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A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying or printing data from, a or a system.similar to a paraphrase of an Oxford English Dictionary definition. Based on OED, B.2.d. (terminal), the paraphrase says that a terminal is a device for feeding data into a computer or receiving its output, especially one that can be used by a person for two-way communication with a computer. The was an example of an early day hardcopy terminal,, and predated the use of a computer screen by decades.; it also had a light-pen.

(1980). 9780932376091, Digital Equipment Corporation/Digital Press.

Early terminals were inexpensive devices but very slow compared to or for input, but as the technology improved and were introduced, terminals pushed these older forms of interaction from the industry. A related development was systems, which evolved in parallel and made up for any inefficiencies of the user's typing ability with the ability to support multiple users on the same machine, each at their own terminal.

The function of a terminal is confined to display and input of data; a device with significant local programmable data processing capability may be called a "smart terminal" or . A terminal that depends on the host computer for its processing power is called a "dumb terminal" or . A personal computer can run terminal emulator software that replicates the function of a terminal, sometimes allowing concurrent use of local programs and access to a distant terminal host system.


History

The terminal of the first working programmable, fully automatic digital Turing-complete computer, the Z3, had a keyboard and a row of lamps to show results.Lippe, Prof. Dr. Wolfram. "Kapitel 14 – Die ersten programmierbaren Rechner (i.e. The first programmable computers)"


Hard-copy terminals
Early user terminals connected to computers were electromechanical /teletypewriters (TeleTYpewriter, TTY), such as the Teletype Model 33 ASR, originally used for or the Friden Flexowriter. Later printing terminals such as the DECwriter LA30 were developed. However printing terminals were limited by the speed at which paper could be printed, and for interactive use the paper record was unnecessary.

By the early 1970s, many users in the computer industry realized that an affordable video data entry terminal could supplant the ubiquitous and permit new uses for computers that would be more interactive. The problem was that the amount of memory needed to store the information on a page of text was comparable to the memory in low end then in use. Displaying the information at video speeds was also a challenge and the necessary control logic took up a rack worth of pre-integrated circuit electronics. One company announced plans to build a video terminal for $15,000 and attracted a large backlog of orders, but folded when their engineering plans, which included fabricating their own ICs, proved too ambitious. Another approach involved the use of the , a specialized CRT developed by that retained information written on it without the need to refresh.

The Datapoint 3300 from was announced in 1967 and shipped in 1969, making it one of the earliest stand-alone display-based terminals. It solved the memory space issue mentioned above by using a digital shift-register design, and using only 72 columns rather than the later more common choice of 80.


Early VDUs
Early video computer displays were sometimes nicknamed "Glass TTYs" ("glass Teletypes") or "Visual Display Units" (VDUs), and used no CPU, instead relying on individual or very primitive LSI chips. Nevertheless, they quickly became extremely popular Input-Output devices on many different types of computer system once several suppliers gravitated to a set of common standards:
  • character set (rather than, say, or anything specific to one company), but early/economy models often supported only capital letters (such as the original ADM-3, the model 6052 – which could be upgraded to a 6053 with a lower-case character ROM – and the Heathkit H9)
  • RS-232 serial ports (25-pin, ready to connect to a modem, yet some manufacturer-specific pin usage extended the standard, e.g. for use with 20-mA current loops)
  • 24 lines (or possibly 25 – sometimes a special status line) of 80 characters of text (the same as punched cards). Later models sometimes had two character-width settings.
  • Some type of blinking cursor that can be positioned (with arrow keys or "home" and other direct cursor address setting codes).
  • Implementation of at least 3 control codes: Carriage Return (Ctrl-M), Line-Feed (Ctrl-J), and Bell (Ctrl-G), but usually many more, such as to provide underlining, dim or reverse-video character highlighting, and especially to clear the display and position the cursor.

Starting with the Datapoint 3300 in 1969, by the late 1970s and early 1980s, there were dozens of manufacturers of terminals, including Lear-Siegler, , , DEC, Hazeltine Corporation, , , , , and , many of which had incompatible command sequences (although many used the early ADM-3 as a starting point).

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 -like systems the 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 (such as 6 for Data General Dasher terminals, 4 for ADM 3A/5/11/12 terminals, 0 or 2 for TTY's with no special features) 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. (Amber was claimed to reduce eye strain). Terminals with modest colour capability were also available but not widely used, for example a colour version of the popular Wyse WY50, the WY350, offered 64 shades on each character cell.


"Intelligent" terminals
An "intelligent" terminal does its own processing, usually implying a microprocessor is built in, but not all terminals with microprocessors did any real processing of input: the main computer to which it was attached would have to respond quickly to each keystroke. The term "intelligent" in this context dates from 1969.Twentieth Century Words; by John Ayto; Oxford Unity Press; page 413

Notable examples include the IBM 2250 and IBM 2260, predecessors to the IBM 3270 and introduced with System/360 in 1964.

From the introduction of the IBM 3270, and the DEC VT100 (1978), the user and programmer could notice significant advantages in VDU technology improvements, yet not all programmers used the features of the new terminals (backward compatibility in the VT100 and later terminals, for example, with "dumb terminals" allowed programmers to continue to use older software).

Some dumb terminals had been able to respond to a few escape sequences without needing microprocessors: they used multiple printed circuit boards with many Integrated Circuits; the single factor that classed a terminal as "intelligent" was its ability to process user-input within the terminal — not interrupting the main computer at each keystroke — and send a block of data at a time (for example: when the user has finished a whole field or form). Most terminals in the early 1980s, such as ADM-3A, TVI912, Data General D2, DEC VT52, despite the introduction of ANSI terminals in 1978, were essentially "dumb" terminals, although some of them (such as the later ADM and TVI models) did have a primitive block-send capability. Common early uses of local processing power included features that had little to do with off-loading data processing from the but added useful features such as printing to a local printer, buffered serial data transmission and serial handshaking (to accommodate higher serial transfer speeds), and more sophisticated character attributes for the display, as well as the ability to switch emulation modes to mimic competitor's models, that became increasingly important selling features during the 1980s especially, when buyers could mix and match different suppliers' equipment to a greater extent than before.

The advance in microprocessors and lower memory costs made it possible for the terminal to handle editing operations such as inserting characters within a field that may have previously required a full screen-full of characters to be re-sent from the computer, possibly over a slow modem line. Around the mid 1980s most intelligent terminals, costing less than most dumb terminals would have a few years earlier, could provide enough user-friendly local editing of data and send the completed form to the main computer. Providing even more processing possibilities, workstations like the TS-800 could run CP/M-86, blurring the distinction between terminal and Personal Computer.

Another of the motivations for development of the was to simplify and reduce the electronics required in a terminal. That also made it practicable to load several "personalities" into a single terminal, so a Qume QVT-102 could emulate many popular terminals of the day, and so be sold into organizations that did not wish to make any software changes. Frequently emulated terminal types included:

  • ADM-3A and later models
  • 910 to 950 (these models copied ADM3 codes and added several of their own, eventually being copied by Qume and others)
  • Digital Equipment Corporation VT52 and VT100
  • D1 to D3 and especially D200 and D210
  • Hazeltine Corporation H1500
  • 4014
  • W50, W60 and W99

The ANSI X3.64 escape code standard produced uniformity to some extent, but significant differences remained. For example, the VT100, H19 in ANSI mode, 970, D460, and QVT-108 terminals all followed the ANSI standard, yet differences might exist in codes from , what character attributes were available, block-sending of fields within forms, "foreign" character facilities, and handling of printers connected to the back of the screen.

Most terminals were connected to or mainframe computers and often had a green or amber screen. Typically terminals communicate with the computer via a via a cable, often using an EIA RS-232 or RS-422 or RS-423 or a current loop serial interface. systems communicated over a using IBM's SNA protocol, but for many DEC, and (and so on) computers there were many visual display suppliers competing against the computer manufacturer for terminals to expand the systems. In fact, the instruction design for the Intel 8008 was originally conceived at as the processor for the Datapoint 2200.


Contemporary
While early had single color green screens, these screens were not terminals. The screen of a PC did not contain any character generation hardware; all video signals and video formatting were generated by the video display card in the PC, or (in most graphics modes) by the CPU and software. An IBM PC monitor, whether it was the green monochrome display or the 16-color display, was technically much more similar to an analog TV set (without a tuner) than to a terminal. With suitable software a PC could, however, emulate a terminal, and in that capacity it could be connected to a mainframe or minicomputer. The Data General One could be booted into terminal emulator mode from its ROM. Eventually microprocessor-based personal computers greatly reduced the market demand for conventional terminals.

In the 1990s especially, "thin clients" and have combined economical local processing power with central, shared computer facilities to retain some of the advantages of terminals over personal computers:

Today, most PC clients provide emulation of the most common terminal, the DEC VT100, using the ANSI escape code standard X3.64, or could run as using software such as Cygwin/X under Microsoft Windows or X.Org Server software under Linux.

Since the advent and subsequent popularization of the personal computer, few genuine hardware terminals are used to interface with computers today. Using the and keyboard, modern operating systems like and the derivatives feature virtual consoles, which are mostly independent from the hardware used.

When using a graphical user interface (or GUI) like the X Window System, one's display is typically occupied by a collection of windows associated with various applications, rather than a single stream of text associated with a single process. In this case, one may use a terminal emulator application within the windowing environment. This arrangement permits terminal-like interaction with the computer (for running a command line interpreter, for example) without the need for a physical terminal device; it can even allow the running of multiple terminal emulators on the same device.


Capabilities

Text terminals

A text terminal, or often just terminal (sometimes text console) is a serial computer interface for text entry and display. Information is presented as . When such devices use a video display such as a , they are called a "video display unit" or "visual display unit" (VDU) or "video display terminal" (VDT).

The is a text terminal used to operate a computer. Modern computers have a built-in keyboard and display for the console. Some operating systems such as and have virtual consoles to provide several text terminals on a single computer.

The fundamental type of application running on a text terminal is a command line interpreter or shell, which prompts for commands from the user and executes each command after a press of Enter. This includes and some interactive programming environments. In a shell, most of the commands are small applications themselves.

Another important application type is that of the . A text editor occupies the full area of display, displays one or more text documents, and allows the user to edit the documents. The text editor has, for many uses, been replaced by the , which usually provides rich formatting features that the text editor lacks. The first word processors used text to communicate the structure of the document, but later word processors operate in a graphical environment and provide a simulation of the formatted output.

Programs such as and control a and the local terminal to let the user interact with remote servers. On the , and work similarly.

In the simplest form, a text terminal is like a file. Writing to the file displays the text and reading from the file produces what the user enters. In operating systems, there are several character special files that correspond to available text terminals. For other operations, there are special , control characters and termios that a program can use, most easily via a library such as . For more complex operations, the programs can use terminal specific system calls. For an application, the simplest way to use a terminal is to simply write and read text strings to and from it sequentially. The output text is scrolled, so that only the last several lines (typically 24) are visible. systems typically buffer the input text until the key is pressed, so the application receives a ready string of text. In this mode, the application need not know much about the terminal. For many interactive applications this is not sufficient. One of the common enhancements is command line editing (assisted with such libraries as ); it also may give access to command history. This is very helpful for various interactive command line interpreters.

Even more advanced interactivity is provided with full-screen applications. Those applications completely control the screen layout; also they respond to key-pressing immediately. This mode is very useful for , and . In addition, such programs control the color and brightness of text on the screen, and decorate it with underline, blinking and special characters (e.g. box drawing characters). To achieve all this, the application must deal not only with plain text strings, but also with control characters and , which allow to move cursor to an arbitrary position, to clear portions of the screen, change colors and display special characters, and also respond to . The great problem here is that there are so many different terminals and terminal emulators, each with its own set of . In order to overcome this, special libraries (such as curses) have been created, together with terminal description databases, such as and .


Dumb terminals
Dumb terminals are those that can interpret a limited number of control codes (, , etc.) but do not have the ability to process special that perform functions such as clearing a line, clearing the screen, or controlling cursor position. In this context dumb terminals are sometimes dubbed glass Teletypes, for they essentially have the same limited functionality as does a mechanical . This type of dumb terminal is still supported on modern systems by setting the environment variable TERM to dumb. Smart or intelligent terminals are those that also have the ability to process escape sequences, in particular the VT52, VT100 or ANSI escape sequences.


Graphical terminals
A graphical terminal can display images as well as text. Graphical terminals are divided into terminals, and .

A vector-mode display directly draws lines on the face of a cathode-ray tube under control of the host computer system. The lines are continuously formed, but since the speed of electronics is limited, the number of concurrent lines that can be displayed at one time is limited. Vector-mode displays were historically important but are no longer used. Practically all modern graphic displays are raster-mode, descended from the picture scanning techniques used for , in which the visual elements are a rectangular array of . Since the raster image is only perceptible to the human eye as a whole for a very short time, the raster must be refreshed many times per second to give the appearance of a persistent display. The electronic demands of refreshing display memory meant that graphic terminals were developed much later than text terminals, and initially cost much more.

Most terminals today are graphical, that is, they can show images on the screen. The modern term for graphical terminal is "". A thin client typically uses a protocol like X11 for -terminals, or RDP for Microsoft Windows. The bandwidth needed depends on the protocol used, the resolution, and the .

Modern graphic terminals allow display of images in color, and of text in varying sizes, colors, and (type faces).

In the early 1990s an industry consortium attempted to define a standard, , that would allow a single CRT screen to implement multiple windows, each of which was to behave as a distinct terminal. Unfortunately like I2O this suffered from being run as a closed standard: non-members were unable to obtain even minimal information and there was no realistic way a small company or independent developer could join the consortium. Possibly because of this the standard disappeared without trace.


Emulation
A terminal emulator is a piece of software that emulates a text terminal. In the past, before the widespread use of local area networking and broadband internet access, many computers would use a serial access program to communicate with other computers via or serial device.

When the first was released, a program called was used to communicate with many computers, including the .

Dec Terminal was one of the first terminal programs for the popular Altair.

The Win32 console on Windows does not emulate a physical terminal that supports so and programs (for logging in textually to remote computers) for Windows, including the Telnet program bundled with some versions of Windows, often incorporate their own code to process escape sequences.

The terminal emulators on most systems, such as, for example, , qterminal, , terminal.app, do emulate physical terminals including support for escape sequences; e.g. xterm can emulate the VT220 and Tektronix 4010 hardware terminals.


Modes
Terminals can operate in various modes, relating to when they send input typed by the user on the keyboard to the receiving system (whatever that may be):
  • Character mode (a.k.a. character-at-a-time mode): In this mode, typed input is sent immediately to the receiving system.
  • Line mode (a.k.a. line-at-a-time mode): In this mode, the terminal provides a local line editing function, and sends an entire input line, after it has been locally edited, when the user presses a key. A so-called "line mode terminal" operates solely in this mode.
  • Block mode (a.k.a. screen-at-a-time mode): In this mode, the terminal provides a local full-screen data function. The user can enter input into multiple fields in a form on the screen (defined to the terminal by the receiving system), moving the cursor around the screen using keys such as and the and performing editing functions locally using , , and so forth. The terminal sends only the completed form, consisting of all the data entered on the screen, to the receiving system when the user presses an key.

There is a distinction between the and the keys. In some multiple-mode terminals, that can switch between modes, pressing the key when not in block mode does not do the same thing as pressing the key. Whilst the key will cause an input line to be sent to the host in line-at-a-time mode, the key will rather cause the terminal to transmit the contents of the character row where the cursor is currently positioned to the host, host-issued prompts and all.

Different computer require different degrees of mode support when terminals are used as computer terminals. The POSIX terminal interface, as provided by Unix and POSIX-compliant operating systems, does not accommodate block-mode terminals at all, and only rarely requires the terminal itself to be in line-at-a-time mode, since the operating system is required to provide canonical input mode, where the terminal device driver in the operating system emulates local echo in the terminal, and performs line editing functions at the host end. Most usually, and especially so that the host system can support non-canonical input mode, terminals for POSIX-compliant systems are always in character-at-a-time mode. In contrast, IBM 3270 terminals connected to systems are always required to be in block mode.


See also


Notes

External links

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