI and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit bus (with parity), running asynchronously at 3.5 MB/s or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 meters (just under 20 feet -- compare that to the 18 inch (0.45 meter) limit of the ATA interface). A variation on the original standard included a high-voltage differential (HVD) implementation whose maximum cable length was many times that of the single-ended versions.
SCSI 2This standard was introduced in 1989 and gave rise to the Fast SCSI and Wide SCSI variants. Fast SCSI doubled the maximum transfer rate to 10 MB/s and Wide SCSI doubled the bus width to 16 bits on top of that (to reach 20 MB/s). However, these improvements came at the minor cost of a reduced maximum cable length to 3 meters. SCSI-2 also specified a 32-bit version of Wide SCSI, which used 2 16-bit cables per bus; this was largely ignored by SCSI device makers because it was expensive and unnecessary, and was officially retired in SCSI-3.
SCSI 3Before Adaptec and later SCSITA codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI and fast-20 SCSI, were introduced in 1992. The bus speed doubled again to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide. The maximum cable length stayed at 3 meters but ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems).
Ultra 2This standard was introduced c. 1997 and featured a low voltage differential (LVD) bus. For this reason ultra-2 is sometimes referred to as LVD SCSI. Using LVD technology, it became possible to allow a maximum bus cable length of 12 meters (almost 40 feet!), with much greater noise immunity. At the same time, the data transfer rate was increased to 80 MB/s. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by ultra-3 (ultra-160) SCSI.
Ultra 3Also known as Ultra-160 SCSI and introduced toward the end of 1999, this version was basically an improvement on the ultra-2 standard, in that the transfer rate was doubled once more to 160 MB/s by the use of double transition clocking. Ultra-160 SCSI offered new features like cyclic redundancy check (CRC), an error correcting process, and domain validation.
Ultra 320This is the ultra-160 standard with the data transfer rate doubled to 320 MB/s. Nearly all new SCSI hard drives being manufactured at the time of this writing (October 2003) are actually ultra-320 devices.
Ultra 640Ultra-640 (otherwise known as Fast-320) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. Ultra-640 doubles the interface speed yet again, this time to 640 MB/s. Ultra640 pushes the limits of LVD signaling; the speed limits cable lengths drastically, making it impractical for more than one or two devices. Because of this, most manufacturers have skipped over Ultra640 and are developing for Serial Attached SCSI instead.
iSCSIiSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged. iSCSI advocates project the iSCSI standard, an embedding of SCSI-3 over TCP/IP, as displacing Fibre Channel in the long run, arguing that Ethernet data rates are currently increasing faster than data rates for Fibre Channel and similar disk-attachment technologies. iSCSI could thus address both the low-end and high-end markets with a single commodity-based technology
Serial SCSIThree recent versions of SCSI SSA, FC-AL and Serial Attached SCSI break from the traditional parallel SCSI standards and perform data transfer via serial communications.
SCSI CompatibilitySCSI devices are generally backward-compatible, i.e., it is possible to connect an ultra-3 SCSI hard disk to an ultra-2 SCSI controller and use it (though with reduced speed and feature set).
Each SCSI device (including the computer's host adapter) must be configured to have a unique SCSI ID on the bus. Also, the SCSI bus must be terminated with a terminator. Both active and passive terminators are in common use, with the active type much preferred (and required on LVD buses). Improper termination is a common problem with SCSI installations.
Terminating SCSISCSI buses must always be terminated at both ends.
There are four ways to terminate:
|Interface||Bus width||Clock speed||Bus bandwidth||Max. cable length||Max. number of devices|
|SCSI||8 bits||5 MHz||5 MB/s||6m||8|
|Fast SCSI||8 bits||10 MHz||10 MB/s||1.5-3m||8|
|Wide SCSI||16 bits||10 MHz||20 MB/s||1.5-3m||16|
|Ultra SCSI||8 bits||20 MHz||20 MB/s||1.5-3m||5-8|
|Ultra Wide SCSI||16 bits||20 MHz||40 MB/s||1.5-3m||5-8|
|Ultra2 SCSI||8 bits||40 MHz||40 MB/s||12m||8|
|Ultra2 Wide SCSI||16 bits||40 MHz||80 MB/s||12m||16|
|Ultra3 SCSI||16 bits||40 MHz DDR||160 MB/s||12m||16|
|Ultra-320 SCSI||16 bits||80 MHz DDR||320 MB/s||12m||16|
|SSA||1 bit||400 MBit||80 MB/s||25m||96|
|FC-AL||1 bit||2GBit||200 MB/s|
per direction; full duplex
|iSCSI||Dependent upon IP network||??|
|SAS 3Gbit||1 bit||N/A||375 MB/s|
per direction; full duplex
|10m||16,256 (128 per expander)|
Centronics 50 Male
Centronics 50 Female
At one time this was one of the most common SCSI connectors. Used for scsi-1 applications: older scanners, controllers, external scsi device cases. The Centronics 50 connector has 50-pins arranged in two rows one on top of the other. The top row has 25 pins and the lower row has 25 pins. Used mostly with older 5 MB SCSI-1 Systems. Often called Low-Density SCSI-1 connector. Most SCSI SLOW (5 Mbyte/sec) computers and host adapters use the Centronics type 50-pin connector. Also some 8-bit Fast computers and host adapters.
Half Pitch 50 Male (Micro DB50 Male)
Used for scsi-2 applications: scanner, removable storage drive, controller, external cdr/cdrw. The Micro DB50 connector has 50-pins arranged in two rows one on top of the other. The top row has 25 pins and the lower row has 25 pins. Most 8-bit SCSI FAST (up to 10 Mbytes/sec) computers and host adapters use this 50-pin High-Density connector. Commonly used on Apples and Mac, and some older Sun 8-bit workstations. This connector is seen increased use on Scanners and Iomega Zip Drives.
Used for parallel, serial or scsi applications: modem, null modem, laplink, printer, scanner, removable storage drive, Apple scsi. The DB25 connector has 25-pins arranged in two rows one on top of the other. The top row has 13 pins and the lower row has 12 pins.
Used for early scsi applications such as older Sun Sparcstations. This SCSI connector was also used by DEC, DG and HP. The DB50 connector has 50-pins arranged in three rows one on top of the other. The top row has 17 pins, the middle row has 16 pins and the lower row has 17 pins.
HP60 Centronics Male
Used for scsi applications: IBM RS-6000. The Micro Centronics 60 connector has 60-pins arranged in two rows one on top of the other. The top row has 30 pins and the lower row has 30 pins.
HP Centronics 68 Male
Used for scsi applications: IBM RS-6000. The Micro Centronics 68 connector has 68-pins arranged in two rows one on top of the other. The top row has 34 pins and the lower row has 34 pins.
Probably the most common SCSI connector used today. Used on all SCSI Wide applications and some old DEC single-ended SCSI this connector provides a highly secure connection. Used for scsi-3 applications: scanner, removable storage drive, controller, external cdr/cdrw, ultra/2. The HP68 connector has 68-pins arranged in two rows one on top of the other. The top row has 34 pins and the lower row has 34 pins.
.8mm Centronics 68 Male
Used for scsi-3 applications: RAID. Also called a Very High-Density connector Interface (VHDC1) or 0.8mm connector. It's similar to the SCSI-3 68 pin connector in that it has 68 pins but a much smaller footprint. The VHDCI.8mm 68-pin connector has 68-pins arranged in two rows one on top of the other. The top row has 34 pins and the lower row has 34 pins.
Used for scsi applications: Apple PowerBook. A non-standard connector created by Apple for reduced mounting space on their PowerBook notebooks. Not suitable for multiple SCSI devices or long cables because there are only 30 pins. The HDI-30 connector has 30-pins arranged in five rows one on top of the other.
Used for internal scsi-1/scsi-2 applications: hard drive, cd-rom, removable storage drive. 50 pin insulation displacement connector (IDC) used on ribbon cables for internal SCSI cabling. Female connector used on cables, male on device or host adapter.
Internal 68 Pin Male
Used for internal scsi-3/ultra2/lvd applications: hard drive, cd-rom, removable storage drive. Used on the P-cable for 16-bit WIDE SCSI. 68-pin version of the 50-pin micro-D high-density connector. Male connector used on cables, female on device or host adapter.
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