SCSI glossary, practical definitions and terminologies for SCSI

Note: This SCSI glossary presents practical definitions and is not exhaustive. It provides a good general understanding of SCSI terminology, and how it relates to Paralan's SCSI product line. For a more comprehensive approach, refer to the latest IEEE release of the SCSI specifications X3.131-199X for formal definitions of SCSI terms or to the ANSI Standards Committee.

Note: Reference list of terms defined in Paralan's SCSI Glossary - Quick List of SCSI Terminologies.

-A-

A-Cable:: The SCSI specifications define an A-Cable for use in Narrow SCSI (50-pin connector) applications. An Alt 1, A-Cable has 50-pin male high-density micro-D shielded or unshielded connectors. The shielded connector on an Alt 1, A-Cable is specified to have latch type fasteners. An Alt 2, A-Cable may be either an unshielded ribbon cable with a female connector having 2 rows of 25 pins on 0.1 inch centers or a shielded round cable with 25 twisted pairs and male centronics-type 50-contact connectors.
Active Negation:: SCSI signals are negative true logic, meaning that the signals are normally asserted for a "0". In the past, to change a "0" to a "1", the signal drive was simply removed and the signal allowed to "drift" to the unasserted state. For Ultra data throughput this method is barely fast enough but it is not fast enough for LVD or faster data throughputs. To overcome this, "active negation" is used to drive the signal to the unasserted state. This allows more rapid transitions for LVD and faster SCSI data throughput. The SCSI specifications recommend active negation for Ultra SCSI devices and specify it for LVD and faster.
ANSI - American National Standards Institute:: SCSI is maintained by the X3T10 committee of ANSI.
Arbitration:: If a SCSI device needs to use the bus, it uses arbitration to determine if it can take control of the bus. If several devices arbitrate at the same time, the device with the highest priority wins.
ASPI - Advanced SCSI Programming Interface:: A SCSI software interface developed by Adaptec, Inc for sending commands to SCSI host adapters. Provides an insulating layer so that peripheral device designers do not have to deal with differences in SCSI host adapters. Refer to CAM, another SCSI software interface.
Asynchronous (ASYNC) transfer:: A method of transferring data that requires that the bus wait for a REQ-ACK handshake for each byte of data. This results in a maximum data transfer rate of 5 MBytes/second that decreases quickly with even small increases in cable length. All SCSI bus negotiations are carried on asynchronously, even on a synchronous SCSI bus. On a synchronous SCSI bus, only actual data transfers are accomplished synchronously.

-B-

B-Cable:: The SCSI-2 specifications defined a B-Cable for use with an A-Cable in Wide (16-bit) SCSI applications. It is identical in construction to the A-Cable. The B-Cable was rendered obsolete by the introduction of the P-Cable in the SCSI-3 specifications.
Bridge:: Refer to iSCSI to SCSI Bridge.
Bridge Controller:: A target controller that uses SCSI for the connection between the initiator and some other bus (or SCSI) to connect to the peripheral device. Bridge controllers are commonly used in RAID arrays or in optical disk or tape libraries.
Burst Rate:: Many SCSI devices can send data much faster over the bus than they can recover them from the medium (see: sustained transfer rate). The data are buffered and sent in a burst at maximum speed over the bus, so the bus is utilized for only a short period of time. It is then released and can be used by other devices.
Bus Mastering:: A peripheral controller, such as a SCSI host adapter, which is capable of transferring data over the host bus without the host CPU's assistance, is called a bus master. Bus mastering delivers maximum relief of the host on I/Os, but it requires extra intelligence on the peripheral controller. (See also SCSI Bus Mastering.)

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-C-

CAM - Common Access Method:: ANSI software interface for SCSI devices. Allows a single device driver to be written for all SCSI controllers. Part of SCSI-3. Refer to ASPI, another SCSI software interface. Also refer to ANSI draft CAM Document X3T9.2/90-186.
CCS - Common Command Set:: In 1985, ANSI Committee X3T9.2 began work on a CCS to provide a common software interface for all disk drives and subsequently issued a supplement to SCSI-1. In SCSI-2, the CCS was greatly expanded to include peripheral devices other than disk drives.
CDB - Command Descriptor Block:: A Command Descriptor Block is a data structure that contains a SCSI command along with some parameters. CDBs have a fixed length of either 6, 10, 12 or 16 bytes. The target fetches the CDB from the initiator during a command phase.
CRC - Cyclic Redundancy Check:: Cyclic Redundancy Check is a means of detecting errors that is much more effective than the simple parity check that SCSI has used for years. CRC detects all single bit errors, all two bit errors, all errors with an odd number of bit errors, and all burst errors up to 32-bits long. CRC uses a 32-bit polynomial checksum to test data integrity. A similar process is used in Fibre Channel, Ethernet and other buses.
Cables:: The single most critical item in the installation of a successful, high-reliability, maximum throughput SCSI system. Do not waste time and money on cheap SCSI cables!
The longer the cables and the faster the data throughput, the more critical cables become. Internal SCSI cables are usually non-shielded ribbon cables and external cables are usually round, shielded cables with shielded connectors. Cable impedance must be matched to the requirements of SCSI devices and cable pairs must be carefully selected for the correct SCSI signal lines.

SCSI specifications recommend the use of twisted pair cable for either ribbon cables or external shielded cables. With the higher data throughput of today's SCSI systems, twisted pairs are strongly recommended. For a correctly made round, shielded cable assembly, SCSI signal pairs must be placed on wire pairs in the cable. Also, sensitive signals such as REQ and ACK must be placed on pairs in the core of the cable with data lines in the second layer of pairs and data lines in the outer layer. Ribbon cables are generally not a problem as the wire location in the ribbon is fixed.
Cable lengths:: The SCSI specifications provide maximum recommended cable lengths for various implementations of SCSI. It is further recognized that in "engineered" installations these cable lengths may be exceeded. Maximum recommended SCSI cable lengths:

SCSI TYPE - SPEED	MAX CABLE LENGTH
SINGLE ENDED - SLOW (FAST-5)	6 Meters / 19.7 Feet
SINGLE ENDED - FAST (FAST-10)	3 Meters / 9.8 Feet
SINGLE ENDED - ULTRA (FAST-20)	1.5 Meters* / 4.9 Feet
DIFFERENTIAL (HVD) - (ANY SPEED)	25 Meters / 82 Feet
LVD - ULTRA2, ULTRA3 or ULTRA4	12 Meters / 39.4 Feet**

*Maximum cable length with eight addresses. May be up to 3 meters with four devices.
**This may be increased to 25 meters (82 Feet) in point to point applications.

Single-ended Cable - Strictly speaking there is a slight difference in cable pair placement between single ended and differential cables. However, careful selection of cable pairs avoids the necessity for manufacturers to build single ended as well as differential cables. Some cable manufacturers save manufacturing cost by deleting up to 25 of the ground wires in the narrow single ended SCSI cable. That drastically changes cable impedance and increases cross talk thereby causing problems, particularly with FAST SCSI and/or long cables. Although they may work over short distances (not more than four feet) they often cause errors resulting in SCSI retries and reduced data throughput. They will not work at all on differential SCSI. To save yourself headaches, purchase "differential" cables and do not use so called "single-ended" SCSI cables.

Command:: A SCSI command is an instruction that an initiator issues to a target specifying the task to be carried out.
Command Queuing:: Refer to Tagged Command Queuing.
Connectors:

Standard Connectors:

IDC header - 50 pin insulation displacement connector (IDC) used with ribbon cables for nonshielded, internal SCSI cabling. Female type used on cables. SCSI terminology: Nonshielded Alternate 2-A Cable
Centronics type - This connector is a 50-pin version of the connector used for parallel printer ports for years. Specified in SCSI-1 and still an acceptable connector for narrow SCSI. Male connector used for cables, female for devices. Uses bail fasteners on the device-mounted female connector to secure the male connector. Often referred to as the "SCSI-1" connector. SCSI terminology: Shielded Alternative 1, A-cable.
50-pin high-density micro-D connector - Pin-and-socket connector. Smaller than the centronics-type. Specified to have spring-latch fasteners. Male connector used on cables; female on devices. SCSI terminology: Shielded (or non-shielded) Alternative 1, A-cable.
68-pin Micro-D high-density - Originally specified in SCSI-3 to allow the use of one cable for wide (16-bit) applications. This cable is called a P-cable. 68-pin version of the 50-pin micro-D high-density connector. Although the 2-56 thumbscrew fastener is specified in SCSI-3, some manufacturers have used the spring-latch fastener. Male connector used on cables; female on devices. Frequently referred to as the "SCSI-3" connector. SCSI terminology: Shielded (or non-shielded) Alternate 3, P-cable.
68-pin VHDCI (Very High Density Cable Interconnect) - Originally specified in SCSI-3. Ribbon contact construction similar to centronics type connector, but on smaller 0.8 mm (0.0315") pitch. Much smaller footprint allows up to four connectors to be mounted on a PC backplate. Called the VHDCI connector. SCSI terminology: Shielded Alternate 4 - P cable.
80-pin SCA (Single Connector Adapter) - Originally specified in the SCSI-3 specifications for device connection on hot-swap backplanes. Device ID is assigned by the backplane and power connections are included on the connector, so using peripherals with an SCA connector in a cabled system requires the use of an SCA to 68 pin (or 50 pin) adapter with provisions for peripheral power and assignment of ID. SCSI terminology: Non-shielded Alternate 4 - P cable.

Non-Standard Connectors:

DB-25 - Not specified in the SCSI specifications, this 25-pin connector can support SLOW 8-bit (narrow) SCSI only and then only with very short cables. Apple Macintosh version became a de facto standard (totally incompatible with the pinout of the old Future Domain 25-pin SCSI connector). Male connector on cable; female on devices.
30-pin HDI - 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.
DB-50, D-sub connector was used on some older Sun and DG computers for Narrow SCSI applications. 50 pins arranged in three rows.
60-pin high-density - A non-standard connector used by IBM. Early in the process of writing the SCSI-2 specification, the ANSI X3T9.2 committee specified a 60-pin connector for 8-bit SCSI that was later abandoned. IBM, however, retained this connector for their PS/2 systems. It is a 60-pin high-density micro-D male connector with spring-latch fasteners. The first 50-pin assignments are identical to the SCSI-2 high-density pinout and pins 51 to 60 are designated as "reserved".

Visual Guide to SCSI Connectors or to Paralan's cable information.

Converter:: An electronic product designed to convert between various SCSI interfaces such as single-ended and HVD; HVD and LVD; or LVD and single ended. Allows placement of single-ended devices on a bus connected to a differential host and vice versa. Properly designed converters may be used back-to-back to extend single-ended or differential SCSI up to 61meters (200 feet). Termed an "expander" in SCSI-3.
Crosstalk:: The wires in a cable are located in very close proximity to each other. This results in a signal in one wire inducing smaller signals in the adjacent wires. Crosstalk can be a problem for SCSI transmissions at higher speeds or over long distances. A good cable quality and the proper layout of the wires inside the cable can minimize the effects of crosstalk.

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-D-

Device:: From the EPI document of SCSI-3: Devices include targets, initiators and bus expanders. The term "SCSI device" is limited to targets and initiators.
Differential SCSI:: Refers to the manner in which SCSI signals are placed on the cable. Single ended drives one signal line against ground while differential SCSI drives two signal lines. The signal is the voltage difference between the two lines. Differential drive has greater noise immunity than single-ended, especially when used with twisted pair cable which converts noise to common mode voltage that is more easily rejected. This greater noise immunity allows substantially longer SCSI cabling of up to 25 meters (82 feet) for HVD and 12 meters (39.4 feet) for LVD versus 6 meters (19.7 feet) or less for single-ended. Differential and single-ended SCSI are not compatible on the same bus segment without an electronic device such as a SCSI converter to convert between differential and single-ended. With rare exception, no software (driver) modifications are necessary for conversion between single-ended and differential. HVD has no requirement for other than passive terminators. Single ended terminators should be active and multimode LVD terminators must be active. HVD SCSI was rendered obsolete in the SPI-3 document of SCSI-3.
DIFFSENSE:: A signal on differential SCSI cabling that is used as an active HIGH enable for HVD differential transceivers. If a single-ended device is connected to the bus, this line is pulled low. This disables the differential transceivers to protect them from trying to drive signals into ground. DIFFSENSE is used by multimode transceivers (LVD/SE) to determine if the device is connected to a single-ended, LVD or HVD SCSI bus. If the voltage on DIFFSENSE is between 0 - 0.5 V the bus is single-ended; if it is between 0.7 – 1.9 V the bus is LVD; and if it is greater than 2.4 V, the bus is HVD.
Disconnect:: The concept of disconnect-reconnect is what allows SCSI to be multi-tasking or multi-threaded. Disconnect is the process of a target (or initiator) disconnecting from the bus when it experiences a delay in completing a task so that another SCSI device can make use of the bus. Also see "reconnect".
Domain:: This term is part of the SCSI-3 structural model. A SCSI domain is an I/O subsystem including all devices that share a common service delivery subsystem. In simpler words: all devices that are connected in a way that they can see each other belong to one SCSI domain. See also: Segment.
Domain Validation:: Domain validation is a method used in Ultra 160 (Fast-80 or Ultra 3) to test for the optimum rate for data exchange. Once the host adapter (initiator) has located a peripheral and negotiated a data transfer rate, the initiator sends a Write Buffer command to the target at that negotiated data transfer rate and then reads it back to determine if what it reads is what it wrote. If not, it will resend the Write Buffer command at the next lower data transfer rate. This will continue until a speed is reached where the test is successful. This is all accomplished automatically.
Double-Speed SCSI:: Refer to Ultra SCSI
Double Transitional Clocking:: Double transition clocking is used to double the data transfer rate from Ultra 2 (Fast-40) to Ultra 3 (Ultra 160 or Fast-80) SCSI without having to increase the clock speed. That means that both the rising and falling edges of the REQ and ACK signals are used to clock data. The REQ and ACK signals run at 40 MHz on Ultra SCSI, so double clocking increases the rate at which data is clocked to 80 MHz. This provides data throughput of 160 Mbytes/sec for wide SCSI.
Double WIDE SCSI:: Refers to 32-bit SCSI.

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-E-

EPI - Enhanced Parallel Interface:: A technical report (X3T10/1143D) of the SCSI committee that describes configurations and extensions which are possible, but not defined, within the existing SCSI standards. This includes extending the SCSI bus length, converting from single-ended to differential SCSI, and hot plugging capabilities.
Expander:: Expanders were introduced in the EPI document of SCSI-3. Expanders are devices for doing things beyond the normal SCSI definitions that do not require a SCSI ID. Examples of expanders are SCSI enhancement devices such as SCSI Converters, SCSI Bus Extenders, SCSI RegeneratoRs, SCSI repeaters and SCSI switches.
Extender:: An electronic product designed to extend the distance at which peripheral devices may be placed from the host computer system. May use standard SCSI cables for parallel SCSI signals or fiber optic or coaxial cables for serial SCSI transmission. Termed an "expander" in SCSI-3.

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-F-

FCP - Fibre Channel Protocol:: A document that shows how to adapt the SCSI-3 protocol to Fibre Channel.
FC-AL - Fibre Channel, Arbitrated Loop:: Refer to Fibre Channel.
FPT:: Refer to Terminator, Force Perfect Termination.
Fairness:: A system included in SCSI-3 which prevents fast devices with a higher priority from "hogging" the bus and preventing devices with a lower priority from gaining control of the bus. Must be used if QAS is implemented.
Fast SCSI:: Defined in the SCSI-2 specifications. Increased the maximum SCSI data throughput from 5 MBytes/second for 8-bit (NARROW) synchronous SCSI-1 to 10 Mbytes/sec. WIDE (16-bit) SCSI synchronous speed increased from 10 Mbytes/second to 20 Mbytes/ second. No effect on asynchronous SCSI speed.
Fast-20:: Refer to Ultra SCSI
Fast-40:: Refer to Ultra2 SCSI
Fast-80:: Refer to Ultra3 SCSI Also known as Ultra160 or Ultra 160/m.
Fast-160:: Refer to Ultra4 SCSI Also known as Ultra320.
Fast-Wide SCSI:: Usually refers to 16-bit (WIDE) SCSI with FAST data transfers of up to 20 MBytes/second. May also refer to 32-bit SCSI FAST data transfers (up to 40 MBytes/ second). Note that 32-bit wide SCSI was obsoleted in the SPI-3 document of SCSI 3.
Fibre Channel:: A high-speed, high-bandwidth serial protocol for channels and networks that interconnect over twisted-pair wires, coaxial cable or fiber optic cable. The "fabric" topology of Fibre Channel offers up to 16 million ports with cable lengths of up to 10 kilometers. SCSI will use the lower cost "Arbitrated Loop" topology (FC-AL) of Fibre Channel. FC-AL using fiber optic media offers speeds of up to 100 MBytes/sec and up to 127 ports all connected in serial with up to 25 meters between ports. Fibre Channel on copper wiring is available in several versions from 12.5 MBytes/sec with up to 100 meters of cable to 100 MBytes/sec with up to 25 meters of cable. Does not require ID switches or terminators. The FC-AL loop may be connected to a Fibre Channel "fabric" for connection to other nodes. SCSI on FC-AL will be expensive and will require some changes to software as well as hardware.
FireWire:: Apple's name for their implementation of IEEE 1394. Refer to High Performance Serial Bus (IEEE 1394).

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-G-

We are unaware of any SCSI terminology for the letter G, but if you know of one e-mail us and we will add it.

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-H-

HBA - Abbreviation for Host Bus Adapter:: See Host Adapter.
HVD - High Voltage Differential:: Differential SCSI scheme that was defined in SCSI-2 and has been in use for years. HVD SCSI signals are TTL (5V). Not directly compatible with single ended or LVD SCSI unless a SCSI Converter is used. See also Differential SCSI and LVD. Note that HVD SCSI was obsoleted in the SPI-3 document of SCSI-3.
High Performance Serial Bus (IEEE 1394):: Serial SCSI in SCSI-3 will include mappings for IEEE 1394 as well as FC-AL and SSA. Designed by Apple as a serial replacement for parallel SCSI and called "FireWire" by them. Uses three twisted pair copper cables and, like other serial SCSI schemes has no terminators and no IDs to contend with. Logically it looks like a bus, just like parallel SCSI. It supports isochronous transfers so it is very attractive to time-dependent data applications such as video and audio. Currently supports transfers at 100 Megabits/second (about 10 MBytes/second) but devices are under development to increase this to 200 or 400 Megabits/second. Cable lengths can be up to 5 meters "per hop" with up to 63 nodes or devices. Just starting to show up in consumer electronics such as home entertainment systems with VCR, video cameras, etc.
Host Adapter:: The SCSI card or circuitry in a host computer that allows the host to communicate with the SCSI devices is called a host adapter. The name adapter (as opposed to controller) is tribute to the fact that all the intelligence is located in the target controller on the peripheral device.
Hot Plugging or Hot Swapping:: The ability to remove and replace SCSI devices on the bus. There are four "levels" or "cases" of hot plugging. Case 4 is true hot plugging as it requires that the bus remain running during the plugging action. Read Paralan's white paper on hot swapping of SCSI devices.

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-I-

ID:: The unique address of a SCSI device. 8-bit SCSI can have up to eight IDs; 16-bit up to sixteen IDs; 32-bit up to 32 IDs. There must be a minimum of one target and one initiator on the bus. SCSI IDs range from #00 to #07 for 8-bit; #00 to #15 for 16-bit; and #00 to #31 for 32 bit systems. The host is usually assigned ID #07 as it is the highest priority ID regardless of the width of the bus.
IEEE - Institute of Electrical and Electronic Engineers:: (Pronounced I triple E.) An organization that maintains electronics standards. See also: ANSI.
Information Units:: Information Units is one of the five optional features of Ultra3 SCSI as defined in the SPI-3 document. It is a form of SCSI packetization or encapsulation that is designed to reduce overhead and speed data transfers. The SCSI command and data phases are reduced from a seven to a three phase process and then further combines the status phase and message phase that occur after a data transfer. Also, the timing on the command phase, status phase and message phase is changed from asynchronous to synchronous which results in another reduction in overhead.
An "SPI information unit" is a data structure that packetizes data, status, command, task attributes and CRC into various formats. An "information unit transfer" is a transfer of data, status, commands, task attributes, task management, and CRC using only SPI information transfers. Ultra160m (U160/m) SCSI does not include the Information Units option.
Initiator:: A device that begins a SCSI transaction by issuing a command to another device giving it a task to perform. Typically a SCSI host adapter is the initiator but targets may also become initiators.
iSCSI:: iSCSI (Internet SCSI) a SCSI transport protocol that maps block-oriented storage data over TCP/IP networks.
iSCSI Bridge™:: Paralan Trademark. See iSCSI to SCSI Bridge.
iSCSI to SCSI Bridge™:: An iSCSI to SCSI Bridge enables the connection of parallel SCSI devices to serial iSCSI on Ethernet (TCP/IP). Facilitates the addition of legacy parallel SCSI (LVD, SE, HVD) storage devices to an IPSAN. Or it can be used to locate legacy devices at any distance from the host.
Isochronous transfer:: A data transfer that is made within a specified timeframe. Very important for time dependent information such as audio and video. IEEE 1394 offers isochronous data transfer.

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-J-

JBOD:: Acronym for Just a Bunch of Disks, an enclosure with a bunch of disk drives in it.

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-K-


We are unaware of any SCSI terminology for the letter K, but if you know of one e-mail us and we will add it.

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-L-

L-Cable:: The L-Cable was proposed for use in 32-bit Wide SCSI applications with a shielded 110-pin male micro-D connector. Meant to eliminate the need for both a P-Cable and a Q-Cable in 32-bit Wide applications. No longer viable as 32-bit SCSI was obsoleted in the SPI-3 document of SCSI-3.
LUN - Logical Unit Number:: A method of expanding the number of SCSI devices that can be placed on one SCSI domain. Logical Unit Numbers address up to seven devices at each SCSI ID on an 8-bit bus or up to fifteen devices at each ID on a 16-bit bus.
LVD - Low Voltage Differential:: An LVD/MSE SCSI device will operate in the LVD mode when connected to an LVD bus segment or as a SE device when connected to a SE bus segment. Also note that when a SE device is connected to an LVD/MSE bus segment, the entire bus segment and any attached LVD/MSE devices will switch to the SE mode with its limitations on data throughput and cable length. This term originated in the SPI-3 document of the SCSI-3 specifications and is synonymous with multimode LVD, which is a term that is no longer used in the SCSI documents.
LVD/MSE - Low Voltage Differential/Multimode Single-ended:: Originally defined in the SPI-2 document of SCSI-3, uses 3.3 volt logic so consumes less power, dissipates less heat and is faster than HVD SCSI.

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-M-

Megatransfer:: A million transfers. Typically expressed as Megatransfers/second. For SCSI 1 Megatransfer/sec is equivalent to 1 Megabyte/sec for narrow (8-bit) SCSI and 2 Megabytes/sec for wide (16-bit) SCSI.
Message:: SCSI Messages are used to allow initiator and target to communicate while processing a SCSI task. SCSI messages are used during the command transfer to negotiate the fast and wide transfer options for the data transfer. An initiator can use a message to abort a command currently executed. A target can use a message to notify the initiator of a parity error.
Multi-threaded SCSI:: The ability of SCSI to initiate and complete more than one task at a time. A result of the "intelligence" of SCSI that allows a device to disconnect from the bus while it is performing a task, such as locating some information at a specific address, and, once it has located the information, reconnect to the bus in order to complete its task. Optimizes the use of the bus bandwidth. Refer to disconnect and reconnect.
Multimode LVD:: Synonymous with LVD/MSE. This term is no longer used in the SCSI specifications documents.

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-N-

NARROW SCSI:: A SCSI implementation that has 8-bit data transfers. Most easily identified by its single 50-pin connector.

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-O-

Offset:: A feature of synchronous SCSI that allows a target to issue multiple REQ signals before receiving ACK signals back from the initiator. Offset defines the number of pending REQ signals that may be issued by the target before receiving corresponding ACK signals from the initiator. This alleviates the asynchronous SCSI problem of large decreases in data throughput with long SCSI cables and allows the data rates associated with SCSI-2 and SCSI-3. Offset may be any number but is usually 8 or 16.

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-P-

P-Cable:: The SCSI-3 specifications defined the P-Cable for use in Wide (16-bit) SCSI applications. Uses 34 twisted pair cable and shielded 68-pin male high-density micro-D connectors with thumbscrew fasteners having a 2-56 thread. Eliminated the need for using both an A-Cable and a B-Cable in 16-bit Wide SCSI applications.
Packetization:: Refer to "Information Units".
Parity checking:: A simple way of detecting errors in SCSI data that is required to be built into all SCSI-2 devices. Counts the number of 1's in a byte of data and sets a parity bit so that the number is always odd or even. SCSI uses odd parity. You can use parity checking only if all devices on the bus use parity checking. Parity checking will be replaced by CRC in Ultra 4 SCSI.

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-Q-

Q-Cable:: The Q-Cable is physically and electrically identical to the P-Cable. Both a Q-Cable and a P-Cable are needed for 32-bit Wide SCSI. The Q-Cable was rendered obsolete when the SPI-3 document of the SCSI-3 specifications obsoleted 32-bit Wide SCSI.
QAS:: Quick Arbitration and Selection, an optional method of arbitration defined in SCSI-3, is designed to eliminate overhead and speed up data transfers on the SCSI bus by skipping over the bus free phase. A device with QAS will arbitrate for the bus immediately after the last device on the bus sends a disconnect instead of waiting for the bus free phase that normally occurs after a disconnect. Included in QAS is the need for SCSI "fairness" (Refer to SPI-3, Annex B) which prevents fast devices with a higher priority from "hogging" the bus and preventing devices with a lower priority from gaining control of the bus. QAS is negotiated in the arbitration phase.

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-R-

RAID:: Originally, Redundant Array of Inexpensive Disks but has evolved to Redundant Array of Independent Disks.
Reconnect:: The concept of disconnect-reconnect is what provides the ability of SCSI to initiate and complete more than one task at a time. A result of the "intelligence" of SCSI that allows an initiator or target to disconnect from the bus while it is performing a task, such as locating some information at a specific address, and, once it has located the information, reconnect to the bus in order to complete its task. Optimizes use of the bus bandwidth. See also "disconnect".
RegeneratoR™:: A Paralan trademarked product name for an electronic device that accepts a given type of SCSI input, such as WIDE differential, and outputs identical signals. Used to multiply the maximum allowable length of SCSI cables from two to ten times depending on the type of SCSI and the number of RegeneratoRs used. Also provides input to output signal isolation to allow "hot swapping" of SCSI devices. Termed an "expander" in SCSI-3.
Repeater:: A SCSI expander that increases the maximum allowable length of SCSI cabling by regenerating the signals. Refer to RegeneratoR™.

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-S-

SASI - Shugart Associates Systems Interface:: The predecessor to the SCSI bus. Developed by Shugart Associates in 1979. The first intelligent hard disk interface designed for smaller computers. Defined single-ended SCSI and offered maximum data throughput of 1.5 Mbytes/second, asynchronous and 8-bit only. Expanded by ANSI Committee X3T9.2 and released as IEEE SCSI (now called SCSI-1) specification number X3.131-1986 in June of 1986.
SCA - Single Connector Attachment:: This 80-pin connector contains the SCSI signals, some control signals including device ID and power. Designed as a SCSI device connector for hot swap backplane systems. SCA-2, an enhanced version, is part of the SPI-2 document.
SCAM - SCSI Configured AutoMatically:: In combination with Intel/Microsoft's Plug-and-Play SCAM allowed users to interconnect SCSI host adapters and peripheral devices with no need to set configuration switches or jumpers. All configuration will be accomplished by the computer. SCAM rendered obsolete in SPI-3 document of SCSI-3.
SCSI - Small Computer System Interface:: An intelligent peripheral I/O interface with a standard, device independent protocol that allows many different peripheral devices to be attached to the host's SCSI port. Allows up to 8, 16 or 32 addresses on the bus depending on the width of the bus. Devices can include multiple hosts (initiators) and peripheral devices (targets) but must include a minimum of one of each. 32 bit SCSI was rendered obsolete in the SPI-3 document of SCSI-3. See SCSI-1 X3.131-1986.
SCSI-1:: A derivative of SASI. Specification first released by ANSI in 1986 as X3.131-1986. Originally included synchronous and asynchronous data transfers at speeds up to 5 Megatransfers per second (5 MBytes/sec for 8 bit). Defined the single-ended narrow interface with the "idc" internal connector and the centronics type external connector. Did not include definitions of a device independent interface.
SCSI-2:: The next generation of SCSI by ANSI Committee X3T9.3 and released as specification X3.131-1994 in January of 1994. Arguably the most significant addition of SCSI-2 is the expanded definition of the common command set (CCS). Defines 16-bit and 32-bit WIDE data bus. Increases the maximum data throughput to 10 Megatransfers /second (10 MBytes/ second for 8-bit; 20 Megabytes/second for 16-bit; and 40 Megabytes/second for 32-bit). Added the smaller 50-pin high-density micro-D connector. Strongly recommends active terminators for single-ended bus. Backward compatible with SCSI-1.
SCSI-3:: An ANSI Committee X3T10 work in progress. With the new SCSI Architecture Model, a "SCSI-3" specification will not be published. Instead, a collection of stand-alone documents, each with its own rev number will be introduced. These documents include enhancements for parallel SCSI as well as definitions for serial SCSI to allow interface to serial buses such as Fibre Channel, FireWire and iSCSI. SCSI-3 defines a single cable (the P-cable) to eliminate the necessity to use two cables (the A-cable and B-cable) for 16-bit SCSI. Parallel SCSI enhancements include Ultra SCSI (also called Fast-20) which doubles SCSI-2 speeds to a maximum data throughput of 20 Megatransfers/sec (20 MBytes/sec for 8-bit SCSI; 40 Mbytes/sec for 16-bit SCSI). The SPI-2 document of SCSI-3 includes Ultra2 SCSI (also called Fast-40), which doubles these data throughputs again and requires low voltage differential (LVD) signaling for 80 Mbyte/sec data transfers. The Ultra160 (also called Fast-80) interface will be found in SPI-3 for 160 Mbyte/sec data transfers. SPI-3 introduces domain validation and obsoletes the 32-bit wide interface, the high voltage differential (HVD) interface and SCAM. SPI-4 enhancements include Ultra320 (Fast-160) for 320 Mbytes/sec data transfers. The EPI document of SCSI-3 includes definitions for expanders including SCSI enhancement devices such as converters. The new interfaces are backward compatible with SCSI-2 as well as SCSI-1 via the single-ended interface.
SCSI Bus Mastering:: A method of controlling arbitration in a SCSI domain such that two segments of a SCSI bus will maintain proper arbitration protocols over distance without having to modify the arbitration timing of the devices on the bus.
SCSI Switch:: SCSI switches enable users to switch one host between two strings of peripherals or to share SCSI peripherals between two hosts.
SE - Abbreviation for single-ended
SPI - The SCSI-3 Parallel Interface (pronounced "spy"):: The sections of the SCSI-3 specification dealing with parallel SCSI.
SSA - Serial Storage Architecture:: An open serial interface standard developed by IBM called Serial Storage Architecture that has been submitted to the ANSI X3T10.1 subcommittee for approval as an ANSI standard. Incorporates a dual port full-duplex module capable of maintaining four conversations simultaneously for a total of up to 80 MBytes/sec. Includes multiple signal paths for fault tolerance and provides hot plugging and automatic configuration when nodes are added. With shielded twisted pair cable, nodes can be up to 20 meters (65.6 feet) apart. The optical fiber implementation extends this to 2.5 Km. A loop configuration can support up to 127 nodes. Uses 9-pin miniature D-shell connectors with two twisted wire pairs. Requires modified system firmware when interfacing to SCSI. Mapping for SSA will be included in SCSI-3.
STA - SCSI Trade Association:: This organization was formed in October 1995, and is focused on developing and marketing parallel SCSI. STA is the response to the serial SCSI hype and its goal is to channel the support for parallel SCSI.
Segment:: This term was introduced in the EPI technical report. A SCSI bus segment is defined as two terminators and all devices between them. See also: domain.
Single-ended SCSI:: Refers to a manner in which the SCSI cable is driven by SCSI devices. Single-ended (SE) SCSI drives one signal line against ground. Susceptibility to noise limits the maximum allowable cable lengths. SLOW SCSI (up to 5 Megatransfers/second) cabling may be up to 6 meters (19.7 feet) long; FAST SCSI (up to 10 Megatransfers/second) cabling up to 3 meters (9.8 feet) long; and Ultra SCSI (up to 20 Megatransfers/ second) cabling up to 1.5 meters (4.9 feet) long with more than 5 active IDs and up to 3 meters (9.84 feet) with fewer than 5 active IDs. Single-ended and differential SCSI (HVD or LVD) are not compatible on the same bus segment without an electronic device such as a SCSI converter to convert between single-ended and differential. With rare exception, no software (driver) modifications are necessary for conversion between single-ended and differential. There are several variations of terminators developed for use with single-ended SCSI (refer to Terminators) and Differential SCSI).
Skew:: Differences in the arrival times (or departure times) of signals in a parallel SCSI system. Skew is caused by small delay differences in the paths of each SCSI signal including small differences in the length of the wire pairs. Deskew delays in the timing specifications compensate for some skew. Skew becomes ever more important with the tighter timing tolerances required by increasing data throughput.
SLOW SCSI:: SCSI systems with up to 5 Megatransfers/second maximum synchronous data throughput.
Stub:: An unterminated SCSI bus section branching off the main SCSI bus. The SCSI specification dictates that a stub is to be no longer than 0.1 meters (4 inches) for single ended or LVD SCSI and no longer than 0.2 meters (8 inches) for HVD SCSI. Stubs cause lots of difficult-to-trace problems. Stubs are unavoidable but keep them to a minimum and remember that SCSI devices have stubs that cannot be removed (the PC board traces from the connector on a disk drive to its electronic circuitry are stubs).
Sustained Transfer Rate:: The sustained transfer rate is the rate at which a SCSI device can read (or write) data from the medium continuously. Using buffers, the actual rate on the SCSI bus is often much higher (also see: burst rate).
Synchronous (SYNC) transfer:: A method of transferring data that does not require that the bus wait for a REQ-ACK handshake for each byte of data. Instead, the target is allowed to send a number of REQ pulses without waiting for return ACK pulses. This number of REQ pulses is called the "offset". Offset is the maximum number of unanswered REQ pulses that can exist at a given time and may be any number although it is normally something like 8. Offset eliminates the requirement to wait for an ACK for every REQ and avoids the effect of propagation delay in the SCSI cable. The result is a more efficient utilization of the SCSI bus bandwidth and gives a maximum data transfer rate of 5 Megatransfers/second for SCSI-1 NARROW, 10 Megatransfers/second for FAST SCSI and 20 Megatransfers/second for Ultra SCSI. All SCSI bus negotiations, even on a synchronous SCSI bus, are carried on asynchronously. On a synchronous SCSI bus, only the actual data transfers are accomplished synchronously.

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Tagged Command Queuing:

A SCSI-2 feature that is used when the initiator wants to send multiple commands to the same SCSI address or LUN. Tagged queues allow the target to store up to 256 commands per initiator. Without tagged queues, targets could support only one command per LUN for each initiator on the bus. Per the SCSI-2 specification, tagged queue support by targets is optional.

Target:

A SCSI device that executes a command from an initiator to perform a task. Typically a SCSI peripheral device is the target but a host adapter may, in some cases, be a target.

Terminator (Terminations):

Electrical circuitry at the end of a cable designed to match impedances for the purpose of preventing the reflection of electrical signals when they reach the end of the cable. In SCSI systems, this electrical circuitry is called a terminator. It should be noted that any SCSI bus segment requires two terminators and only two terminators. Not one, not three, but two terminators. Also, the terminators must be installed at the very ends of the SCSI cable, not at devices in the middle of the bus. Terminators require power that is usually provided by the host adapter on the TERMPWR line(s) on the bus. Many SCSI devices power their own terminators. There are four basic types of SCSI termination: Passive, active, FPT, and LVD (including LVD/MSE):

Passive - Termed "Alternative 1" in SCSI-2 - The simplest form of terminator consisting of a 220 ohm resistor from TERMPWR to the signal line and a 330 ohm resistor from the signal line to ground. Low cost but has the disadvantage that any fluctuations in the TERMPWR voltage will show up on the signal lines of the bus which may cause data errors. SCSI-2 recommends the use of active terminators whenever possible for single-ended SCSI. Differential (HVD) SCSI uses only passive terminators.
Active - Termed "Alternative 2" in SCSI-2 - Because fluctuations in the TERMPWR voltage supplied to passive terminators shows up as fluctuations in signal levels, active terminators include a voltage regulator to reduce the effect of fluctuations in TERMPWR to insignificance. Uses only a 110 ohm resistor from the regulator to the signal line which is a much closer match to the SCSI cable impedance. Results in more stable SCSI signals, less signal reflection and fewer data errors.
Force Perfect Termination (FPT) - Not recognized in the SCSI specifications. Single-ended termination method utilizing diode switching and biasing to actively compensate for impedance mismatches between the SCSI cabling and the peripheral device. You should be aware that there are several designs of FPT that may not be totally compatible. Also, our customers have found that, generally speaking, FPT likes to "talk" only to FPT.
LVD - Terminator - A form of active termination necessary for the LVD SCSI bus. Defined in the SPI-2 and SPI-3 documents of the SCSI-3 specifications. There are LVD only terminators and "multimode LVD" or "LVD/MSE" terminators. The latter should be used if there is a possibility that a single-ended device will be connected to the multimode LVD bus.
LVD/MSE - LVD that uses "Multimode" transceivers. Depending on the voltage level appearing on the DIFFSENSE pin of the cable, the Multimode transceivers of LVD/MSE will be automatically configured for LVD or single-ended (SE). Most new SCSI designs will include Multimode transceivers.

Term power:

The voltage (+5 VDC) placed on the TERMPWR line(s) of the SCSI bus used to power terminators. SCSI requires that the host adapter provide term power. Many peripheral devices are also capable of providing term power. Having more than one device on the bus providing term power does no harm and is often desirable to reduce problems of voltage "droop" caused by IR losses in long SCSI cables.

Training pattern:

A data pattern sent by the SCSI controller that is used by the peripheral devices to compensate for skew in the SCSI cabling. Introduced in SPI-4 (Ultra320).

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Ultra 320:: Refer to Ultra4.
Ultra SCSI:: Also called "Fast-20". An enhancement of SCSI that results in doubling the FAST SCSI data throughput speeds to 20 MBytes/sec for 8-bit and 40 MBytes/sec for 16-bit. Reduces maximum allowable single-ended SCSI cable length to 1.5 meters (4.9 feet) for five to eight addresses and 3 meters (9.8 feet) for four or fewer addresses. Maximum allowable differential (HVD) SCSI cable length is 25 meters (82 feet). Defined in the SPI document of the SCSI-3 specification.
Ultra2 SCSI:: Also called "Fast-40". An enhancement of SCSI that results in doubling the Ultra SCSI data throughput speeds to 40 Megatransfers/sec (40 MBytes/sec for 8-bit and 80 MBytes/sec for 16-bit). The SCSI specification recognizes only LVD Ultra2 SCSI and single-ended is not defined at this speed. Maximum allowable Ultra2 cable length is 12 meters (39.4 feet) with more than two active SCSI IDs or 25 meters point to point. Backward compatible through the single-ended interface using Ultra or slower speeds and cable lengths.
Ultra3 SCSI:: Also called "Fast-80" or "Ultra160". Described in SPI-3, Ultra160 again doubles the data throughput speeds to 80 Megatransfers/sec (160 MBytes/sec for 16-bit). Single-ended not defined at this speed. Requires LVD signalling. SPI-3 also obsoletes HVD, SCAM and the 8-bit and 32-bit wide bus. U160/m is a sub-set of Ultra160 SCSI that includes double transition clocking, CRC and domain validation. Maximum allowable Ultra3 cable length is 25 meters (82 feet) point to point or 12 meters (40 feet) with more than two active SCSI IDs. Backwards compatible through the single-ended interface using Ultra or slower speeds and cable lengths.
Ultra4 SCSI:: Also called "Ultra 320" or "Fast-160". Again doubles the data throughput to 320 MBytes/sec. Defined in the SPI-4 document of the SCSI-3 specifications, Ultra 320 requires LVD signaling. To reduce overhead and increase data throughput Ultra 320 will include QAS and Information Units (packetizing). Maximum allowable Ultra4 cable length is 12 meters (39.4 feet) with more than two active SCSI IDs and up to 25 meters (82 feet) point to point. Ultra320 is backward compatible through the single-ended interface using Ultra or slower speeds and cable lengths. As with Ultra3 SCSI, single-ended is not defined at Ultra4 throughputs and HVD is obsolete.
Ultra 160:: Refer to Ultra3.
Universal LVD:: A term that was at one time proposed for the LVD/MSE SCSI interface. Not used in the SCSI specifications.
Universal SCSI™:: A SCSI interface developed by Paralan Corporation that senses whether it is connected to a single-ended (SE), HVD or LVD bus segment and automatically sets itself for compatible signal levels and correct termination.

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VHDCI:: The 68-pin Very High Density Cable Interconnect (VHDCI) connector introduced in the SPI-3 document of SCSI-3. The VHDCI connector is a very small connector that allows placement of four wide SCSI connectors on the back of a single PC card slot. Physically, it looks like a miniature Centronics type connector. It uses the regular 68-contact pin assignment.

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WIDE SCSI:: A SCSI implementation that has 16-bit data transfers. Most easily identified by its single 68-pin connector. May also refer to 32-bit SCSI data transfers but 32-bit WIDE SCSI was obsoleted in the SPI-3 document of SCSI-3.

Write Blocker:: Prevents any attempt to write to a drive while allowing complete access to download, view and investigate the drive under examination. Used for computer forensic protection.See Paralan's SCSI Write Blocker

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X3T10:: (formerly X3T9.2) - The name of the ANSI Committee assigned to the task of writing the SCSI specifications.

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We are unaware of any SCSI terminology for the letter Y, but if you know of one e-mail us and we will add it.

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We are unaware of any SCSI terminology for the letter Z, but if you know of one e-mail us and we will add it.

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