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The VT520 is an ANSI standard computer terminal introduced by Digital Equipment Corporation (DEC) in 1993 and 1994.[1] The VT520 supports monochrome graphics using the ReGIS vector or Sixel bitmap systems. The VT525 added color support, while the VT510 was a single-session, text-only version with a built-in monitor. The VT525 appears to be the most popular model in the series.

The VT500s replaced all previous models of DEC's VT line, at that time consisting of the VT420 text and VT340 graphics terminals. It was introduced in an era when the market was being flooded by low-cost IBM PC clones which could perform the same functions using a terminal emulator while also running other software. DEC introduced the VT500s only a short time before selling off their entire terminal division in August 1995. This brought the VT series to a close, after a total of about six million terminals had been sold.

The VT520 is still available from Boundless Technologies.


In terms of major features, DEC's terminal line reached its peak with the VT300 series of 1988. The high-end models, the VT330 and VT340, included the abilities to display bitmap graphics using the sixel format, vector graphics using ReGIS or Tektronix 4010 emulation, terminal-side buffering and editing, and added the new ability to support two separate terminal sessions using a system known as TD/SMP.[a] The base-model VT320 was a simple text-only version that lacked TD/SMP support, and this was replaced by the VT420 in 1990, adding this feature.

By the mid-1990s the price of low-end PCs was rapidly falling to the under-$1000 price point. When equipped with a terminal emulator, these machines could perform all the functions of a DEC terminal, as well as running software locally. The terminal market began to crash, but remained important to DEC's core minicomputer business. DEC responded by introducing the VT500 series as simplified and lower-cost options. The VT510 was introduced in 1993 as an all-in-one unit like their previous designs, replacing DECs proprietary keyboards with a PS/2 port and adding standard RS-232 ports in addition to their proprietary MMJ serial ports. The 520 and 525 dispensed with the monitor as well, packaging the system into a pizza box case and working with a user-supplied monitor connected on an SVGA port.

Like all models of the VT series, the VT500's primary purpose is to act as an ANSI standard terminal. The VT510 supported only a single session, while the 520 and 525 supported up to four sessions, up from two in the earlier VT series. The user can flip between the sessions using control sequences on the keyboard (typically F4), or display multiple sessions at the same time by splitting the screen horizontally or vertically. All models have multiple character sets in ROM, supporting DEC, international and PC characters. They can also replace any of these by downloading custom characters using sixels, and perform single-character swaps using the National Replacement Character Set, swapping $ with £ for use with UK keyboards for instance.

The 500s included two MMJ connectors for serial connections, as well as a RS-232C port and a Centronics port for printing. The speed of the serial ports was increased to 115.2 kbps, up from 38.4 kbps on the VT300s. Any one of the serial ports could support two sessions using TD/SMP. Like earlier models of the VT line, the 500s could be put into modes emulating the VT100 and VT52, but added a wide variety of other emulations for Wyse, ADDS, Televideo and other terminals. The 500s also directly supported ANSI commands for color, like the Wyse, in addition to the custom escape sequences used for color support on previous VT models.

Another new feature was the inclusion of a set of desk accessories running on the terminal's CPU. These included a calculator, alarm clock, calendar and a character set viewer.

Terminal emulator specifications may refer to VT500 instead of VT510, VT520 and VT525 in the statements about their compatibility.


  1. ^ On the terminals, TD/SMP was known as SSU.


  1. ^ Paul Williams,Meet the Family,vt100.net, retrieved June 16, 2007