National Replacement Character Set
| Company type | Technology/Standard |
|---|---|
| Industry | Computer terminals, Character encoding |
| Founded | 1983 |
| Founder | Digital Equipment Corporation (DEC) |
| Products | Character sets for DEC VT200 series terminals |
The National Replacement Character Set (NRCS) was introduced in later DEC computer terminal models, beginning with the VT200 series launched in 1983. NRCS provided the capability to substitute individual characters within a character set dynamically, enabling terminals to adapt character sets for various local languages without the need to alter ROM contents or maintain multiple sets. This feature was also embraced by numerous third-party terminals and terminal emulators compatible with VT200 protocols.
Description
The ASCII standard uses 7 bits to represent 128 characters, including 96 printable symbols such as alphabets, digits, and basic punctuation marks. However, it does not accommodate additional characters like accented letters or international currency signs prevalent in European languages. To overcome this, regional ASCII variants were developed by replacing specific characters—for example, the UK version substituted ASCII’s hash symbol (#) with the pound sign (£). Traditionally, supporting these variations required distinct terminal or printer models equipped with unique ROM glyphs, some of which were formalized under ISO/IEC 646.
With the advent of 8-bit clean serial connections, the character set was extended to 256 symbols by adding characters beyond the original 128 (positions 128–255), allowing for more complete Western European alphabets. This extension led to standards such as ISO/IEC 8859-1 ("ISO Latin"), which includes 191 characters, and its broader superset Windows-1252.
NRCS was designed to meet the demand for multiple country-specific terminals by permitting redefinition of certain basic 7-bit ASCII characters using DEC’s Multinational Character Set (MCS), itself based on ISO Latin. Instead of storing all localized variants in ROM, terminals contained only standard ASCII and MCS glyphs and generated local adaptations at runtime. For instance, rather than manufacturing separate UK-specific ASCII terminals, NRCS allowed the hash character to be replaced with the pound symbol dynamically—so pressing Shift 3 on a UK-configured terminal would produce £ instead of #.
Configuration of NRCS could be performed manually through setup commands or automatically by recognizing keyboard model codes during system startup, facilitating effortless localization when keyboards with region-specific layouts were connected.
NRC Sets
Starting with the VT220 series, DEC terminals supported a total of twelve NRC sets in addition to standard ASCII:
... [Table continues with other language sets, each emphasizing relevant code pages and standards] ...| Character set | Code page | Standard | 0x23 | 0x40 | 0x5B | 0x5C | 0x5D | 0x5E | 0x5F | 0x60 | 0x7B | 0x7C | 0x7D | 0x7E |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Standard ASCII | 367 | ASCII, ISO 646-US IR 6 | # | @ | [ | \ | ] | ^ | _ | ` | { | | | } | ~ |
| United Kingdom | 1101 | DEC, ISO | £ | @ | [ | \ | ] | ^ | _ | ` | { | | | } | ~ |
| Denmark/Norway (Alternate) | 1107 | DEC, ISO | # | @ | Æ | Ø | Å | ^ | _ | ` | æ | ø | å | ~ |
| Denmark/Norway | 1105 | DEC | # | Ä | Æ | Ø | Å | Ü | _ | ä | æ | ø | å | ü |
See also
- 7-bit DEC Greek alphabet
- 7-bit DEC Hebrew encoding (also known as SI 960, HEBREW-7)
- 7-bit DEC Turkish character set (TR7DEC)
- 7-bit DEC Cyrillic encoding (KOI-7)