SCSI Glossary



A-Cable-A 50-conductor cable that consists of 8 data signals, parity, and 9 control signals. It's associated with both SCSI-1 and SCSI-2.


Active Terminator-A terminator that can compensate for variations in the terminator power supplied by the host adapter through means of a built-in voltage regulator. See also Forced-Perfect Terminator Passive Terminator.


B-Cable-A 68-conductor wide bus option that adds 24 data lines, 3 parity lines, and 2 control signals to the A-Cable. It's associated with 16- and 32-bit Wide and Fast Wide SCSI-2.


Bus-A common pathway, or channel, between multiple devices. Buses are generally hardware, although software can be designed and linked via a so-called "software bus." The computer's internal bus is known as the local bus, or processor bus. It connects the CPU to its main memory and to other buses. The bus is the entire collection of cables and/or wires that make up the path or channel.


Bus Slots.--The computer's bus is comprised of a series of receptacles, connectors, or slots into which expansion boards (video display, disk controllers, etc.) or printers or scanners are plugged.


Controller Card-Controller cards are printed circuit cards that plug directly into the computer's motherboard. The controller cards enable the computer to control and communicate with the control devices. SCSI cards are hard-disk controller cards.


B-Sub-This is a widely used family of D-shaped connectors. They are designated as DB with a number, such as DB25 or DB9. The number represents the number of pins in the connector.


Differential SCST-A type of SCSI that Carries every signal on two wires and senses the signals by measuring the difference in voltage between the two wires. This means that, unlike Single-Ended SCSI, its maximum distance is 25 meters (82 ft.), no matter what the data rate.


EIDE (Enhanced IDE-AN extension to the I DE hardware interface that supports more devices and increased transfer rates. Enhanced IDE, or EIDE, supports multiple channels, each able to connect two devices; the ATA-2 interface, which increases transfer rates to 1 1 MB/sec; and hard disks greater than 528 MB. It also supports the ATAPI interface, which connects CD-ROMS, tape drives, and optical disks. Starting in 1994, almost all PCs were shipped with Enhanced IDE BIOSs, host adapters and drives. Many motherboards provide primary and secondary IDE channels for up to four devices total. Also see IDE.


FPT (Forced Perfect Terminator-A high-quality type of single-ended SCSI terminator, developed by IBM, with special circuitry that compensates not only for variations in terminator power but also for variations in bus impedance. See also Active Terminator Passive Terminator.


Fast-20-A type of SCSI, introduced in the SCSI-3 specification, in which the data rate is quadrupled to 20 MBytes per second for narrow SCSI or 40 MBytes per second for wide SCSI. Also known as Fast-20 or Double Speed SCSI.


Fast 40 type of SCSI in which the data rate is increased to 40 MBytes per second for narrow SCSI or 80 MBytes per second for wide SCSI. Also known as Fast-40.


Fast SCSI type of SCSI, introduced in the SCSI-2 specification, in which the data rate is increased to 1 0 MBytes per second for narrow SCSI or 20 MBytes per second for wide SCSI.


HO Connector-This is a high-density connector in which the pins are spaced close together to save space. High-density A-Gable connectors have the same number of pins as low-density A-cable

connectors, but are smaller than the low-density ones.


IDE (integrated Drive electronics) type of hardware interface widely used to connect hard disks to a PC. IDE is popular because of its lower cost and is increasingly being used to connect CD-ROMs and tape drives. Starting out with 40-MB capacities years ago, IDE hard disks up to 1 GB are now common.


Passive Terminator-A crude type of single-ended SCSI terminator that can't compensate for variations in terminator power or bus impedance. No longer recommended by ANSI, it's adequate for most simple SCSI-1 applications. See also Active Terminator Forced-Perfect Terminator.


SCSI (Small Computer Standard Interface;

pronounced "scuzzy" SCSI )is a hardware interface that enables the connection of up to 7, 1 5, or 31 peri pheral devices (hard disk, CD-ROM, scanner, etc.) to a host adapter. The host adapter connects the host system to the SCSI bus, and performs low layers of protocol when accessing the SCSI bus.


SCSI-2-The second generation of SCSI. Introduced as Fast SCSI and Wide SCSI. The approval of SCSI-2 also introduced mandatory parity checking.


SCSI-3-The third generation of SCSI. Introduced as Fast-20 and Fast-40. It's not yet officially approved.


Terminator-A device, usually having its own connector, that provides termination for a signal line, or several signal lines, at the end of a cable. It's an electrical circuit attached to each end of the SCSI bus to minimize signal reflections and extraneous noise. See also Active Terminator and Forced-Perfect Terminator.


Wide SCSI-A type of SCSI that uses a 16- or 32-bit bus. It can transmit twice as much information as narrow SCSI.