Editing Computer terminal (section)

===Video display unit{{anchor|Video display unit|VDU|VDUs|Video display units (VDUs)}}===
A '''video display unit''' (VDU) displays information on a screen rather than printing text to paper and typically uses a [[cathode-ray tube]] (CRT). 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 [[Digital Equipment Corporation|DEC]], [[Engineering Research Associates|ERA]], [[IBM]] and [[UNIVAC]]; military computers supporting specific defence applications such as [[Ballistic Missile Early Warning System|ballistic missile warning systems]] and radar/air defence coordination systems such as [[Back-Up Interceptor Control|BUIC]] and [[Semi-Automatic Ground Environment|SAGE]].
[[File:IBM 2260.jpg|thumb|180px|[[IBM 2260]]]]

Two early landmarks in the development of the VDU were the [[Univac Uniscope]]<ref>{{cite web|url=https://archive.computerhistory.org/resources/text/Remington_Rand/UNIVAC.UNISCOPE_300.ca1960.102646317.pdf | title=Uniscope brochure|access-date=2021-05-23}}</ref><ref>{{cite manual
 | title       = Uniscope 100 - Display Terminal - General Description
 | id          = UP-7701
 | version     = Rev. 2
 | year        = 1973
 | url         = http://bitsavers.org/pdf/univac/terminals/Uniscope_100/UP-7701r2_Uniscope_100_Display_Terminal_General_Description_1973.pdf
 | section     = 5. Functional Description
 | section-url = http://bitsavers.org/pdf/univac/terminals/Uniscope_100/UP-7701r2_Uniscope_100_Display_Terminal_General_Description_1973.pdf#page=30
 | pages       = 24–27
 | publisher   = [[Sperry Rand Corporation]]
 | access-date = December 3, 2023
 }}
</ref><ref>{{cite manual
 | title       = Uniscope 300 General Description - Visual Communications Terminal
 | id          = UP-7619
 | year        = 1968
 | url         = http://bitsavers.org/pdf/univac/terminals/Uniscope_300/UP-7619_Uniscope_300_General_Description_1968.pdf
 | section     = 5. Operation
 | section-url = http://bitsavers.org/pdf/univac/terminals/Uniscope_300/UP-7619_Uniscope_300_General_Description_1968.pdf#page=31
 | pages       = 5-1-5-5
 | publisher   = [[Sperry Rand Corporation]]
 | access-date = December 3, 2023
 }}
</ref> and the [[IBM 2260]],<ref>{{cite manual
 | title       = IBM System/360 Component Description: - IBM 2260 Display Station - IBM 2848 Display Control 
 | id          = A27-2700-4
 | date        = January 1969
 | edition     = Fifth
 | url         = http://bitsavers.org/pdf/ibm/2260/A27-2700-4_2260_2848_Component_Description_Jan69.pdf
 | series      = Systems Reference Library 
 | publisher   = [[IBM]]
 | access-date = December 3, 2023
 }}
</ref> 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|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 communication|asynchronous]] serial communication between the [[IBM 2848|2848]] and the host. The 2265, related to the 2260, also used asynchronous serial communication.

The [[Datapoint 3300]] from [[Datapoint|Computer Terminal Corporation]], announced in 1967 and shipped in 1969, was a character-mode device that emulated a [[Teletype Model 33|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.

{{multiple image
 | align = right
 | total_width = 360

 | image1 = ADM 3A (82481457).jpg
 | alt1 = ADM-3A
 | caption1 = [[ADM-3A]]

 | image2 = Terminal-dec-vt52.jpg
 | alt2 = VT52
 | caption2 = [[VT52]]

 | image3 = RT-11 help.jpg
 | alt3 = VT100
 | caption3 = [[VT100]]

}}
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".<ref>{{cite web |title=glass tty |quote=has a display screen ... behaves like a teletype |url=http://www.catb.org/jargon/html/G/glass-tty.html}}</ref> 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|time sharing computers]]. Important early products were the [[ADM-3A]], [[VT52]], and [[VT100]]. These devices used no complicated [[Central processing unit|CPU]], instead relying on individual [[logic gate]]s, [[Large Scale Integration|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 (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 [[Digital current loop interface|current loops]])
* 24 lines (or possibly 25 – sometimes a special status line) of 72 or 80 characters of text (80 was the same as [[IBM]] punched cards). Later models sometimes had two character-width settings.
* Some type of 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|Carriage Return]] (Ctrl-M), [[Line feed|Line-Feed]] (Ctrl-J), and [[Bell character|Bell]] (Ctrl-G), but usually many more, such as [[escape sequence]]s to provide underlining, dim or reverse-video character highlighting, and especially to clear the display and position the cursor.

The experimental era of serial VDUs culminated with the [[VT100]] in 1978. By the early 1980s, there were dozens of manufacturers of terminals, including [[ADM-3A|Lear-Siegler]], [[Applied Digital Data Systems|ADDS]], Data General, [[Digital Equipment Corporation|DEC]], [[Hazeltine Corporation]], [[Heathkit|Heath/Zenith]], [[Hewlett-Packard]], IBM, [[TeleVideo]], Volker-Craig, 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 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 (such as 6 for Data General Dasher terminals, 4 for ADM 3A/5/11/12 terminals, 0 or 2 for TTYs 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 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 keyboard|Model M]] shipped on IBM PCs from 1985, and through it all later computer keyboards.

Although [[flat-panel display]]s 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.