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 3 MBytes/second that is decreased substantially by even small increases in cable length. All SCSI bus negotiations, even on a synchronous SCSI bus, are carried on asynchronously. On a synchronous SCSI bus, only actual data transfers are accomplished synchronously.


The single most critical item in the installation of a successful, high-reliability, maximum throughput SCSI system. The longer the cables and the faster the data throughput, the more critical cables become. Internal SCSI cables are usually unshielded 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. Do not waste time and money on cheap SCSI cables!
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:



SINGLE ENDED - SLOW 6 Meters / 19.7 Feet
FAST 3 Meters* / 9.8 Feet
ULTRA 1.5 Meters / 4.9 Feet
DIFFERENTIAL 25 Meters / 82 Feet

*1 Maximum cable length with eight addresses. May be up to 3 meters with four devices.

Single-ended Cable - To put it bluntly, there is no such thing as a "single-ended SCSI cable". Some cable manufacturers save manufacturing cost by deleting up to 25 of the ground wires in the SCSI cable. That drastically changes cable impedance and causes 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, do not use so called "single-ended" SCSI cables.

CAM - Common Access Method
Proposed 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.
Command Queuing
Refer to Tagged Command Queuing.
An electronic product designed to convert between single-ended and differential SCSI signals. 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 61 meters (200 feet). Termed an "expander" in SCSI-3.


Differential SCSI
Refers to the manner in which the SCSI cable is driven by the host and peripheral devices. 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 cables of up to 25 meters (82 feet) versus 6 meters (19.7 feet) or less for single-ended. Maximum low voltage differential (LVD) SCSI cable length is 12 meters (39.4 ft). Differential and single-ended SCSI are not compatible on the same bus 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. Differential has no requirement for other than passive terminators.
A signal on differential SCSI cabling (Pin 21 on NARROW and pin 16 on WIDE) that is used as an active HIGH enable for differential drivers. If a single-ended device is connected to the bus, this line is pulled low. This disables the differential drivers to protect them from trying to drive signals into ground. Used by Universal transceivers (LVD/SE) to determine if the SCSI device is connected to single-ended or differential cabling.
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 device can make use of the bus. Also see "reconnect".

Double-Speed SCSI
Refer to Ultra SCSI
Refers to 32-bit SCSI.


A device to be officially introduced in an X3T10 Committee report. 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.
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.


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.
Refer to Ultra SCSI
Refer to Ultra-2 SCSI
Usually refers to 16-bit (WIDE) SCSI with FAST data transfers of up to 40 MBytes/second. May also refer to 32-bit SCSI FAST data transfers (up to 80 MBytes/ second).
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.
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.
Apple's name for their implementation of IEEE 1394. Refer to High Performance Serial Bus (IEEE 1394).
Refer to Terminator, Force Perfect Termination.


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.
Hot Plugging or Hot Swapping
The ability to remove and replace devices from the SCSI 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.
HVD - High Voltage Differential
Differential SCSI scheme that has been in use for years. Terminators run on 5 VDC. See also LVD.


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 0 to 7 for 8-bit, 0 to 15 for 16-bit and 0 to 31 for 32 bit systems. The host is usually assigned ID #7.
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.
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.


LUN - Logical Unit Number
A method to expand the number of devices that can be placed on one SCSI bus domain. Logical Unit Numbers address up to eight devices at each SCSI ID.
LVD - Low Voltage Differential
Refer to "LVD" under "Terminator".


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.


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


Parity checking
A simple way of detecting errors in SCSI data that is required to be built into all SCSI-2 devices and will be continued in future issues of SCSI. 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.


Redundant Array of Inexpensive Disks.
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".
A Paralan trademarked product name for a 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.


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.
SCAM - SCSI Configured Auto-Magically.
In combination with Intel/Microsoft's Plug-and-Play will allow 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. To be defined in SCSI-3.
SCSI - Small Computer Systems 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. See SCSI-1 X3.131-1986.
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 Mega words per second (5 MBytes/sec for 8 bit). Defined single-ended and differential. Did not include definitions of a device independent interface.
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 Megawords /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.
An ANSI Committee X3T10 work in progress. Will include enhancements for parallel SCSI as well as definitions for serial SCSI to allow interface to serial buses such as Fibre Channel, SSA and Firewire. Parallel SCSI enhancements include UltraSCSI (also called Fast-20 and DoubleSpeed SCSI) which doubles SCSI-2 speeds to maximum data throughput of 20 Megawords/second (20 MBytes/sec for 8-bit SCSI; 40 MBytes/sec for 16-bit SCSI; 80 MBytes/sec for 32-bit SCSI). May include Ultra-2 SCSI which doubles these data throughputs again and requires low voltage differential (LVD) inputs. Also 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. Expected to include SCAM. Reportedly will include definitions for expanders including SCSI enhancement devices such as converters and specifications on active negation terminators for single-ended devices. Will be backward compatible with SCSI-2 as well as SCSI-1.
SPI - The SCSI-3 Parallel Interface
The sections of the SCSI-3 specification dealing with parallel SCSI. (pronounced "spy").
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.
Single-ended SCSI
Refers to the manner in which the SCSI cable is driven by the host and peripheral devices. Single-ended SCSI drives one signal line against ground. Susceptibility to noise limits the maximum allowable cable lengths. SLOW SCSI (up to 5 Megawords/second) cables may be up to 6 meters (19.7 feet) long; FAST SCSI (up to 10 Megawords/second) cables up to 3 meters (9.8 feet) long; and UltraSCSI (up to 20 Megawords/ second) cables up to 1.5 meters (4.9 feet) long. Single-ended and differential SCSI are not compatible on the same bus 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).
SCSI systems with up to 5 Megawords/second maximum synchronous data throughput.
An unterminated SCSI bus segment 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 SCSI and no longer than 0.2 meters (8 inches) for differential 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).
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 Megawords/second for SCSI-1 NARROW, 10 Megawords/second for FAST SCSI and 20 Megawords/second for UltraSCSI. 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.


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.
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.
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. LVD requires a 3.3 VDC TERMPOWER instead of 5 VDC.
Terminator (Terminations)
Electrical circuitry at the end of a cable to prevent 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 five basic types of SCSI termination: Active, active negation, FPT, LVD (including LVD/SE) and passive.


Ultra SCSI
A proposed 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 eight addresses and 3 meters (9.8 feet) for four addresses. Maximum allowable differential SCSI cable length is 25 meters (82 feet). The official definition of UltraSCSI will be released when the SCSI-3 specification is released.
Ultra/Wide SCSI
This is a 16-bit implementation of Ultra SCSI which has a data thourghput of 40 MBytes/sec. It is a combination of the Ultra and Wide SCSI standaard.
Ultra-2 SCSI
A proposed enhancement of SCSI that results in doubling the UltraSCSI data throughput speeds to 40 MBytes/sec for 8-bit and 80 MBytes/sec for 16-bit. Requires LVD. The SCSI specification will recognize only differential Ultra-2 SCSI. Maximum allowable Ultra-2 cable length is 12 meters (39.4 feet).


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 is not yet commonly encountered.


(formerly X3T9.3) - The name of the ANSI Committee assigned to the task of writing the SCSI specifications.

Note:This glossary is of practical definitions and is not exhaustive. It includes definitions of SCSI terms you are likely to encounter that relate to SCSI products. The definitions given are not necessarily those of the ANSI Standards Committee. Because it is meant to be a glossary of SCSI terms that you are likely to encounter today but may not know the definition of, it does not necessarily contain standard electrical/electronic definitions such as "impedance". Refer to the latest IEEE release of the SCSI specifications X3.131-199X for formal definitions of SCSI terms.