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DEC VT420.

The VT420 was an ANSI standard computer terminal introduced in 1990 by Digital Equipment Corporation (DEC). The 420 was the only model in the 400 series, replacing the VT320. There were no color or graphics-capable 400 series terminals; the VT340 remained in production for those requiring ReGIS and Sixel graphics and color support. The entire lineup of VT300s and VT420 was eventually replaced by the relatively unknown VT500 series starting in 1993.

ANSI escape sequences are a standard for in-band signaling to control the cursor location, color, and other options on video text terminals and terminal emulators. Certain sequences of bytes, most starting with Esc and '[', are embedded into the text, which the terminal looks for and interprets as commands, not as character codes.

Computer terminal computer input/output device; an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system

A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying or printing data from, a computer or a computing system. The teletype was an example of an early day hardcopy terminal, and predated the use of a computer screen by decades.

Digital Equipment Corporation (DEC), using the trademark Digital, was a major American company in the computer industry from the 1950s to the 1990s.



The VT420 was essentially an updated VT320, adding the multi-session capabilities originally introduced on the VT330 and VT340.

Those two models had included a system known as TD/SMP [lower-alpha 1] which allowed two sessions to be multiplexed over a single serial connection to a compatible terminal server. Alternately, the two sessions could be supported by separate serial connections on those models with multiple MMJ ports. Using either method, the VT330/340/420 could either show the two sessions behind each other, using a key sequence to flip back and forth, or split the screen horizontally to display the sessions one above the other, or vertically side-by-side.

TD/SMP, short for Terminal Device/Session Management Protocol, was a terminal multiplexer system introduced by DEC on their VT330/340 terminals in 1987. The terminal-side was referred to as SSU. TD/SMP allowed data from two separate host sessions to be sent to a compatible computer terminal over a single serial port. The format was patented and never described in depth, limiting it to DEC's own terminal servers and terminals.

A terminal server enables organizations to connect devices with an RS-232, RS-422 or RS-485 serial interface to a local area network (LAN). Products marketed as terminal servers can be very simple devices that do not offer any security functionality, such as data encryption and user authentication. The primary application scenario is to enable serial devices to access network server applications, or vice versa, where security of the data on the LAN is not generally an issue. There are also many terminal servers on the market that have highly advanced security functionality to ensure that only qualified personnel can access various servers and that any data that is transmitted across the LAN, or over the Internet, is encrypted. Usually companies which need a terminal server with these advanced functions want to remotely control, monitor, diagnose and troubleshoot equipment over a telecommunications network.

Modified Modular Jack

The Modified Modular Jack (MMJ) is a small form-factor serial port connector developed by Digital Equipment Corporation (DEC). It uses a modified version of the 6P6C modular connector with the latch displaced off-center so standard modular connectors found on Ethernet cables or phone jacks cannot accidentally be plugged in. MMJ connections are used on Digital minicomputers, such as the PDP-11, VAX and Alpha systems, and to connect terminals, printers, and serial console servers.

The VT420 also added a number of more minor features. One was to add a number of PC character sets, allowing the terminal to be used with a variety of PC programs. Another allowed the terminal to generate the proper character sequences to produce rectangular-area commands. For instance, one could select a rectangular area and fill it with a particular character, or blank it out. This was in addition to the terminal-side editing system introduced on the VT300s.

The VT420 had a total of 5 sets of 94 characters for normal VT operation, another 3 sets of 128 PC characters, and 1 set of 96 characters containing various graphics and math symbols. Like all models since the VT200 series, the user could also upload a custom character set of their own design using the Sixel system. Likewise, it also supported the National Replacement Character Set system, which swapped out single characters in 7-bit modes to allow basic changes like swapping the # for the £ for use on UK systems.


The VT220 is an ANSI standard computer terminal introduced by Digital Equipment Corporation (DEC) in 1983. The VT240 added monochrome ReGIS vector graphics support to the base model, while the VT241 did the same in color. The 200 series replaced the successful VT100 series, providing more functionality in a much smaller unit with a much smaller and lighter keyboard. Among its major upgrades was a number of international character sets, as well as the ability to define new character sets.

Sixel DEC created six pixel vertical format element that can be used to create pictures on printing or terminal screens

Sixel, short for "six pixels", is a bitmap graphics format supported by terminals and printers from DEC. It consists of a pattern six pixels high and one wide, resulting in 64 possible patterns. Each possible pattern is assigned an ASCII character, making the sixels easy to transmit on 7-bit serial links.

The National Replacement Character Set, or NRCS for short, was a feature supported by later models of Digital's (DEC) computer terminal systems, starting with the VT200 series in 1983. NRCS allowed individual characters from one character set to be replaced by one from another set, allowing the construction of different character sets on the fly. It was used to customize the character set to different local languages, without having to change the terminal's ROM for different counties, or alternately, include many different sets in a larger ROM. Many 3rd party terminals and terminal emulators supporting VT200 codes also supported NRCS.

All DEC terminals that came after the VT100, including the VT420, are able to emulate their ancestors. The VT420 had VT100 and VT52 emulating modes.

VT100 computer terminal

The VT100 is a video terminal, introduced in August 1978 by Digital Equipment Corporation (DEC). It was one of the first terminals to support ANSI escape codes for cursor control and other tasks, and added a number of extended codes for special features like controlling the status lights on the keyboard. This led to rapid uptake of the ANSI standard, becoming the de facto standard for terminal emulators.

VT52 CRT-based computer terminal introduced by Digital Equipment Corporation (DEC) in July 1974

The VT50 was a CRT-based computer terminal introduced by Digital Equipment Corporation (DEC) in July 1974. It provided a display with 12 rows and 80 columns of upper-case text, and used an expanded set of control characters and forward-only scrolling based on the earlier VT05. DEC documentation of the era refers to the terminals as the DECscope, a name that was otherwise almost never seen.

The screen itself was a 14" flat CRT with a resolution of 800 (horizontal) by 400 (vertical) pixels. A variety of glyphs were available that provided 80 or 132 characters across, and 24, 36 or 48 lines of text vertically. The screen had room for 25 lines at normal font sizes, but the last line was normally used for status indications, like ⇪ Caps Lock. The MMJ ports could operate at speeds up to 38,400 bit/s, double that of the VT300s maximum 19,200 bit/s.


  1. Known as SSU when being referring to on the terminal side of the same system.

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