1. SCSI(Small Computer System Interface)

2. What is SCSI?  SCSI (Small Computer System Interface) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners, printers, and optical drives (CD, DVD, etc.). The SCSI standards promote device independence, which means that, at least in theory, almost any type of hardware can be connected via SCSI. standards peripheral devicescommandsprotocolselectricalopticalinterfaces hard diskstape drivesscannersprintersoptical drivesCDDVDstandards peripheral devicescommandsprotocolselectricalopticalinterfaces hard diskstape drivesscannersprintersoptical drivesCDDVD SCSI is most commonly pronounced "scuzzy"[1][2].[1][2]

3. About SCSI  A computer is full of busses -- highways that take information and power from one place to another. For example, when you plug an MP3 player or digital camera into your computer, you're probably using a universal serial bus (USB) port. Your USB port is good at carrying the data and electricity required for small electronic devices that do things like create and store pictures and music files. But that bus isn't big enough to support a whole computer, a server or lots of devices simultaneously. universal serial bus universal serial bus  For that, you'd need something more like SCSI. SCSI originally stood for Small Computer System Interface, but it's really outgrown the "small" designation. It's a fast bus that can connect lots of devices to a computer at the same time, including hard drives, scanners, CD- ROM/RW drives, printers and tape drives. Other technologies, like serial-ATA (SATA), have largely replaced it in new systems, but SCSI is still in use. This article will review SCSI basics and give you lots of information on SCSI types and specifications. hard drivesscannersCD- ROM/RW drivesprinterstape driveshard drivesscannersCD- ROM/RW drivesprinterstape drives

4. About SCSI  WHAT IS SCSI ?  SCSI stands for (S)mall (C)omputer (S)ystems (I)nterface. The official name of the SCSI standard is: ANSI X3.131 - 1986. The SCSI interface is a local bus type interface for connecting multiple devices (up to eight), designated as either initiators (drivers) or targets (receivers). There are two electrical alternatives for this standard:  Single-ended type and  Differential type  Single-ended and Differential devices are different and MAY NOT be mixed on the same bus; however, LVD (or 'Low Voltage Differential') SCSI can be used on the same SCSI bus with 'SE' type devices, if:  Your SCSI card supports this; and  You are using a multi-mode type terminator, sometimes designated as 'LVD/SE' type.  In a SCSI environment, devices are daisy - chained together using a common cable. Both ends of the cable must be terminated. All signals are common between all SCSI devices.

5.  SINGLE - ENDED vs. DIFFERENTIAL  The set of signals for Single - Ended devices is very different from the signals for Differential devices. For Differential devices all signals consist of two lines denoted +SIGNAL and - SIGNAL, while for Single - Ended devices all signals consist of one line (SIGNAL).  Single - Ended (SE) and Differential devices CAN NOT be mixed on the same SCSI cable - except for LVD/SE, mentioned above. The designation 'Differential' generally refers to High-Voltage differential, which is mainly used on older, larger systems such as IBM mainframes. This type of SCSI is not common with personal computers.  SINGLE - ENDED CABLES:  Single - Ended cables connect up to eight drivers and receivers.  A 50 conductor flat cable or 25 signal twisted-pair cable should be used. The maximum cable length shall be 6 meters (primarily for connection within a cabinet). A stub length of no more than 0.1 meters is allowed.  DIFFERENTIAL CABLES:  Differential cables connect up to eight Differential drivers and receivers. A 50 - conductor cable or 25 - signal twisted - pair cable shall be used. The maximum cable length shall be 25 meters (primarily for connection outside of a cabinet).  A stub length of no more than 0.2 meters is allowed.

6.  History  SCSI is based on "SASI", the "Shugart Associates System Interface", introduced by the company of the same name in 1979. The Shugart SASI controller provided an interface between a hard disk's serial analog interface (called RLL) and a host computer, which needed to read sectors (blocks) of data. SASI interfaces were 5.25"x8" in size, mounted usually on top of a hard disk. SASI was used in mini- and microcomputers like the Apple II. SASI defined the interface as using a 50-pin flat ribbon connector. Shugart AssociatesApple IIShugart AssociatesApple II  Some say SCSI used to be spelled SC/ASI at some point in history.[citation needed] The "small" part is historical; since the mid-1990s, SCSI has been available on even the largest of computer systems. citation neededcitation needed  Since its standardization in 1986, SCSI has been commonly used in the Apple Macintosh and Sun Microsystems computer lines. Apple switched to IDE around 1998, and Sun has switched its lower end range to SATA. SCSI has never been popular in the low-priced IBM PC world, owing to the lower cost and adequate performance of its ATA hard disk standard. SCSI drives and even SCSI RAIDs became common in PC workstations for video or audio production, but the appearance of large cheap SATA drives (e.g., 800 gigabytes) means that SATA is rapidly taking over this market. Apple MacintoshSun MicrosystemsIBM PCATARAIDsworkstationsApple MacintoshSun MicrosystemsIBM PCATARAIDsworkstations  At this time, SCSI is popular on high-performance workstations and servers. RAIDs on servers almost always use SCSI hard disks, though a number of manufacturers offer SATA-based RAID systems as a cheaper option. Desktop computers and notebooks more typically use the ATA/IDE or the newer Serial ATA interfaces for hard disks, and USB and FireWire connections for external devices. serversDesktop computersnotebooksIDESerial ATAUSBFireWireserversDesktop computersnotebooksIDESerial ATAUSBFireWire

7. Types of SCSI  SCSI-1  SCSI-2  SCSI-3  LVD SCSI

8.  SCSI interfaces  SCSI is available in variety of interfaces. The first, still very common, was parallel SCSI (also called SPI). It uses a parallel electrical bus design. The traditional SPI design is making a transition to Serial Attached SCSI, which switches to a serial point-to-point design but retains other aspects of the technology. iSCSI drops physical implementation entirely, and instead uses TCP/IP as a transport mechanism. Finally, many other interfaces which do not rely on complete SCSI standards still implement the SCSI command protocol parallel SCSI parallelelectrical busSerial Attached SCSIserialpoint-to-pointiSCSITCP/IP SCSI command protocolparallel SCSI parallelelectrical busSerial Attached SCSIserialpoint-to-pointiSCSITCP/IP SCSI command protocol

9. SCSI interface overview  Parallel SCSI Interface Alternat ive names Specificat ion document Connector Wi dth (bit s) Clock [3] [3] Maximum Throughp ut [4] [4] Length (single ended) [5] [5] Length LVD Length HVD Devices [6] [6] SCSI-1 IDC50; Centronics C50 85 MHz5 MB/s6 mNA25m8 Fast SCSISCSI-2 IDC50; Centronics C50 810 MHz10 MB/s1.5-3 mNA25m8 Fast-Wide SCSI SCSI-2; SCSI-3 SPI 2 x 50-pin (SCSI- 2); 1 x 68-pin (SCSI- 3) 1610 MHz20 MB/s1.5-3 mNA25m16 Ultra SCSIFast-20 SCSI-3 SPI IDC50820 MHz20 MB/s1.5-3 mNA25m8 Ultra Wide SCSI SCSI-3 SPI 68-pin1620 MHz40 MB/s1.5-3 mNA25m16 Ultra2 SCSIFast-40 SCSI-3 SPI-2 50-pin840 MHz40 MB/sNA12m25m8 Ultra2 Wide SCSI SCSI-3 SPI-2 68-pin; 80-pin SCA-2 1640 MHz80 MB/sNA12m25m16 Ultra3 SCSI Ultra- 160 SCSI-3 SPI-3 16 40 MHz DDR DDR 160 MB/s25m12mNA16 Ultra-320 SCSI 16 80 MHz DDR 320 MB/sNA12mNA16 Ultra-640 SCSI 16 160 MHz DDR 640 MB/s ??16

10. Fiber, serial and iSCSI Fiber, serial and iSCSI Interface Alternative names Specification document Connector Width (bits) Clock [7] [7] Maximum Throughput [8] [8] Length [9] [9] Devices [10] [10] SSA1200 MHz40 MB/s [11] [12] [11] [12] 25 m96 SSA 401400 MHz80 MB/s [11] [12] [11] [12] 25 m96 FC-ALFC-AL 1Gb11 GHz100 MB/s [13] [12] [13] [12] 500m/3km [14] [14] 127 FC-AL 2Gb12 GHz200 MB/s [13] [12] [13] [12] 500m/3km [14] [14] 127 FC-AL 4Gb14 GHz400 MB/s [13] [12] [13] [12] 500m/3km [14] [14] 127 SAS13 GHz300 MB/s [13] [12] [13] [12] 6 m16,256 [15] [15] iSCSIImplementation/network dependant

11.  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. iSCSIparadigmTCP/IPFibre Channel Ethernettechnologiescommodity iSCSIparadigmTCP/IPFibre Channel Ethernettechnologiescommodity  [edit] Serial SCSI edit  Four recent versions of SCSI, SSA, FC-AL, IEEE1394, 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. Serial SCSI has number of advantages over parallel SCSI—faster data rates, hot swapping, and improved fault isolation. Serial SCSI devices are more expensive than the equivalent parallel SCSI devices, but this is likely to change soon. SSAFC-ALIEEE1394Serial Attached SCSIparallel SCSIparallel interfacehot swappingSSAFC-ALIEEE1394Serial Attached SCSIparallel SCSIparallel interfacehot swapping

12.  The following varieties of SCSI are currently implemented:  · SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps dataMBpsdataMBps  · SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI.  · Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers.  · Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10 MBps.  · Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.  · Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.  · SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI.  · Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.  · Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.

13.  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. parallel SCSIparallel 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. initiatortargetcommandCDBinitiatortargetcommandCDB  At the end of the command sequence the target returns a Status 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. Status CodeCheck ConditionSCSI Request Sense commandKCQContingent Allegiance ConditionStatus CodeCheck ConditionSCSI Request Sense commandKCQContingent 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: SCSI commandsSCSI commands

14.  Test unit ready: ask the device if it is ready for data transfers (disk spun up, media loaded...) Test unit ready Test unit ready  Inquiry: return basic device information, also used to "ping" the device since it does not modify sense data Inquiry  Request sense: give any error codes from the previous command that returned an error status Request sense Request sense  Send diagnostic and Receive diagnostic results: run a simple self-test, or a specialised test defined in a diagnostic page Send diagnosticReceive diagnostic resultsdiagnostic page Send diagnosticReceive diagnostic resultsdiagnostic page  Start/Stop unit: spin disks up and down, load/unload media Start/Stop unit Start/Stop unit  Read capacity: return storage capacity Read capacity Read capacity  Format unit Format unit Format unit  Read (four variants) Read  Write (four variants) Write  Log sense: return current information from log pages Log senselog pages Log senselog pages  Mode sense: return current device parameters from mode pages Mode sensemode pages Mode sensemode pages  Mode select: set device parameters in a mode page Mode select Mode select

15.  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 "direct access" (i.e. disk type) 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. Logical Unit NumberLBARead(6)Write(6)Read(10)Read(12)Read LongWrite(10)Write(12) Write LongLogical Unit NumberLBARead(6)Write(6)Read(10)Read(12)Read LongWrite(10)Write(12) Write Long  A "sequential access" (i.e. tape-type) device does not have a specific capacity because it typically depends on the length of the tape, which is not known exactly. Reads and writes on a sequential access device happen at the current position, not at a specific LBA. The block size on sequential access devices can either be fixed or variable, depending on the specific device. (Earlier devices, such as 9-track tape, tended to be fixed block, while later types, such as DAT, almost always supported variable block sizes.) 9-track tapeDAT9-track tapeDAT

16. Difference bet n IDE & SCSI  About IDE IDE, or more formally, IDE/ATA, is the most common system for connecting a hard drive to a PC. In modern systems (to which this discussion is limited), they plug directly into the mother board through a 40 pin cable. Most mother boards offer 2 separate IDE channels and thus 2 connectors on the board. Each connector can support 2 IDE devices, be they disk drives, CD drives, tape drives, removable drives and so on. If a channel has 2 devices on it, one must be designated a master and the other a slave. This is done simply by moving or removing a jumper on the drive itself. As a result of this configuration, any system can have 4 IDE devices connected to it. Using an external controller board connected to the PCI bus supporting 2 additional channels, up to 8 devices and be supported on a PC. This is the limit, and attempting to add the other 4 devices with an extra controller will consume more interrupts and other system resources. This contrasts with modern SCSI which can have up to 15 devices on a controller and occupies the same amount of system resources regardless of the number of devices connected up to that limit. The History of IDE IDE replaces older interfaces such as ST-506 and ESDI. Through the years, many changes have been made to the IDE standard as defined by ANSI. The original standard, call simply ATA called for 2 devices on the same channel configured as master and slave. It also defined PIO modes 0, 1 and 2 and DMA single word modes 0, 1 and 2 and multiword mode 0. However, this standard had problems. Often drives by different manufacturers wouldn't work if combined on a single channel as master and slave. ATA-2 added the faster PIO modes 3 and 4 (mode 4 being the common default PIO mode for modern PCs), faster DMA multiword modes 1 and 2, the ability to do block mode transfers, Logical Block Addressing or LBA, and improved support for the "identify drive" command that allows the system to interrogate the drive for manufacturer, model and geometry. The terms "Fast ATA and Fast ATA-2" are the inventions of Seagate and Quantum. They are not really standards and only denote drives that are compliant to all or part of the ATA-2 standard. ATA-3, however, was a real standard that improved reliability and defined the SMART feature in disk drives. It was followed by the current Ultra ATA or UATA. UATA also goes by many other names like UDMA, DMA-33/66 and ATA- 33/66. UATA isn't really a new standard, and UATA drives are still backward compatible with ATA and ATA-2 systems. Ultra ATA is the term given to drives that support the new DMA modes that provide up to 33 MB/s (UDAM-33) or up to 66 MB/s (UDMA-66) transfer rates with 100 MB/s just over the next hill. Both UDMA versions support CRC error checking that assures data integrity through the IDE cable, which was a source of serious problems in previous standards. Note that the UDMA-66 standard calls for an 80 conductor cable instead of the 40 conductor cable used up to and through UDMA-33. EIDE or Enhanced IDE is a designation created by Western Digital to describe its newer line of high speed drives. It really isn't a standard at all, but just a marketing tool. However, it has taken on common public use to refer to all high speed drives and the systems that support them. IDEATAPCISCSI ANSIPIODMAblock modeLBASeagateQuantumSMARTCRCWestern Digital IDEATAPCISCSI ANSIPIODMAblock modeLBASeagateQuantumSMARTCRCWestern Digital

17.  Fibre Channel Communications  Communications between two FC-AL devices occurs as a sequence of primitives and frames sent from one device to another. Each primitive is a series of four characters that serve to identify the primitive, and which are distinguishable from other data that may be sent along the loop. Primitives are used to control the passage of data between two devices. For example, an ARB primitive is used to arbitrate for control of the loop, while an OPEN primitive is used to open a connection to another device.  Actual data is transmitted from one device to another in the form of frames. Each frame consists of a frame header of 24 bytes followed by a payload of up to 2048 bytes. The payload contains the data being transmitted, while the frame header contains context information regarding the payload's data, such as the IDs of the devices involved.  Fibre Channel SCSI Is Still SCSI  Despite the significant differences between the physical characteristics of FC-AL and parallel SCSI, Fibre Channel disk devices use a version of the SCSI protocol mapped into the frame structure of FC, resulting insignificant similarities to parallel SCSI. Most notably, all parallel SCSI commands and much of the message function remain intact in Fibre Channel. This fact allows most Fibre Channel SCSI implementations to be based upon a previous parallel SCSI implementation, salvaging much of the earlier programming effort. To illustrate these similarities, a comparison of command execution in parallel SCSI and FC-AL SCSI is in order.  In FC-AL, command execution comprises a sequence of primitives and frames. The ARB primitive is sent by the initiator to gain control of the loop, much like arbitration in parallel SCSI is used to gain control of the bus. An OPEN primitive is likewise analogous to the parallel SCSI selection, in that a connection between the initiator and the specified target device is established.  After the connection is established, the OPENed device (target) sends one or more R_RDY primitives to the OPENing device (initiator). Each R_RDY primitive sent by the target grants one "credit" to the initiator, allowing one frame to be sent to the target. This is analogous to the command phase in parallel SCSI.  After sending the command frame, the initiator closes the connection by sending a CLOSE primitive. The target responds with its own CLOSE primitive, freeing the loop for other activity, much as the analogous disconnect does for parallel SCSI.  When the target is ready to send the requested data to the initiator, the process is similar to that described above, except that the roles are reversed. The target ARBs for the loop and then OPENs a connection to the initiator. One or more R_RDY primitives from the initiator give the target credit to send one or more data frames, after which the target and initiators exchange CLOSE primitives, again freeing the loop.  Command execution is completed when the target sends a response frame to the initiator. The sequence of primitives immediately before and after transmission of the response frame is the same as outlined above for the data frame.  Having completed the command, the initiator can process the data and response information the same way that it would have for a parallel SCSI command execution. This is possible because every important aspect of SCSI command execution is mapped into a Fibre Channel frame.

18.  SCSI Mapping into FC Frames  The SCSI Access Model (SAM) describes command execution in terms of four functions: Command Service Request, Data Delivery Request, Data Delivery Action, and Command Service Response. Each of these functions is mapped into a Fibre Channel frame type, allowing an entire SCSI command execution to be mapped into frames.  A Command Service Request is transmitted from an initiator to a target by sending a command frame. The command frame payload contains the SCSI Command Descriptor Block, as well as the Logical Unit Number, the expected data transfer length, and a control field.  A Data Delivery is the transfer of actual data from one device to another. It occurs within one or more data frames, depending upon the length of the transfer. The Command Service Response is transmitted from the target to the initiator at the completion of command execution. It contains the SCSI status for the command, and also contains any SCSI sense information that may have been generated for the command.  Prior to a Data Delivery Action, there may or may not be a Data Delivery Request sent by the target to the initiator. If present, the Data Delivery Request is in the form of a transfer ready frame. The frame payload indicates the relative offset of the first data frame to follow, and also the total number of bytes that can be transferred as a burst of one or more data frames. For disk drives, the transfer ready frames are typically used during write commands, but not read commands.  Conclusion  Even though the physical differences between Fibre Channel and Parallel SCSI are substantial, the mapping of SCSI functionality into Fibre Channel frames simplifies the migration dramatically. FC-AL devices can leverage off of existing parallel SCSI devices, and designers can learn Fibre Channel beginning with the parallel SCSI concepts that are already well known.

19. Reference  www.help2engg.com

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