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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

As of 2003, there have only been three SCSI standards: SCSI-1, SCSI-2, and SCSI-3. All SCSI standards have been modular, defining various capabilities which manufacturers can include or not. Individual vendors and SCSITA have given names to specific combinations of capabilities. For example, the term "Ultra SCSI" is not defined anywhere in the standard, but is used to refer to SCSI implementations that signal at twice the rate of "Fast SCSI." Such a signalling rate is not compliant with SCSI-2 but is one option allowed by SCSI-3. Similarly, no version of the standard requires low-voltage-differential (LVD) signalling, but products called Ultra-2 SCSI include this capability. This terminology is helpful to consumers, because "Ultra-2 SCSI" device has a better-defined set of capabilities than simply identifying it as "SCSI-3."

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 1

The 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 2

This 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 3

Before 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 2

This 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 3

Also 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 320

This 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 640

Ultra-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.

iSCSI

iSCSI 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 SCSI

Three 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 Compatibility

SCSI 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 SCSI

SCSI buses must always be terminated at both ends.

There are four ways to terminate: The mainstream implementations of SCSI (in chronological order) are as follows, using common parlance:
SCSI interface overview
Interface Bus width Clock speed Bus bandwidth Max. cable length Max. number of devices
SCSI8 bits5 MHz5 MB/s6m8
Fast SCSI8 bits10 MHz10 MB/s1.5-3m8
Wide SCSI16 bits10 MHz20 MB/s1.5-3m16
Ultra SCSI8 bits20 MHz20 MB/s1.5-3m5-8
Ultra Wide SCSI16 bits20 MHz40 MB/s1.5-3m5-8
Ultra2 SCSI8 bits40 MHz40 MB/s12m8
Ultra2 Wide SCSI16 bits40 MHz80 MB/s12m16
Ultra3 SCSI16 bits40 MHz DDR160 MB/s12m16
Ultra-320 SCSI16 bits80 MHz DDR320 MB/s12m16
SSA1 bit400 MBit80 MB/s25m96
FC-AL1 bit2GBit200 MB/s
per direction; full duplex
?127
iSCSIDependent upon IP network??
SAS 3Gbit1 bitN/A375 MB/s
per direction; full duplex
10m16,256 (128 per expander)


SCSI Connectors


Centronics 50 Male
Centronics 50 Male

Centronics 50 Female
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.
HP50 Male
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.
DB25 Male
DB25 Male

DB25 Female
DB25 Female

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.
DB50 Male
DB50 Male

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
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
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.
HP68 Male
HP68 Male

HP68 Female
HP68 Female

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
.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.
HDI-30 Male
HDI-30 Male

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.
IDC50 Male
IDC50 Male

IDC50 Female
IDC50 Female

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 Male
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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