Editing VT220 (section)

==Software==
The VT220 was designed to be compatible with the [[VT100]], but added features to make it more suitable for an international market. This was accomplished by including a number of different [[character set]]s that could be selected using a series of ANSI commands.

Glyphs were formed within a 10 by 10 grid. The terminal shipped with a total of 288 characters in its ROM, each one formed from an 8 by 10-pixel glyph. Using only 8 of the columns left space between the characters. The characters included the 96 printable ASCII characters, 67 Display Controls, 32 DEC Special Graphics, and a backward question mark used to represent undefined characters.

The VT200s included the ability to make minor changes to the character set using the [[National Replacement Character Set]] (NRCS) concept. When operating on an [[8-bit clean]] link up to 256 character codes were available, which included a full set of European characters. However, when operating on a typical 7-bit link, only 128 were available, and only 96 of these produced display output, while the rest were [[control character]]s. This was not enough characters to handle all European languages. Most terminals solved this by shipping multiple complete character sets in [[ROM]], but there was a cost in doing so.

DEC's solution to this problem, NRCS, allowed individual characters glyphs in the base set of 96 7-bit characters to be swapped out. For instance, the British set made a single substitution, replacing the US's hash character, {{keypress|#}}, with the pound sign, {{keypress|£}}. The terminal included 14 such replacement sets, most of which swapped out about a dozen characters.<ref>[https://www.censoft.com/support/help/ttus_help/ttusNational_Replacement_Character_S.htm "National Replacement Character Set (NRCS)"][sic]</ref> This eliminated the need to ship 14 versions of the terminal or to include 14 different 7-bit character sets in ROM.

Additionally, the VT200s allowed for another 96 characters in the [[Dynamically Redefined Character Set]] (DRCS), which could be downloaded from the host computer. Data for the glyphs was sent by encoding a set of six vertical pixels into a single character code, and then sending many of these ''[[sixel]]s'' to the terminal, which decoded them into the character set memory. In later models, the same sixel concept would be used to send [[bitmapped graphics]] as well. Character graphics were a common example of these downloaded sets.<ref>[https://vt100.net/dec/vt320/soft_characters VT320 Soft Character Sets]</ref>