computer terminal

Any device which can be attached to a computer, possibly over a telecommunications line, to allow a user to interact with the computer. Examples are visual display units and some kinds of personal computers using appropriate software while linked to another computer.

A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system. Typically it provides a text terminal interface over a serial line.

Historical

Early user terminals connected to computers were generally electromechanical teleprinters (TTYs), such as the model 33 Teletype. By the early 1970s, many in the computer industry realized that an affordable video data entry terminal could supplant the then ubiquitous punch cards and permit new uses for computers that would be more interactive. 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.

Early video computer displays were sometimes nicknamed "Glass TTYs" and used individual logic gates, with no CPU. Most terminals were connected to mainframe computers and often had a green or amber screen. Typically terminals communicate with the computer via a serial line, often using the RS-232 serial interface.

Later, so called intelligent terminals were introduced, such as the VT52 and VT100 made by DEC, both of which are still widely emulated in software. Notable non-VT100 computer terminal types include the IBM 3270, various Wyse models (whose Wyse 60 was a best-seller—many are still in use), and the Tektronix 4014, but during the late 1970's there were dozens of manufacturers of terminals, many of which had incompatible command sequences.

While early IBM PCs had single color green screens, these were not terminals. With suitable terminal software PCs could, however, emulate a terminal, if connected to a mainframe computer. Eventually microprocessor-based personal computers greatly reduced the market demand for terminals.

Graphical terminals

Most terminals today are GUI-based, and can show a picture on the screen.

The graphical terminals has taken over for the text based terminals most places, and in fact led to renewed interest in thin clients.

The X11 display system for Unix is built around a server/client architecture, and was one of the first possibilities for transporting graphical applications over a network, or later, the Internet.

Contemporary

Since the advent and subsequent popularization of the personal computer, few genuine hardware terminals are used to interface with computers today.

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.

Technical discussion

For an application, the simplest way to use a terminal is to simply write and read text strings to and from it sequentially. In this mode, the application needs not to know much about the terminal.

To achieve all this, the application must deal not only with plain text strings, but also with control characters and escape sequences, 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 function keys.

The great problem here is that there are so many different terminals and terminal emulators, each with its own set of escape sequences. In order to overcome this, special libraries (such as curses) have been created, together with terminal description databases, such as termcap and terminfo. Unfortunately, the libraries, the databases and the terminal emulators themselves are too often buggy, so it is not unusual to see the display imperfect or garbled, or functional keys not working.

In recent years, the general switching of users to GUI has lessened the attention paid to terminal-handling libraries and to terminal emulation, and almost stalled the debugging efforts.