SCSI

SCSI stands for "Small Computer System Interface", and is a standard interface and command set for transferring data between devices on both internal and external computer buses. SCSI is usually pronounced "scuzzy".

SCSI is most commonly used for hard disks and tape storage devices, but also connects a wide range of other devices, including scanners, printers, CD-ROM drives, CD recorders, and DVD drives. In fact, the entire SCSI standard promotes device independence, which means that theoretically SCSI can be used with any type of computer hardware.

Since its standardization in 1986, SCSI has been commonly used in the Apple Macintosh and Sun Microsystems computer lines. It has never been popular in the IBM PC world, due to the lower cost and adequate performance of its ATA hard disk standard. The introduction of USB, FireWire, and ATAPI made SCSI a relatively unattractive proposition on PC due to its high cost and rising complexity.

At this time, SCSI is popular on high-performance workstations, servers, and high-end peripherals; and RAID arrays on servers almost always use SCSI hard disks. Desktop computers and notebooks more typically use the ATA/IDE or the newer SATA interfaces for hard disks, and USB or FireWire connections for external devices.

History

In 1979, Shugart Associates introduced an interface called SASI (Shugart Associates System Interface). At the same time, NCR Corporation's Peripherals division (now Engenio), had developed a more sophisticated product called BYSE, and was developing an ASIC to implement it. In late 1981, NCR and Shugart agreed to merge the best features of the two solutions, and to jointly promote the concept as an ANSI standard. After several committee meetings and after a number of other companies decided to adopt the combined standard, it received the new name "SCSI." In 1986, with SCSI already in widespread use, ANSI approved the SCSI spec (as X3.131-1986). Since then, SCSI has developed as an industry-wide standard, capable of being applied to virtually any computer system (there were even SCSI implementations for the venerable Commodore 64 and Apple II home computers). The first working SCSI ASIC was donated by NCR to the Smithsonian Museum.

Standards

SCSI has the unfortunate distinction of having among the most confusing set of standards names of anything in the computer field, with the probable exception of 3D video cards. There are a dozen SCSI interface names, most with ambiguous wording (Quick, which is faster: Wide SCSI or Fast SCSI?); three SCSI standards, each of which has a collection of modular, optional features; several different connector types; and three different types of voltage signalling. The leading SCSI card manufacturer, Adaptec, has manufactured over 100 varieties of SCSI cards over the years. In actual practice, many experienced technicians simply refer to SCSI devices by their bus bandwidth(ie SCSI 320 or SCSI 160) in Megabytes per second.

SCSI has evolved since its introduction. Before summarizing the evolution, a distinction should be made between the terminology used in the SCSI standard itself, as promulgated by the T10 committee of INCITS, and common parlance, as codified by the SCSI trade association, SCSITA.

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".

No version of the standard has ever specified what kind of connector should be used. The connectors used by vendors have tended to evolve over time. Although SCSI-1 devices typically used bulky Blue Ribbon ("Centronics") connectors, and SCSI-2 devices typically "Mini-D" connectors, it is not correct to refer to these as "SCSI-1" and "SCSI-2" connectors.

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
SCSI	8 bits	5 MHz	5 MB/s	6 m	8
Fast SCSI	8 bits	10 MHz	10 MB/s	1.5-3 m	8
Wide SCSI	16 bits	10 MHz	20 MB/s	1.5-3 m	16
Ultra SCSI	8 bits	20 MHz	20 MB/s	1.5-3 m	5-8
Ultra Wide SCSI	16 bits	20 MHz	40 MB/s	1.5-3 m	5-8
Ultra2 SCSI	8 bits	40 MHz	40 MB/s	12 m	8
Ultra2 Wide SCSI	16 bits	40 MHz	80 MB/s	12 m	16
Ultra3 SCSI	16 bits	40 MHz DDR	160 MB/s	12 m	16
Ultra-320 SCSI	16 bits	80 MHz DDR	320 MB/s	12 m	16
SSA	1 bit	400 MBit	80 MB/s	25 m	96
FC-AL	1 bit	2 GBit	200 MB/s per direction; full duplex	?	127
iSCSI	Dependent upon IP network				??
SAS 3Gbit	1 bit	N/A	375 MB/s per direction; full duplex	10 m	16,256 (128 per expander)

SCSI-1

The original standard that was derived from SCSI and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit parallel 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 25 meters.

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 single-ended 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 (SAS) break from the traditional parallel SCSI standards and perform data transfer via serial communications. Although much of the documentation of SCSI talks about the parallel interface, most contemporary development effort is on serial SCSI.

SCSI command protocol

In addition to many different hardware implementations, the SCSI standards also include a complex set of command protocol definitions. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with iSCSI and serial SCSI.

In SCSI terminology, communication takes place between an initiator and a target. The initiator sends a command to the target which then responds. SCSI commands are sent in a Command Descriptor Block (CDB). The CDB consists of a one byte operation code followed by five or more bytes containing command-specific parameters.

At the end of the command sequence the target returns a Sense Code byte which is usually 00h for success, 02h for an error (called a Check Condition), or 08h for busy. When the target returns a Check Condition in response to a command, the initiator usually then issues a SCSI Request Sense command in order to obtain a Key Code Qualifier (KCQ) from the target. The Check Condition and Request Sense sequence involves a special SCSI protocol called a Contingent Allegiance Condition.

There are 4 categories of SCSI commands: N (non-data), W (writing data from initiator to target), R (reading data), and B (bidirectional). There are about 60 different SCSI commands in total, with the most common being:

• Test unit ready - "ping" the device to see if it responds
• Inquiry - return basic device information
• Request sense - give any error codes from the previous command
• Send diagnostic - run a simple self-test, or a specialised test defined in a diagnostic page
• Start/Stop unit
• Read capacity - return storage capacity
• Format unit
• Read (4 variants)
• Write (4 variants)
• Log sense - return current information from log pages
• Mode sense - return current device parameters from mode pages
• Mode select - set device parameters in a mode page

Each device on the SCSI bus is assigned at least one logical unit number (LUN). Simple devices have just one LUN, more complex devices may have multiple LUNs. A storage device consists of a number of logical blocks, usually referred to by the term Logical Block Address (LBA). A typical LBA equates to 512 bytes of storage.

The usage of LBAs has evolved over time and so four different command variants are provided for reading and writing data. The Read(6) and Write(6) commands contain a 21-bit LBA address. The Read(10), Read(12), Read Long, Write(10), Write(12), and Write Long commands all contain a 32-bit LBA address plus various other parameter options.

Compatibility

SCSI devices are generally backward-compatible. For example, 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).

As new SCSI connectors have been introduced, design features have been included to avoid accidental mis-plugging of incompatible devices. For example it is not possible to plug a parallel SCSI disk into an FC-AL backplane, nor to connect a cable between an SSA initiator and an FC-AL enclosure. However there are some compatibility issues with parallel SCSI busses which are described in the rest of this section.

Ultra-2, ultra-160 and ultra-320 devices may be freely mixed on the parallel LVD bus with no compromise in performance, as the host adapter will negotiate the operating speed and bus management requirements for each device. Single-ended devices should not be attached to the LVD bus, as doing so will force all devices to run at the slower single-ended speed. Support for single-ended interfaces has been deprecated in the SPI-5 standard (which describes Ultra-640), so future devices may not be electrically backward compatible.

Modern Single Connector Attachment (SCA) parallel SCSI devices may be connected to older controller/drive chains by using SCA adapters. Although these adapters often have auxiliary power connectors, use caution: it is possible to destroy the drive by connecting external power. Always try the drive without auxiliary power first.

Each SCSI device (including the computer's host adapter) must be configured to have a unique SCSI ID on the bus. Also, any parallel SCSI bus must be terminated at both ends with the correct type of 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 parallel SCSI installations.

It is possible to convert a wide parallel bus to a narrow one, with wide devices closer to the adapter. To do this properly requires a cable which terminates the wide part of the bus. This is sometimes referred to as a cable with high-9 termination. Specific commands allow the host to determine the active width of the bus. This arrangement is discouraged.

SCSI device identification

In serial SCSI installations there is an automated process of "discovery" of the IDs. SSA initiators "walk the loop" to determine what devices are there and then assign each one a 7-bit "hop-count" value. FC-AL initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN (World Wide Name). These discovery processes occur at power-on time and also if the bus topology changes later, for example if an extra device is added.

For parallel SCSI busses, the initiator (controller) SCSI ID is usually set by a physical jumper or switch. The target (disk-drive) SCSI IDs are either set by physical jumpers or by control signals which vary for each connector on a backplane.

SCSI enclosure services

In larger SCSI servers, the disk-drive devices are housed in an intelligent enclosure that supports SCSI Enclosure Services (SES). The initator can communicate with the enclosure using a specialised set of SCSI commands to access power, cooling, and other non-data characteristics.

External links

• T10 Technical Committee (SCSI standards)
• SCSITA terminology
• "Storage Cornucopia" SCSI links, maintained by a consultant
• SCSI/iSCSI/RAID/SAS Information Sheet
• SCSI basics
• WWW Virtual Library for SCSI