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. Early terminals were inexpensive devices but very slow compared to punched cards or paper tape for input, but as the technology improved and video displays were introduced, terminals pushed these older forms of interaction from the industry. A related development was timesharing 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 fat client. A terminal that depends on the host computer for its processing power is called a "dumb terminal" or thin client. 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.
A Teletype Model 33 ASR teleprinter, usable as a terminal
Early user terminals connected to computers were electromechanical teleprinters/teletypewriters (TeleTYpewriter, TTY), such as the Teletype Model 33 ASR, originally used for telegraphy or the Friden Flexowriter. Later printing terminals such as the DECwriter 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 in the computer industry realized that an affordable video data entry terminal could supplant the ubiquitous punched cards 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 minicomputers 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 storage tube, a specialized CRT developed by Tektronix that retained information written on it without the need to refresh.
The Datapoint 3300 from Computer Terminal Corporation 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 video computer displays were sometimes nicknamed "Glass TTYs" ("glass Teletypes") or "Visual Display Units" (VDUs), and used individual logic gates, with no CPU or very primitive LSI chips, yet quickly became extremely popular Input-Output devices on many different types of computer system once several suppliers gravitated to a set of common standards:
- ASCII character set (rather than, say, EBCDIC or anything specific to one company), but early/economy models often supported only capital letters (such as the original ADM-3, the Data General 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 (sometimes with two character width settings).
- Some type of blinking cursor that can be positioned (with arrow keys and/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 Escape sequences to provide underlining, dim and/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, Data General, DEC, Hazeltine Corporation, Heath/Zenith, Hewlett Packard, IBM, Televideo, and Wyse, 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 beteween 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 (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.
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.
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 Televideo 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.
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 intelligent terminals, costing less that 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 Televideo TS-800 could run CP/M-86, blurring the distinction between terminal and Personal Computer.
Another of the motivations for development of the microprocessor 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:
- Lear Siegler ADM-3A and later models
- Televideo 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
- Data General D1 to D3 and especially D200 and D210
- Hazeltine Corporation H1500
- Tektronix 4014
- Wyse W50, W60 and W99
The ANSI X3.64 escape code standard produced uniformity to some extent, but significant differences remained (for example, the VT100, Heathkit H19 in ANSI mode, Televideo 970, Data General D460, and Qume QVT-108 terminals all followed the ANSI standard, yet differences might exist of codes from function keys, 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 minicomputers or mainframe computers and often had a green or amber screen. Typically terminals communicate with the computer via a serial port via a null modem cable, often using an EIA RS-232 or RS-422 or RS-423 or a current loop serial interface. IBM systems communicated over a coaxial cable using IBM's SNA protocol, but for many DEC, Data General and NCR (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 Computer Terminal Corporation as the processor for the Datapoint 2200.