This glossary provides definitions and explanations or common terms and abbreviations used in connection with video terminals and printers. Basic communication terms are also included.


ACK0, ACK1 (Affirmative Acknowledgement)
The replies (DLE sequences in binary synchronous communications) indicate that the previous transmission block was accepted by the receiver and that it is ready to accept the next block of the transmission. Use of ACK0 and ACK1 alternately provides sequential checking control for a series of replies. ACK0 is also an affirmative (ready to receive) reply to a station selection (multipoint), or to an initialization sequence (line bid) in point-to-point operation. See also: DLE (data link escape) and binary synchronous communications. Compare with negative acknowledgement and WACK.
Acoustic coupler
A device that converts electric signals into audio signals, enabling data to be transmitted over the public telephone network via a conventional telephone handset.
See: analog-to-digital converter.
Pertaining to a character set that contains both letters and numbers.
Pertaining to signals, and other quantities, that can occur anywhere on some continuous scale.
Analog-to-digital converter
An A/D (or ADC) converter is a device that converts continuous electrical signals from sensors (analog signals) to a digital form that can be handled by a computer. Used primarily for monitoring instruments or processors, or as part of a control arrangement. Compare with: digital-to-analog converter.
American National Standards Institute.
The ability of a terminal to transmit a stored message that identifies it. This message may be sent from the keyboard or transmitted automatically when the terminal receives an enquiry character (ENQ). See also: ENQ.
American Standard Code for Information Interchange. The code that has assigned a binary number to each alphanumeric character and several nonprinting characters used to control printers and communication devices. The binary number (code) assigned to each alphanumeric character is called ASCII code. ASCII characters are seven or eight bits long and may have an additional parity bit for error detection. See also: bit, control character, parity.
ASCII keyboard
A keyboard that sends an ASCII character to a computer when the user presses the corresponding key. See: ASCII.
Aspect ratio
The ratio of horizontal to vertical dots per inch in graphic printing devices (video displays or printers). See: dot matrix printer and video display terminal.
(Automatic Send/Receive). Refers to terminals, usually printing terminals, that have papertape or other local storage.
A communications method in which data is sent as soon as it is ready, as opposed to methods in which data is sent at fixed intervals. It is commonly used at speeds of 110 to 19,200 b/s. Compare with: synchronous.
Asynchronous transmission
Transmission in which time intervals between transmitted characters may be of unequal length. Transmission is controlled by start and stop elements at the beginning and end of each character. Also called start/stop transmission. Compare with: synchronous transmission.
Auto repeat
If a key on a terminal keyboard is held down for a half-second or so, it begins to repeat automatically until it is released.
Automatic Dialing Unit (ADU)
A device capable of automatically generating dialing digits.
Automatic Calling Unit (ACU)
A dialing device supplied by the communication common carriers that permits a business machine to dial calls automatically over the communication networks. See: common carrier.


Band printer
A lineprinter that uses a rotating steel band as the character transfer device.
Baud rate
The speed at which data is transmitted over a communications link. Usually measured in bits per second.
Baudot code
A code for the transmission of data in which five bits represent one character. It is named for Emile Baudot, a pioneer in printing telegraphy. The name is usually applied to the code used in many teleprinter systems, which was first used by Murray, a contemporary of Baudot.
Bidirectional printing
A printing terminal technique to increase printing throughput by printing lines from right to left in addition to printing other left to right, thus using the carriage return time. If the printer is buffered, the data may be send in the usual order even if it is not to be printed right to left.
Binary digit (bit)
In binary notation either of the characters 0 or 1. "Bit" is the commonly used abbreviation for binary digit.
Binary Synchronous Communications (BSC)
A uniform discipline, using a defined set of control characters and control character sequences, for synchronized transmission of binary coded data between stations in a data communications system. Also called BISYNC.
IBM's 1968 Binary Synchronous Communication Protocol (BSC), still widely used today.
Abbreviation for binary digit.
Bit-map graphics
A technology that allows control of individual pixels on a display screen to produce graphics elements of superior resolution, permitting accurate reproduction of arcs, circles, sine waves, or other curved images that block-addressing technology cannot accurately display. See also: pixels.
Bit transfer rate
The number of bits transferred per unit time, usually expressed in bits per second (b/s, formerly bps).
A group of bits transmitted as a unit, over which a coding procedure is usually applied for synchronization or error control purposes. See also: error control.
Block check character (BCC)
The result of a transmission verification algorithm accumulated over a transmission block, and normally appended at the end, for example, CRC or LRC. See: block.
Block mode terminal
A block mode terminal saves up to a screenful of keystrokes before sending them in a block to the computer. See: block.
When a message is sent to all devices connected to a network, it is said to be broadcast to them.
A place where data can be stored temporarily. Terminals can store data in a buffer if data is received faster than it can be processed or displayed.
A binary element string operated upon as a unit and usually shorter than a computer word; usually eight bits.


Carriage Return/Line Feed (CR/LF)
Two print functions often done together. LF rolls the paper up one line and CR moves the printhead to the left margin.
A continuous frequency capable of being modulated or impressed with a signal.
Carrier system
A means of obtaining a number of communication channels over a single path by modulating each channel upon a different carrier frequency and demodulating at the receiver to restore the signals to their original form.
Cathode-ray tube (CRT)
A television-like picture tube used in video terminals. See: video terminal.
Committee Consultatif Internationale de Telegraphie et Telephonie. An international consultative committee that sets international communications usage standards.
Abbreviation for characters per inch. Formerly cpi.
Abbreviation for characters per second. Formerly cps.
A single printable letter (A through Z), numeral (0 through 9), or symbol, (for example, % . $ ,) used to represent data. Text symbols also include those that are not visible as characters, such as, a space, a tab, or a carriage return.
Character code
A code that assigns numerical values to characters, such as ASCII code. See ASCII.
Character printer
A printer that prints one character at a time like a typewriter. Compare with: lineprinter.
Abbreviation for centimeters per second.
  1. A set of unambiguous rules specifying the way in which data may be represented, for example, the set of correspondences in Standard Code for Information Interchange.
  2. In data communications, a system of rules and conventions according to which the signals representing data can be formed, transmitted, received, and processed.
  3. In data processing, to represent data or a computer program in a symbolic form that can be accepted by a data processor.
Common carrier
In data communications, a public utility company that is recognized by an appropriate regulatory agency as having a vested interest and responsibility in furnishing communication services to the general public, for example, Western Union and the Bell System.
Communication control character
In ASCII, a functional character intended to control or facilitate transmission over data networks. There are 10 control characters specified in ASCII that form the basis for character-oriented communications control procedures. See: control character.
Communication link
The physical connection, typically a phone line, between a terminal and a computer or another physical device.
The ability of an instruction, source language, or peripheral device to be used on more than one computer.
Computer network
An interconnection of computer systems, terminals, and communications facilities.
A communications device that provides communications capability between many low-speed, usually asynchronous channels and one or more high-speed, usually synchronous channels. Usually different speeds, codes, and protocols can be accommodated on the low-speed side. The low-speed channels usually operate in contention requiring buffering. The concentrator may have the capability to be polled by a computer, and may in turn poll terminals. See also: asynchronous, protocol, and synchronous.
The addition of equipment to leased voice-grade lines to provide specified minimum values of line characteristics required for data transmission, for example, equalization and echo suppression. See also: echo suppression and voice-grade channel.
A particular selection of host computer, peripherals, and interfacing equipment that are functioning together. Also a list of devices and computers of a computer system.
Control character
  1. A character whose occurrence in a particular context initiates, modifies, or stops a control function.
  2. In the ASCII code, any of the 32 characters in the first two columns of the standard code table.
See also: communication control character
Control unit
A device, usually incorporated into a terminal, that allows data to be encoded and decoded for transmitting and receiving.
A console is another name for a terminal. A "system console" or an "operator's console" is a special, privileged terminal used to control the computer system.
Pertaining to a mode of processing that involves step-by-step interaction by means of keyboard and display between the user at a terminal and a computer. See also: interactive.
Conversational mode
A procedure for communication between a terminal and the computer in which each entry from the terminal elicits a response from the computer and vice versa.
An interface to transform information from one form to another. See: analog-to-digital converter, digital-to-analog converter, and interface.
CPU (Central Processing Unit)
Commonly called a "computer". A set of electronic components that control the transfer of data and perform arithmetic and logic calculations.
See: cyclic redundancy check.
CRT terminal
Another name for a video terminal. See: cathode-ray tube.
Current loop interface
Current loop interfaces are used to connect terminals directory to computers and are primarily used for long direct-wired hookups or for use where there may be a lot of electrical noise, as in factories. Also called 20mA interface.
On a video terminal screen, the cursor is a distinctive mark (such as a flashing square or underline) that indicates where the next character will be displayed.
Cyclic redundancy check (CRC)
An error detection scheme in which the check character is generated by taking the remainder after dividing all the serialized bits in a block of data by a predetermined binary number. Compare with: block character check, longitudinal redundancy check, and vertical redundancy check.


A Data Access Arrangement is a protective circuit, sometimes built within a modem and sometimes housed in a small box that is between the modem and the telephone system. The FCC requires a DAA to protect the telephone network from possible damage caused by failure of the data equipment attached directly to the telephone line. Acoustic couplers are not attached directly to the telephone line, and therefore do not require DAAs. See also: acoustic coupler and modem.
A printhead that forms full characters rather than characters formed of dots. It is shaped like a wheel with many spokes, with a letter, numeral, or symbol at the end of each spoke. The print method used is similar to that of a regular typewriter.
Daisywheel printing
A type of printing that uses a circular wheel containing fully formed characters on the end of petals radiating out from the center of the wheel. Compare with: dot-matrix printing and thermal printing.
A large collection of organized data that is required to perform a task. Typical examples are personnel files or stock quotations.
Data collection
A mode of data processing whereby data collected by a terminal, group of terminals, light pen, or card reader is sent back to the CPU.
Data communication
The interchange of data messages from one point to another over communications channels.
Data distribution
A mode of data processing whereby a terminal or group of terminals serves as a receptacle for data supplied by the CPU.
Data integrity
A performance measure based on the rate of undetected errors. See: error.
Data-Phone Digital Service (DDS)
A communications service of the Bell System in which data is transmitted in digital rather than analog form, thus eliminating the need for modems.
Data set
  1. A modem. See also: modem.
  2. A collection of data records, with a logical relation of one to another.
Direct Access Vertical Format Unit. Programmable handling of varied form lengths to allow rapid paper slewing within individual forms. See also: Slew speed. Compare with: EVFU.
DDCMP (Digital Data Communications Message Protocol)
A uniform discipline for the transmission of data between stations in a point-to-point or multipoint data communications system. The method of physical data transfer used may be parallel, serial synchronous, or serial asynchronous.
Digital networks.
The process of retrieving an original signal from a modulated carrier wave. This technique is used in modems to make communication signals compatible with computer signals. Compare with: modulation.
Pertaining to the detection and isolation of malfunctions or mistakes.
The use of a dial or pushbutton telephone to initiate a station-to-station telephone call. Dial-up allows an electrical connection of a terminal with a CPU over voice-grade phone lines via a modem.
Digital-to-analog convertor
A D/A converter (or DAC) is an interface that converts data in a digital form to data in analog form. Used to permit analog output from a digital computer. Compare with: analog-to-digital converter.
Display unit
A device that provides a visual representation of data. See also: video terminal.
Direct cursor addressing
A video terminal feature that allows the computer to move the cursor anywhere on the screen to write the next group of data. See also: cursor.
Direct memory access (DMA)
A facility that permits I/O transfers directly into or out of memory without passing through the processor's general registers; performed either independently of the processor or on a cycle-stealing basis.
DLE (Data Link Escape)
A control character used exclusively to provide supplementary line control signals (contol character sequences or DLE sequences). These are two-character sequences where the first character is DLE. The second character varies according to the function desired and the code used. See: control character.
DNA (Digital Network Architecture)
A hardware and software scheme for interconnecting Digital's computers in a network. It is composed of three elements: Data Access Protocol (DAP), Network Services Protocol (NSP), and Digital Data Communications Message Protocol (DDCMP). See: DDCMP.
Dot density
The number of dots per unit of measure in dot-matrix printing. Usually expressed as dots per character (dots/char) or dots per inch (dots/in). See: dot matrix printing. Compare with: text density.
Dot-matrix printing
A printing technique that forms characters from a two-dimensional array of dots. More dots in a given space produce images with greater resolution. See also: band printer, daisywheel printing, laser printing, and thermal printing.
Abbreviation for dots per inch. Formerly dpi.
Abbreviation for dots per second. Formerly dps.
The time interval during which a device is inoperative.
Draft mode
A print mode characterized by high-speed, lower resolution printing.
Draft-quality printer
A printer, usually high-speed, that produces characters that are very legible, but of less than typewriter quality. Typically used for internal documents for which type quality is not a major factor. Compare with: letter-quality printer.
In communications, pertaining to a simultaneous, two-way independent transmission in both directions. Also called full-duplex. Compare with: half-duplex and simplex.
Duty cycle
For printers, this usually refers to the ratio of time spent printing to the total number of power-on hours.


EBCDIC (Extended Binary Coded Decimal Interchange Code)
An eight bit character code used primarily in IBM equipment. The code provides for 256 different bit patterns. Compare with: ASCII.
The printing of characters typed by the operator by an I/O device, such as a terminal or CRT. When the host computer sends back a character just received to the terminal that sent it, the character is said to have been "echoed".
Echo check
A method of checking the accuracy of transmission of data in which the received data is returned to the transmitting end for comparison with the original data.
Echo suppressor
A device used to suppress the effects of an echo.
A program that interacts with the programmer to enter new programs into the computer and edit them as well as modify existing programs. Editors are language-independent and can edit anything in alphanumeric representation.
Electronic Industries Association (EIA)
A standards organization specializing in the electrical and functional characteristics of interface equipment.
EIA RS232-C interface
A standard means for connecting terminals and computer to modems. It is useful for short (generally less than 10 m/50 feet) direct-wired hookups. The full EIA interface also provides several control and status signals between the modem and the digital device.
Engaged signal (UK)
An audible signal indicating that the required circuit or intermediate apparatus used in setting up the connection is busy, that is, not available.
ENQ (Enquiry)
Used as a request for response to obtain identification and/or an indication of status. In binary synchronous (BSC) transmission, ENQ is transmitted as part of an initialization sequence (line bit) in point-to-point operation, and as the final character of a selection or polling sequence in multipoint operation. See also: binary synchronous communications.
The science of human engineering that combines the study of human-body mechanics and physical limitations with industrial psychology to aid in new product design.
Any discrepancy between a computed, observed, or measured quantity and the true, specified, or theoretically correct value or condition. Systematic error: A constant error or one that varies in a systematic manner (for example, equipment misalignment). Random error: An error that varies in a random fashion (for example, an error resulting from radio static).
Error control
A technique whereby errors occurring in the transmission of data characters can be caught, based upon one of several calculations performed on the bit string. In some systems, refinements are added that will correct the detected errors, either by operations on the received data or by transmission from the source. See: error and parity check.
Escape sequence
A special sequence of ASCII characters beginning with the escape character (ESC) used to send special text-formatting or editing commands to terminals. See also: ASCII.
Electronic Vertical Format Unit. Programmable handling of varied form lengths to allow rapid paper slewing within individual forms. Compare with: DAVFU. See also: slew speed.


Fanfold paper
A continuous sheet of paper whose pages are folded accordion-style and separated by perforations.
Fill character
Fill characters and nonprinting characters are transmitted to allow the receiver enough time to process data already sent. The ASCII characters NUL and DEL are most commonly used as fill characters.
Flight timing
The speed and time interval in which a hammer strikes the print mechanism of a lineprinter.
Lineprinter mechanism that senses whether a 64- or 96-character set print band is in use, and prints accordingly.
A complete set of letters, numerals, and symbols of the same typestyle of a given typeface. Examples of typefaces are Baskerville, Century, and Helvetica. Examples of fonts are Baskerville Italic, Baskerville Bold, and Baskerville Bold Italic. Fonts may be a piece of hardware, such as a daisywheel, or they may be resident in the software.
See: block.
Full-duplex (FDX)
Full-duplex describes a communication channel on which simultaneous two-way communication is available. Compare with: half-duplex and simplex.
A print type normally associated with typewriters and letter-quality printers. These form a full impression of the character upon impact, as opposed to the type of character formed by a dot-matrix printer.
Fully-formed character printing
A type of printing that uses typewriter-style fonts to form printed characters. Compare with: dot-matrix printing, laser printing, and thermal printing.


The use of lines and figures to display data, as opposed to the use of printed characters. See: bit-map graphics.


Half-duplex (HDX)
Half-duplex describes a communication channel on which only one-way communication is permitted at a time. The line can be "turned around" to allow data to flow the other way. Some half-duplex links provide a special "reverse channel" in the direction opposite to the flow of data that permits transmission of control signals only. Compare with: full-duplex and simplex.
Hardcopy refers to paper printout, as opposed to video displays which cannot be saved. Compare with: softcopy.
The physical equipment that makes up a computer system. Compare with: software.
Hertz (Hz)
A unit of frequency equal to one cycle per second. Cycles are referred to as Hertz in honor of the experimenter Heinrich Hertz.
Host computer
A computer attached to a network that provides such services as computation, database access, or special programs or programming languages. See: network.
Host interface
The interface between a communication processor and a host computer. See: interface.


The organizational content of a signal.
  1. Data to be processed.
  2. The process of transferring data to memory from a mass storage device or from other peripheral devices that read data from other media (a papertape reader or ADC, for example).
  3. The process of transferring data onto a mass storage device device from a papertape reader, ADC, or other devices that read other media.
  4. The peripheral device used in the transfer described above.
Compare with: output.
I/O (Input/Output)
Pertaining to devices that accept data for transmission to a computer system (input) or that accept data from a computer system for transmission to a user or process. Devices that perform both these functions are known as I/O devices (for example, terminals).
Abbreviation for inches per second. Formerly ips.
Impact printer
A printer that forms characters on paper by striking the paper with a character-forming element.
Impact matrix printhead
A printhead that prints characters by hitting a column of wires onto a ribbon to form a character on the paper behind the ribbon. See also: nine-wire printhead and printhead. Compare with: daisywheel.
Integral modem
A modem built into a terminal rather than packaged separately. See modem.
Capable of carrying on a dialog through a keyboard with the user, rather than simply responding to commands. See also: conversational.
Interactive terminal
A terminal capable of eliciting immediate response to individual user requests, which may instantly update the database of a central computer, for example.
  1. A shared boundary defined by common physical interconnection characteristics, signal characteristics, and meanings of interchanged signals.
  2. Equipment or a device that makes interoperation between two systems possible, for example, a hardware component or a common storage register.
  3. A shared logical boundary between two software components.


In the computer field, loosely two to the tenth power, which is 1,024 in decimal notation. Hence, a 4K memory has 4096 words.
Equals 1,024 bytes. Also known as Kbytes.
Abbreviation for KBytes per second.
Kbyte (KB)
See: KB.
On a typing/printing device, the array of buttons which cause letters to be generated when pushed.
A keypad is a small auxiliary keyboard, often used for entering numeric data, for editing, or for similar special functions.
Keyboard Send/Receive. Refers to a terminal with a keyboard and printer (or display) but no local storage medium. Compare with: receive-only.


Laser printing
Light Amplification by Stimulated Emission of Radiation. Electrophotographic technology used in many industries for its speed, quality, and precision.
Abbreviation for lines per centimeter.
Letter-quality printer
Printer often used to produce high-quality documents. It produces printing comparable in quality to that of a typewriter. Compare with: draft-quality printer.
Light pen
A device resembling a pencil or stylus which emits a light and reads the reflectance. Used to input information to a CRT display system or send coded information to the host.
Abbreviation for lines per inch. Formerly lpi.
  1. The portion of a circuit external to the apparatus that consists of the conductors that connects a telegraph or telephone set to the exchange or that connects two exchanges.
  2. The group of conductors on the same overhead route in the same cable.
  3. The information deposited during one horizontal pass of a printer.
Line Feed (LF)
The printer operation which advances the paper by one line.
A high-speed printer that prints an entire line of characters at a time. While one line is being printed, the next line is held in a buffer until the device is ready to print it. Compare with: character printer.
Abbreviation for lines per minute. Formerly lpm.
Hardwired connection of a computer to another computer, terminal, or peripheral device, such as in a local area network. Compare with: remote.
Longitudinal redundancy check (LRC)
An error checking technique based on an accumulated exclusive-OR of transmitted characters. An LRC character is accumulated at both the sending and receiving stations during the transmission of a block. This accumulation is called Block Character Check (BCC), and is transmitted as the last character in the block. The transmitted BCC is compared with the accumulated BCC character at the receiving station for an equal condition. An equal comparison indicates a good transmission of the previous block. Compare with: block character check, cyclic redundancy check, and vertical redundancy check.


Equals 1,0242.
One million bytes. Also known as Mbytes.
Abbreviation for Mbytes per second.
Message switching
A mode of data processing whereby the CPU is used as a switching center where data is distributed or switched between remote terminals.
Contraction of Modulator/Demodulator. Modems convert digital data from a terminal or CPU into analog signals for transmission over telephone lines and convert the receiver data back to digital format.
The process by which some characteristic of a high-frequency carrier signal is varied in accordance with another, a lower frequency "information" signal. This technique is used in modems to make business-machine signals compatible with communication facilities.
Abbreviation for millisecond(s), one-thousandth of a second.
Mean time between failure. Usually given in hours.
Mean time to repair. Usually given in hours.
The ability of a dot-matrix printer to print at different print speed/dot density combinations. This permits varying print qualities and graphics printing. See: dot-matrix printing.
A device used for multiplexing. It may or may not be a program stored in the computer. Also a device for connecting a number of communications lines to a computer.
A division of a transmission facility into two or more channels.
Multipoint line
A single communications line to which more than one terminal is attached. Use of this type of line normally requires some kind of polling mechanism, addressing each terminal with a unique ID. Also called multidrop. Compare with: point-to-point connection.


Negative acknowledgement (NAK)
Indicates that the previous transmission block was in error and that the receiver is ready to accept a retransmission of the erroneous block. NAK is also the "not ready" reply to a station selection (multipoint) or to an initialization sequence (line bid) in point-to-point operation. See: block. Compare with: ACK0, ACK1, and WACK.
  1. A series of points interconnected by communications channels.
  2. The switched telephone network is the network of telephone lines normally used for dialed telephone calls.
  3. A private network is a network of communications channels confined to the use of one customer.
Nine-wire pinhead
An impact matrix printhead that can print a column of nine dots at a time along the vertical axis. Other common designs have seven or eighteen dot printing capability. Compare: daisywheel.
An end point of any branch of a network, or a junction common to two or more branches of a network.
Undesirable electrical or acoustical disturbances in a communications system. Noise can generate errors in transmission.
Nonimpact printing
Technique whereby the print mechanism does not physically touch the paper during printing. Compare with: band printer, daisywheel printing and dot-matrix printing.
Nontransparent mode
Transmission of characters in a defined character format, for example, ASCII or EBCDIC, in which all defined control characters and control sequences are recognized and treated as such. See also: ASCII, control characters, and EBCDIC.


Pertaining to equipment or devices not under direct control of the central processing unit. May also be used to describe terminal equipment not connected to a transmission line. Compare: on-line.
Pertaining to equipment, devices, and events in direct communication with the CPU and thereby under its control in some way. Regarding terminals, a terminal is said to be "on-line" when it is ready to send or receive data via a communication link.
One-way-only operation
A mode of operation for a data link in which data is transmitted in a preassigned direction over one channel. Also called simplex operation. Compare with: two-way alternate operation and two-way simultaneous operation.
Operating system
A collection of computer programs that control the overall operation of a computer and perform such tasks as assigning places in memory to programs and data, processing interrupts, scheduling jobs, and controlling the overall input/output of the system. See: program.
  1. Data that has been processed.
  2. The state or sequence of states occurring on a specified output channel.
  3. The device or collective set of devices used for taking data out of a device.
  4. A channel for expressing a state of a device or logic element.
  5. The process of transferring data from an internal storage device to an external storage device.
Compare with: input.


Packet switching
A data transmission process, which utilizes addresses packets, in which a channel is occupied only for the duration of transmission of the packet. Note: In certain data communication networks the data may be formatted into a packet or they may be divided and then formatted into a number of packets (either by the data terminal equipment or by equipment within the network) for transmission and multiplexing purposes. See also: multiplexing.
Parallel transmission
Method of data transfer in which all bits of a character or a byte are transmitted simultaneously either over separate communication lines or on different carrier frequencies on the same communication line. See also: bit and byte. Compare with: serial transmission.
A common technique for error detection in data transmission. Parity check bits are added to the data so that each group of data bits include an even number of "ones" for even parity and an odd number for odd parity.
Parity check
Addition of noninformation bits to data, making the number of ones in each grouping either always odd for odd parity or always even for even parity. This permits single error detection in each group. See also: error and error control.
A word or string of characters that is recognizable by the system and that permits a user access to protected storage, files, or input orr output devices.
Picture elements. Definable locations on a display screen that are used to form images on the screen. Pixels refer to the basic unit of graphics resolution. See: bit-map graphics.
A hard surface roller found in impact printers against which the print mechanism strikes.
Used generically throughout Digital's literature to indicate a parameter that can be changed.
Point-to-point communication
  1. A network configuration in which a connection is established between two, and only two, terminal installations. The connection may include switching facilities.
  2. A circuit connecting two points without the use of any intermediate terminal or computer.
Compare with: multipoint line.
A technique for determining the order in which nodes take turns accessing the network. This is done so that access collision can be avoided.
A port is the place of hookup on the CPU, where physical connection is made between the central computer and a terminal, printer, modem, another computer, or a communications line.
A device that produces a paper copy of computer output. There are a variety of printing technologies: band, matrix, and non-impact. Unlike a terminal, there is virtually no communication from printer to CPU. See: draft-quality printer and letter-quality printer.
The element in a printer that forms a printed character. See: daisywheel.
An informal expression referring to almost anything printed by a computer peripheral device; any computer-generated hardcopy.
Printing terminals
Printing terminals, also called hardcopy terminals display data on paper by one of several printing techniques, including impact (band, daisywheel, or dot-matrix), thermal, laser or inkjet. Impact printers can generally make multiple copies on a single pass, while nonimpact technologies cannot. See also: band printer, daisywheel printing, dot-matrix printing, laser printing, and thermal printing.
See daisywheel.
The complete sequence of instructions and routines needed to solve a problem or to execute directions in a computer. See also: operating system.
A formal set of conventions governing the format and relative timing of message exchange between two communicating processes.
Private Switched Telephone Network. Generic term for European telephone carriers. See: carrier.


Receive-only (RO)
A receive-only terminal cannot transmit data back over the communication link, but merely prints, punches, or displays what is sent to it. RO terminals often have no keyboard except for a few control keys such as paper feed and XON/XOFF. Compare with: KSR.
ReGIS (Remote Graphics Instruction Set)
Digital's graphics command interface to terminals for putting shapes on the terminal screen. Digital's VT125 terminal contains a ReGIS interpreter.
Not hard-wired; communicating via switched lines such as telephone lines. Usually refers to peripheral devices (for example, printers or video terminals) that are located at a site away from the CPU. Compare with: local.
Response time
The elapsed time between the generation of the last character of a message at a terminal and the receipt of the first character of the reply. It includes terminal delay, network delay, and service node delay.
Reverse video
A feature on a display unit that produces the opposite combination of characters and background from that which is usually employed, that is, white characters on a black screen, if having black characters on a white screen is normal. Part or all of the data may be displayed as black on white instead of the usual white on black.
A characteristic of a keyboard that can continue to send the proper codes when several keys are held down at one time. A keyboard with two-key rollover will handle depression of two keys at a time. N-key rollover describes keyboards with no limit to the number of keys that can be depressed without interfering with code generation.


When a video terminal's screen is full, a new line of data can be displayed by adding it at the bottom of the screen and shifting all the previous lines upward, discarding the top line. This process is known as scrolling. When the upward movement is continuous rather than in line steps, it is called smooth scrolling.
Selective addressing
On a multidrop line, selective addressing allows a sender to address a message to a particular receiver (or group of receivers) by sending the intended destination before the message. See: multipoint.
A procedure whereby a program or peripheral checks its own operation. With printing terminals, an off-line generation of print patterns is used to verify the functioning and registration of the print mechanism. With video terminals, a series of test patterns is displayed on the screen as the self-test procedure is performed. See also: diagnostic.
Serial transmission
A method of information transmission in which each bit of information is sent sequentially on a single path rather than simultaneously as in parallel transmission. Compare with: parallel transmission.
In communications, pertaining to a channel that operates in one direction only, as in a public address system or a commercial music radio. Compare with: full-duplex, half-duplex.
Simplex mode
Operation of a channel in one direction only, with no capability for reversing direction.
Slew speed
Speed with which a lineprinter skips over lines that contain no print.
IBM's System Network Architecture. Similar to the Digital Network, it provides a common method of support for a wide range of communication activities sharing a single communications network. SNA is composed of three elements: Virtual Telecommunication Access Method (VTAM), Network Control Program/Virtual Storage (NCP/VS) and Synchronous Data Link Control (SDLC). Compare with: DNA (Digital Network Architecture).
Alphanumeric or graphical data (or both) presented in nonpermanent form, on a video terminal screen. Compare with: hardcopy.
A set of computer programs, procedures, rules and associated documentation concerned with the operation of network computers, e.g., compilers, monitors, editors, utility programs. Compare with: hardware. See also: operating system and program.
  1. A cylindrical coil of insulated wire in which an axial electromagnetic field is established by a flow of electric current.
  2. An assembly often used as a switch which consists essentially of a coil and a metal core free to slide along the coil axis under the influence of the magnetic field.
A technique in which data bits are sent at precisely timed intervals. Synchronous channels are capable of higher data rates than asynchronous ones, often running at 56,000 bits per second. Compare with: asynchronous.
Synchronous idle (SYN)
Character used as a time-fill in the absence of any data or control character to maintain synchronization. The sequence of two continuous SYNs is used to establish synchronization (character phase) following each line turnaround. See: BISYNC.
Synchronous transmission
Transmission in which the data characters and bits are transmitted at a fixed rate with the transmitter and receiver synchronized. This eliminates the need for start/stop elements, thus providing greater efficiency. Compare with: asynchronous transmission.


A system of communication for the transmission of graphic symbols, usually letters or numerals, by use of a signal code.
Term used to refer to the equipment used in a printing telegraphy system: a teletypewriter.
Trademark of Teletype Corporation. Usually refers to one of their series of teleprinters.
Teletypewriter exchange service (TWX)
A public teletypewriter exchange (switched) service in the United States and Canada, formerly belonging to AT&T Company, now owned by the Western Union Company. Both Baudot and ASCII-coded machines are used. See: ASCII, baud rate.
Telex service
A Western Union worldwide teletypewriter exchange service that uses the public telegraph network. Baudot equipment is used.
An input-output device generally used for communication between the operator or user of a computer system and the system itself. See: hardcopy, softcopy.
Terminal installation
  1. The totality of equipment at a user's installation including data terminal equipment, data communication equipment, and necessary support facilities. See also: terminal.
  2. A set composed of data terminal, a signal convertor, and possibly intermediate equipment; this set may be connected to a data processing machine or may be part of it.
Terminal noise
Electromagnetic noise emitted from hot bodies; sometimes called Johnson noise.
  1. A sequence of characters that form part of a transmission sent from the data source to the data sink, which contains the information to be conveyed. It may be preceded by a header and followed by an "End of Text" signal.
  2. In ASCII as well as in general communications usage, a sequence of characters treated as an entity if preceded by a "Start of Text" and followed by an "End of Text" control character.
Text density
The number of printed characters per line of text in a document.
Thermal printing
Thermal printing produces dot-matrix characters on special paper that blackens when heated. Since the paper deteriorates with time, thermal printed output is not suitable for permanent records. Compare with: daisywheel printing, dot-matrix printing, and laser printer.
Thermal transfer printing
A method of thermal printing which can print on plain paper, as well as specially coated thermal paper. See: thermal printing.
A mode of data processing that allows many terminal users to utilize a computer's resources to perform a variety of tasks simultaneously.
An attachment used to move paper through a printer. The roller that moves the paper has sprockets on each end that fit into the fanfold paper's matching pattern of holes. See: fanfold paper.
Transparent mode
Transmission of binary data with the recognition of most control characters suppressed. In binary synchronous communications, entry to and exit from the transparent mode is indicated by a Data Link Escape (DLE) character sequence. See: DLE.
Turnaround time
  1. The elapsed time between submission of a job to a computing center and the return of the results.
  2. In communications, the actual time required to reverse the direction of transmission from sender to receiver or vice versa when using a two-way alternate circuit. Time is required by line propagation effects, modem timing, and computer reaction. See: two-way alternate operation.
Two-way alternate operation
A mode of operation of a data link in which data is transmitted in both directions, one way at a time. Also called: half-duplex operation (U.S.). Compare with: one-way only operation and two-way simultaneous operation.
Two-way simultaneous operation
A mode of operation of a data link in which data is transmitted simultaneously in both directions over two channels. Note: One of the channels is equipped for transmission in one direction while the other is equipped for transmission in the opposite direction. Also called full-duplex or duplex. Compare with: one-way only operation and two-way alternate operation.
See teletypewriter exchange service.
See font.


Unattended operation
The automatic operation of a station that permits the transmission and reception of messages on an unattended basis.
User-defined key (UDK)
A key that remembers and stores a series of keystrokes, which allows the user to save the keystrokes needed to perform a specific operation, and the initiate them in the proper sequence by pressing only one key.


Vertical redundancy check (VRC)
A check or parity bit added to each character in a message so that the number of bits in each character, including the parity bit, is odd (odd parity) or even (even parity). Compare with: block character check, cyclic redundancy check, and longitudinal redundancy check. See also: parity and parity check.
Voice-grade channel
A channel used for speech transmission, usually with an audio frequency range between 300 and 3,400 Hertz. It is also used for transmission of analog and digital data. Up to 10,000 bits per second can be transmitted on a voice-grade channel.
Video Terminal.
Video display terminal
See: video terminal.
Video terminal
A terminal using a television-like screen for displaying information. Video terminals' advantages include silent operation, the ability to operate at very high data rates, and the capability to put new data anywhere on the screen and to erase data already displayed.


WACK (Wait Before Transmitting Positive Acknowledgement)
In binary synchronous communications, this DLE sequence is sent by a receiving station to indicate that it is temporarily not ready to receive data. Compare with: ACK0, ACK1, and negative acknowledgement. See also: binary synchronous communications and DLE.
WATS (Wide Area Telephone Network)
A service provided by telephone companies in the United States that permits a customer to make calls to or from telephones in specific zones for a flat monthly charge. The monthly charges are based on the size of the zone instead of number of calls. WATS may be used on a measured-time or full-time basis.
The greatest number of bits a computer is capable of handling in any one operation. Usually subdivided into bytes. See: bit and byte.
Word processing system
A system that processes text, performing such functions as paragraphing, paging, left and right justification, rearrangement of lines, and printing the text.
The automatic shifting of words from a line that is too long to the next line.


XON and XOFF are the names of two ASCII characters used in Digital systems and elsewhere for controlling the flow of data across a full-duplex communications channel. If the receiver sends the transmitter an XOFF character, the transmitter stops sending until the receiver signals it to resume by sending the transmitter an XON character.