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Biography

Small Computer System Interface ( SCSI , ) is a set of standards for physically connecting and transferring data between computers and peripheral device s. The SCSI standards define SCSI command|commands , protocols, and electrical and optical Interface (computer science)|interfaces . SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD-ROM|CD optical drive|drive s, although not all controllers can handle all devices. The SCSI standard defines command sets for specific SCSI Peripheral Device Type|peripheral device types ; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.

SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host. SCSI uses handshake signals between devices, SCSI-1, SCSI-2 have the option of parity error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a Cyclic redundancy check|CRC32 checksum. The SCSI protocol defines communication from host to host, host to a peripheral device, peripheral device to a peripheral device. However most peripheral devices are exclusively SCSI target s, incapable of acting as SCSI initiator s—unable to initiate SCSI transactions themselves. Therefore peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications. The Symbios Logic 53C810 chip is an example of a Peripheral Component Interconnect|PCI host interface that can act as a SCSI target.

History

SCSI was derived from "SASI", the " Shugart Associates System Interface", developed c. 1978 and publicly disclosed in 1981.ANSI Draft SASI Standard, Rev D, February 17, 1982, pg. ii states, "9/15/81 first presentation to ANSI committee X3T9-3 (2 weeks following announcement in Electronic Design)." A SASI controller provided a bridge between a hard disk drive's low-level interface and a host computer, which needed to read blocks of data. SASI controller boards were typically the size of a hard disk drive and were usually physically mounted to the drive's chassis. SASI, which was used in mini- and early microcomputers, defined the interface as using a 50-pin flat ribbon connector which was adopted as the SCSI-1 connector. SASI is a fully compliant subset of SCSI-1 so that many, if not all, of the then-existing SASI controllers were SCSI-1 compatible.ANSI SCSI Standard, X3.131-1986, June 23, 1986, 2nd, foreword.

Larry Boucher is considered to be the "father" of SASI and SCSI due to his pioneering work first at Shugart Associates and then at Adaptec .

Until at least February 1982, ANSI developed the specification as "SASI" and "Shugart Associates System Interface;"Working document for ANSI meeting on March 3, 1982, "SASI SHUGART ASSOCIATES SYSTEM INTERFACE, Revision D, February 17, 1982" however, the committee documenting the standard would not allow it to be named after a company. Almost a full day was devoted to agreeing to name the standard "Small Computer System Interface," which Boucher intended to be pronounced "sexy", but ENDL's Dal Allan pronounced the new acronym as "scuzzy" and that stuck.

A number of companies such as NCR Corporation , Adaptec and Optimem were early supporters of the SCSI standard. The NCR facility in Wichita, Kansas is widely thought to have developed the industry's first SCSI chip; it worked the first time.

The "small" part in SCSI is historical; since the mid-1990s, SCSI has been available on even the largest of computer systems.

Since its standardization in 1986, SCSI has been commonly used in the Amiga , Macintosh|Apple Macintosh and Sun Microsystems computer lines and PC server systems. Apple started using Parallel ATA (also known as IDE ) for its low-end machines with the Macintosh Quadra 630 in 1994, and added it to its high-end desktops starting with the Power Macintosh G3 in 1997. Apple dropped on-board SCSI completely (in favor of IDE and FireWire ) with the (Blue & White) Power Mac G3 in 1999. Sun has switched its lower end range to Serial ATA (SATA). SCSI has never been popular in the low-priced IBM PC world, owing to the lower cost and adequate performance of ATA hard disk standard. However, SCSI drives and even SCSI RAID s became common in PC workstations for video or audio production.

Recent versions of SCSI – Serial Storage Architecture (SSA), SCSI-over- Fibre Channel Protocol (FCP), Serial Attached SCSI (SAS), Automation/Drive Interface - Transport Protocol (ADT), and USB Attached SCSI (UAS) – 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. Serial SCSI has a number of advantages over parallel SCSI: faster data rates, hot swapping (some but not all parallel SCSI interfaces support it), and improved fault isolation. The primary reason for the shift to serial interfaces is the clock skew issue of high speed parallel interfaces, which makes the faster variants of parallel SCSI susceptible to problems caused by cabling and termination.
iSCSI preserves the basic SCSI paradigm , especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP .

SCSI is popular on high-performance workstations and servers. RAIDs on servers have almost always used SCSI hard disks, though a number of manufacturers now offer SATA-based RAID systems as a cheaper option. Instead of SCSI, desktop computers and notebooks more typically use Serial ATA|ATA interfaces for internal hard disk drives, and USB, eSATA, and FireWire connections for external devices.

SCSI interfaces had become impossible to find for laptop computers. Adaptec had years before produced PCMCIA SCSI interfaces, but when PCMCIA was superseded by the ExpressCard discontinued their PCMCIA line without supporting ExpressCard. Ratoc produced USB and Firewire to SCSI adaptors, but ceased production when the integrated circuit s required were discontinued. Drivers for existing PCMCIA interfaces were not produced for newer operating system s.

Interfaces

SCSI is available in a variety of interfaces. The first, still very common, was parallel SCSI (now also called SPI), which uses a parallel communications|parallel electrical bus design. As of 2008, SPI is being replaced by Serial Attached SCSI (SAS), which uses a Serial communications|serial design but retains other aspects of the technology. Many other interfaces which do not rely on complete SCSI standards still implement the #SCSI command protocol|SCSI command protocol ; others (such as iSCSI ) drop physical implementation entirely while retaining the SCSI architectural model . iSCSI, for example, uses TCP/IP as a transport mechanism.

SCSI interfaces have often been included on computers from various manufacturers for use under Microsoft Windows , Mac OS , Unix , Commodore Amiga and Linux operating systems, either implemented on the motherboard or by the means of plug-in adaptors. With the advent of Serial Attached SCSI|SAS and Serial ATA|SATA drives, provision for SCSI on motherboards is being discontinued. A few companies still market SCSI interfaces for motherboards supporting PCIe and PCI-X.

Parallel SCSI

Interface	Alternative names	Specification documentSpecifications are maintained by the T10 subcommittee of the International Committee for Information Technology Standards .	Connector	Width (bits)	MHz for SPI, or bitrate (per second) for serial interfaces	Maximum						Electrical
Interface	Alternative names		Connector	Width (bits)	MHz for SPI, or bitrate (per second) for serial interfaces	Bandwidth ( MB/ s )In megabyte s per second, not megabit s per second	Bandwidth ( Mbit/ s )In megabit s per second, not megabyte s per second	Length (single ended)For daisy-chain designs, length of bus, from end to end; for point-to-point, length of a single link	LVD LVD cabling may be up to 25m when only a single device is attached to the host adapter	HVD	DevicesIncluding any host adapters (i.e., computers count as a device)	O & #93;	V & #93;
SCSI-1	Narrow SCSI	The SCSI-1 specification has been withdrawn and is superseded by SCSI-2. The SCSI-3 SPI specification has been withdrawn and is superseded by SPI-2. The SCSI-3 SPI-3 and SPI-4 specifications have been withdrawn and are superseded by SPI-5.	IDC50; Centronics C50	8	5& nbsp;MHz	5 MB/ s	40 Mbit/ s	6 m	NA	25m	8	nowrap> SE 90 ± 6 Ocite web	5
Fast SCSI		SCSI-2 (1994)	IDC50; Centronics C50	8	10& nbsp;MHz	10 MB/ s	80 Mbit/ s	3 m	NA	25m	8	nowrap> SE 90 ± 6 O	5
Fast-Wide SCSI		SCSI-2; SCSI-3 SPI (1996)	2 x 50-pin (SCSI-2); 1 x 68-pin (SCSI-3)	16	10& nbsp;MHz	20 MB/ s	160 Mbit/ s	3 m	NA	25m	16	nowrap> SE 90 ± 6 O	5
Ultra SCSI	Fast-20	SCSI-3 SPI	IDC50	8	20& nbsp;MHz	20 MB/ s	160 Mbit/ s	1.5 m	NA	25m	8	nowrap	5
Ultra SCSI	Fast-20	SCSI-3 SPI	IDC50	8	20& nbsp;MHz	20 MB/ s	160 Mbit/ s	3 m	NA	NA	4	nowrap	5
Ultra Wide SCSI	rowspan=3	SCSI-3 SPI	68-pin	16	20& nbsp;MHz	40 MB/ s	320 Mbit/ s	NA	NA	25m	16	nowrap	5
	1.5 m							NA	NA	8
	3 m							NA	NA	4
Ultra2 SCSI	Fast-40	SCSI-3 SPI-2 (1997)	50-pin	8	40& nbsp;MHz	40 MB/ s	320 Mbit/ s	NA	12m	25m	8	nowrap> Low-voltage differential signaling
Ultra2 Wide SCSI		SCSI-3 SPI-2	SCA / SCA-2)	16	40& nbsp;MHz	80 MB/ s	640 Mbit/ s	NA	12m	25m	16	nowrap> Low-voltage differential signaling
Ultra3 SCSI	Ultra-160; Fast-80 wide	SCSI-3 SPI-3 (1999)	SCA / SCA-2)	16	DDR	160 MB/ s	1280 Mbit/ s	NA	12m	NA	16	nowrap> Low-voltage differential signaling
Ultra-320 SCSI	Ultra-4; Fast-160	SCSI-3 SPI-4 (2002)	SCA / SCA-2)	16	80& nbsp;MHz DDR	320 MB/ s	2560 Mbit/ s	NA	12m	NA	16	nowrap> Low-voltage differential signaling
Ultra-640 SCSI	Ultra-5	SCSI-3 SPI-5 (2003)	68-pin; 80-pin	16	160& nbsp;MHz DDR	640 MB/ s	5120 Mbit/ s				16

Other SCSI interfaces

Interface	Alternative names	Specification document	Connector	Width (bits)	Clock	Maximum
Interface	Alternative names	Specification document	Connector	Width (bits)	Clock	Throughput (MB/ s)	Throughput (Mbit/ s)	Length	Devices
SSA				1	200& nbsp;MHz	spatial reuse Duplex (telecommunications)>full duplex	320 Mbit/ s	25 m	96
SSA 40				1	400& nbsp;MHz	80 MB/ s	640 Mbit/ s	25 m	96
FC-AL 1Gb				1	1& nbsp;GHz	100 MB/ sper direction	800 Mbit/ s	500 meters for Multi-mode optical fiber>multi-mode , 3 kilometers for Single-mode optical fiber	127
FC-AL 2Gb				1	2& nbsp;GHz	200 MB/ s	1600 Mbit/ s	500m/ 3& nbsp;km	127
FC-AL 4Gb				1	4& nbsp;GHz	400 MB/ s	3200 Mbit/ s	500m/ 3& nbsp;km	127
SAS 1.1				1	3& nbsp;GHz	300 MB/ s	2400 Mbit/ s	6 m	16,256128 per expander
SAS 2.0				1	6& nbsp;GHz	600 MB/ s	4800 Mbit/ s	6 m	16,256128 per expander
iSCSI			Implementation- and network-dependent

Cabling

SCSI Parallel Interface

Internal parallel SCSI cables are usually ribbon cables|ribbons , with two or more 50–, 68–, or 80–pin connectors attached. External cables are typically shielded (but may not be), with 50– or 69–pin connectors at each end, depending upon the specific SCSI bus width supported. The 80–pin Single Connector Attachment (SCA) is typically used for hot-pluggable devices, where external cables are not usually required.

Serial attached SCSI

Serial attached SCSI uses a Serial Attached SCSI#Connectors|modified Serial ATA data and power cable.

iSCSI

iSCSI (Internet Small Computer System Interface) usually uses Ethernet connectors and cables as its physical transport, but can run over any physical transport capable of transporting Internet Protocol|IP .

USB Attached SCSI

USB Attached SCSI allows SCSI devices to use the Universal Serial Bus .

Automation/Drive Interface

The Automation/Drive Interface - Transport Protocol (ADT) is used to connect removable media devices, such as tape drives, with the controllers of the libraries (automation devices)
in which they are installed. The ADI standard specifies the use of RS-422 for the physical connections. The second-generation ADT-2 standard defines iADT, use of the ADT protocol
over IP (Internet Protocol) connections, such as over Ethernet . The Automation/Drive Interface - Commands standards (ADC, ADC-2, and ADC-3) define SCSI commands for these installations.

SCSI command protocol

In addition to many different hardware implementations, the SCSI standards also include an extensive set of command 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. Other technologies which use the SCSI command set include the ATAPI|ATA Packet Interface , USB mass storage device class|USB Mass Storage class and Serial Bus Protocol 2|FireWire SBP-2 .

In SCSI terminology, communication takes place between an SCSI initiator|initiator and a SCSI target|target . The initiator sends a SCSI command|command to the target, which then responds. SCSI commands are sent in a Command Descriptor Block ( SCSI CDB|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 SCSI Status Code|status code byte, such as 00h for success, 02h for an error (called a SCSI check condition|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|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 SCSI contingent allegiance condition|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 command s in total, with the most commonly used being:

SCSI Test Unit Ready Command|Test unit ready : Queries device to see if it is ready for data transfers (disk spun up, media loaded, etc.).

SCSI Inquiry Command|Inquiry : Returns basic device information.

SCSI Request Sense Command|Request sense : Returns any error codes from the previous command that returned an error status.

SCSI Send Diagnostic Command|Send diagnostic and SCSI Receive Diagnostic Results Command|Receive diagnostic results : runs a simple self-test, or a specialised test defined in a SCSI diagnostic pages|diagnostic page .

SCSI Start Stop Unit Command|Start/Stop unit : Spins disks up and down, or loads/unloads media (CD, tape, etc.).

SCSI Read Capacity Command|Read capacity : Returns storage capacity.

SCSI Format Unit Command|Format unit : Prepares a storage medium for use. In a disk, a low level format will occur. Some tape drives will erase the tape in response to this command.

SCSI Read format capacities : Retrieve the data capacity of the device.

SCSI Read Commands|Read (four variants): Reads data from a device.

SCSI Write Commands|Write (four variants): Writes data to a device.

SCSI Log Sense Command|Log sense : Returns current information from SCSI log pages|log pages .

SCSI Mode Sense Command|Mode sense : Returns current device parameters from SCSI mode pages|mode pages .

SCSI Mode Select Command|Mode select : Sets device parameters in a mode page.

Each device on the SCSI bus is assigned a unique SCSI identification number or ID. Devices may encompass multiple logical units, which are addressed by logical unit number (LUN). Simple devices have just one LUN, more complex devices may have multiple LUNs.

A "direct access" (i.e. disk type) storage device consists of a number of logical blocks, addressed by Logical Block Address ( SCSI LBA|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 SCSI Read Commands|Read(6) and SCSI Write Commands|Write(6) commands contain a 21-bit LBA address. The SCSI Read Commands|Read(10) , SCSI Read Commands|Read(12) , SCSI Read Commands|Read Long , SCSI Write Commands|Write(10) , SCSI Write Commands|Write(12) , and SCSI Write Commands|Write Long commands all contain a 32-bit LBA address plus various other parameter options.

The capacity of a "sequential access" (i.e. tape-type) device is not specified because it depends, amongst other things, on the length of the tape, which is not identified in a machine-readable way. Read and write operations on a sequential access device begin at the current tape position, not at a specific LBA. The block size on sequential access devices can either be fixed or variable, depending on the specific device. Tape devices such as half-inch IBM 9 track|9-track tape , Digital Data Storage|DDS (4& nbsp;mm tapes physically similar to digital audio tape|DAT ), Exabyte (company)|Exabyte , etc., support variable block sizes.

Device identification

In modern SCSI transport protocols, there is an automated process for "discovery" of the IDs. SSA initiators "walk the loop" to determine what devices are connected and then assigns each one a 7-bit "hop-count" value. Fibre Channel – Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN ( World Wide Name ). For iSCSI, because of the unlimited scope of the (IP) network, the process is quite complicated. These discovery processes occur at power-on/initialization time and also if the bus topology changes later, for example if an extra device is added.

On a parallel SCSI bus, a device (e.g. host adapter, disk drive) is identified by a "SCSI ID", which is a number in the range 0–7 on a narrow bus and in the range 0–15 on a wide bus. On earlier models a physical jumper or switch controls the SCSI ID of the initiator ( host adapter ). On modern host adapters (since about 1997), doing I/O to the adapter sets the SCSI ID; for example, the adapter often contains a BIOS program that runs when the computer boots up and that program has menus that let the operator choose the SCSI ID of the host adapter. Alternatively, the host adapter may come with software that must be installed on the host computer to configure the SCSI ID. The traditional SCSI ID for a host adapter is 7, as that ID has the highest priority during bus arbitration (even on a 16 bit bus).

The SCSI ID of a device in a drive enclosure that has a backplane is set either by jumpers or by the slot in the enclosure the device is installed into, depending on the model of the enclosure. In the latter case, each slot on the enclosure's back plane delivers control signals to the drive to select a unique SCSI ID. A SCSI enclosure without a back plane often has a switch for each drive to choose the drive's SCSI ID. The enclosure is packaged with connectors that must be plugged into the drive where the jumpers are typically located; the switch emulates the necessary jumpers. While there is no standard that makes this work, drive designers typically set up their jumper headers in a consistent format that matches the way that these switches implement.

Note that a SCSI target device (which can be called a "physical unit") is often divided into smaller "logical units." For example, a high-end disk subsystem may be a single SCSI device but contain dozens of individual disk drives, each of which is a logical unit (more commonly, it is not that simple—virtual disk devices are generated by the subsystem based on the storage in those physical drives, and each virtual disk device is a logical unit). The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the logical unit number (LUN) identifies a disk device within the subsystem.

It is quite common, though incorrect, to refer to the logical unit itself as a "LUN."
Accordingly, the actual LUN may be called a "LUN number" or "LUN id".

Setting the bootable (or first) hard disk to SCSI ID 0 is an accepted IT community recommendation. SCSI ID 2 is usually set aside for the floppy disk drive while SCSI ID 3 is typically for a CD-ROM drive.

Device Type

While all SCSI controllers can work with read/write storage devices, i.e. disk and tape, some will not work with some other device types; older controllers are likely to be more limited, sometimes by their driver software, and more Device Types were added as SCSI evolved. Even CD-ROMs are not handled by all controllers. Device Type is a 5-bit field reported by a SCSI Inquiry Command ; defined SCSI Peripheral Device Type s include, in addition to many varieties of storage device, printer, scanner, communications device, and a catch-all "processor" type for devices not otherwise listed.

SCSI enclosure services

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

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