Computer terminal
A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. Most early computers only had a front panel to input or display bits and had to be connected to a terminal to print or input text through a keyboard. Teleprinters were used as early-day hard-copy terminals and predated the use of a computer screen by decades. The computer would typically transmit a line of data which would be printed on paper, and accept a line of data from a keyboard over a serial or other interface. Starting in the mid-1970s with microcomputers such as the Sphere 1, Sol-20, and Apple I, display circuitry and keyboards began to be integrated into personal and workstation computer systems, with the computer handling character generation and outputting to a CRT display such as a computer monitor or, sometimes, a consumer TV, but most larger computers continued to require terminals.
Early terminals were inexpensive devices but very slow compared to punched cards or paper tape for input; with the advent of time-sharing systems, terminals slowly pushed these older forms of interaction from the industry. Related developments were the improvement of terminal technology and the introduction of inexpensive video displays. Early Teletypes only printed out with a communications speed of only 75 baud or 10 5-bit characters per second, and by the 1970s speeds of video terminals had improved to or. Similarly, the speed of remote batch terminals had improved to at the beginning of the decade and by the end of the decade, with higher speeds possible on more expensive terminals.
The function of a terminal is typically confined to transcription and input of data; a device with significant local, programmable data-processing capability may be called a "smart terminal" or fat client. A terminal that depends on the host computer for its processing power is called a "dumb terminal" or a thin client. In the era of serial terminals there was a conflicting usage of the term "smart terminal" as a dumb terminal with no user-accessible local computing power but a particularly rich set of control codes for manipulating the display; this conflict was not resolved before hardware serial terminals became obsolete.
The use of terminals decreased over time as computing shifted from command line interface to graphical user interface and from time-sharing on large computers to personal computers and handheld devices. Today, users generally interact with a server over high-speed networks using a Web browser and other network-enabled GUI applications.
Today, a terminal emulator application provides the capabilities of a physical terminal allowing interaction with the operating system shell and other CLI applications. xterm, GNOME Console, Konsole, Terminal, Windows Terminal, and PuTTY are examples of modern terminal emulators.
History
The console of Konrad Zuse's Z3 had a keyboard in 1941, as did the Z4 in 1942–1945. However, these consoles could only be used to enter numeric inputs and were thus analogous to those of calculating machines; programs, commands, and other data were entered via paper tape. Both machines had a row of display lamps for results.In 1956, the Whirlwind Mark I computer became the first computer equipped with a keyboard-printer combination with which to support direct input of data and commands and output of results. That device was a Friden Flexowriter, which would continue to serve this purpose on many other early computers well into the 1960s.
Categories
Hard-copy terminals
Early user terminals connected to computers were, like the Flexowriter, electromechanical teleprinters/teletypewriters, such as the Teletype Model 33, originally used for telegraphy; early Teletypes were typically configured as Keyboard Send-Receive or Automatic Send-Receive. Some terminals, such as the ASR Teletype models, included a paper tape reader and punch which could record output such as a program listing. The data on the tape could be re-entered into the computer using the tape reader on the teletype, or printed to paper. Teletypes used the current loop interface that was already used in telegraphy. A less expensive Read Only configuration was available for the Teletype.Custom-designs keyboard/printer terminals that came later included the IBM 2741 and the DECwriter. Respective top speeds of teletypes, IBM 2741 and the LA30 were 10, 15 and 30
characters per second. Although at that time "paper was king" the speed of interaction was relatively limited.
The DECwriter was the last major printing-terminal product. It faded away after 1980 under pressure from video display units, with the last revision abandoning the classic teletypewriter form for one more resembling a desktop printer.
Printing terminals required that the print mechanism be away from the paper after a pause in the print flow, to allow an interactively typing user to see what they had just typed and make corrections, or to read a prompt string. As a dot-matrix printer, the DECwriter family would move the print head sideways after each pause, returning to the last print position when the next character came from the remote computer.
Video display unit
A video display unit displays information on a screen rather than printing text to paper and typically uses a cathode-ray tube. VDUs in the 1950s were typically designed for displaying graphical data rather than text and were used in, e.g., experimental computers at institutions such as MIT; computers used in academia, government and business, sold under brand names such as DEC, ERA, IBM and UNIVAC; military computers supporting specific defence applications such as ballistic missile warning systems and radar/air defence coordination systems such as BUIC and SAGE.Two early landmarks in the development of the VDU were the Univac Uniscope and the IBM 2260, both in 1964. These were block-mode terminals designed to display a page at a time, using proprietary protocols; in contrast to character-mode devices, they enter data from the keyboard into a display buffer rather than transmitting them immediately. In contrast to later character-mode devices, the Uniscope used synchronous serial communication over an EIA RS-232 interface to communicate between the multiplexer and the host, while the 2260 used either a channel connection or asynchronous serial communication between the 2848 and the host. The 2265, related to the 2260, also used asynchronous serial communication.
The Datapoint 3300 from Computer Terminal Corporation, announced in 1967 and shipped in 1969, was a character-mode device that emulated a Model 33 Teletype. This reflects the fact that early character-mode terminals were often deployed to replace teletype machines as a way to reduce operating costs.
The next generation of VDUs went beyond teletype emulation with an addressable cursor that gave them the ability to paint two-dimensional displays on the screen. Very early VDUs with cursor addressibility included the VT05 and the Hazeltine 2000 operating in character mode, both from 1970. Despite this capability, early devices of this type were often called "Glass TTYs". Later, the term "glass TTY" tended to be restrospectively narrowed to devices without full cursor addressibility.
The classic era of the VDU began in the early 1970s and was closely intertwined with the rise of time sharing computers. Important early products were the ADM-3A, VT52, and VT100. These devices used no complicated CPU, instead relying on individual logic gates, LSI chips, or microprocessors such as the Intel 8080. This made them inexpensive and they quickly became extremely popular input-output devices on many types of computer system, often replacing earlier and more expensive printing terminals.
After 1970 several suppliers gravitated to a set of common standards:
- ASCII character set, but early/economy models often supported only capital letters
- RS-232 serial ports
- 24 lines of 72 or 80 characters of text. Later models sometimes had two character-width settings.
- Some type of cursor that can be positioned.
- Implementation of at least 3 control codes: Carriage Return, Line-Feed, and Bell, but usually many more, such as escape sequences to provide underlining, dim or reverse-video character highlighting, and especially to clear the display and position the cursor.
The great variations in the control codes between makers gave rise to software that identified and grouped terminal types so the system software would correctly display input forms using the appropriate control codes; In Unix-like systems the termcap or terminfo files, the stty utility, and the TERM environment variable would be used; in Data General's Business BASIC software, for example, at login-time a sequence of codes were sent to the terminal to try to read the cursor's position or the 25th line's contents using a sequence of different manufacturer's control code sequences, and the terminal-generated response would determine a single-digit number that would be available to programs to say which set of codes to use.
The great majority of terminals were monochrome, manufacturers variously offering green, white or amber and sometimes blue screen phosphors.. Terminals with modest color capability were also available but not widely used; for example, a color version of the popular Wyse WY50, the WY350, offered 64 shades on each character cell.
VDUs were eventually displaced from most applications by networked personal computers, at first slowly after 1985 and with increasing speed in the 1990s. However, they had a lasting influence on PCs. The keyboard layout of the VT220 terminal strongly influenced the Model M shipped on IBM PCs from 1985, and through it all later computer keyboards.
Although flat-panel displays were available since the 1950s, cathode-ray tubes continued to dominate the market until the personal computer had made serious inroads into the display terminal market. By the time cathode-ray tubes on PCs were replaced by flatscreens after the year 2000, the hardware computer terminal was nearly obsolete.