National Replacement Character Sets

j4james: NRC in Historical Context

I think it's worth mentioning how the National Replacement Character Sets (NRCs) work, because that's something that I think is not immediately obvious if you aren't familiar with the history of these terminals.

My understanding (which may be wrong) is that most if not all NRC sets will not work by default. You first have to enable National Replacement Character Set Mode (DECNRCM), and that itself only works if your keyboard dialect is set to something other than North American. Then even with DECNRCM enabled, you'll still only have access to one of the NRC sets - the one that matches your keyboard dialect.

The point of these sets was for use on 7-bit systems, so once DECNRCM is enabled, everything is 7-bit. If you've got a British keyboard, for example, then Shift+3 is labelled as producing a pound symbol (£), but the keyboard can't generate an 0xA3 codepoint on a 7-bit system. So instead it generates 0x23, and relies on the fact that the British NRC set has 0x23 mapped to £. That way it'll at least look like a £ when you type it, even though the backend system is still receiving an ASCII # character.

So unless you're tied to a 7-bit system with a specific non-ASCII locale, and you're also using a matching keyboard, there's not much use for these character sets. And I'm assuming that's why they only allow for one at a time (if my understanding is correct).

(Thanks go to @j4james who kindly shared that bit of history.)

The VT340 and NRC Sets

National Replacement Character Sets appear to be an older method which the VT340 merely provides as a backwards compatibility convenience. In fact, it DEC seems to gone out of their way to make it difficult to accidentally start using it. Any software which queries the VT340's capabilities will find that National Replacement Character Mode (DECNRCM) is "Permanently Disabled".

Without the manual, is not obvious from the VT340 Set-Up menu how to even enable NRC — the page which lets one pick "Latin1" or "Multilingual Character Set" has nothing about it. The trick is NRC Sets are implicitly chosen by changing the Keyboard type. Any keyboard type other than the default ("North American") simultaneously picks an NRC set.

Keyboard	NRC Set
United Kingdom	United Kingdom
Danish	Norwegian/Danish
Dutch	Dutch
Finnish	Finnish
Flemish	French
French/Belgian	French
French Canadian	French Canadian
German	German
Italian	Italian
Norwegian	Norwegian/Danish
Portuguese	Portuguese
Spanish	Spanish
Swedish	Swedish
Swiss (French)	Swiss
Swiss (German)	Swiss

The VT340 has tables and fonts for twelve different NRC regions and can show only one of them at a time. Each one alters up to 12 different characters from ASCII. For example, the # character displays as a £ for the United Kingdom NRC Set.

The "extra" characters provided by the VT340's NRC Sets are:

¡, £, §, ¨, °, ¼, ½, ¾, ¿, Ã, Ä, Å, Æ, Ç, É, Ñ, Õ, Ö, Ø, Ü, ß, à, à, ã, ä, å, æ, ç, è, é, ê, ì, î, ñ, ò, ô, õ, ö, ø, ù, û, ü, ÿ

As mentioned in the charset README, all of these characters are already available in the Latin-1 character set, so there is no real advantage to using them.

If you have files encoded in an NRC, you may be able to convert them to Unicode using iconv. If you have a program that outputs NRC, try using the luit program to wrap around it.

The twelve NRCs builtin to the VT340 and the escape codes for loading them.

Character Set	G0	G1	G2	G3	International Registry
DEC Great Britain	ESC(A	ESC)A	ESC*A	ESC+A	IR-004
German	ESC(K	ESC)K	ESC*K	ESC+K	IR-021
French (France)	ESC(R	ESC)R	ESC*R	ESC+R	IR-025
Italian	ESC(Y	ESC)Y	ESC*Y	ESC+Y	IR-015
Spanish	ESC(Z	ESC)Z	ESC*Z	ESC+Z	IR-017
DEC Dutch	ESC(4	ESC)4	ESC*4	ESC+4	None
DEC Finnish	ESC(5	ESC)5	ESC*5	ESC+5	None
DEC Portuguese	ESC(%6	ESC)%6	ESC*%6	ESC+%6	None
DEC Norwegian/Danish	ESC(6	ESC)6	ESC*6	ESC+6	None
DEC Swedish	ESC(7	ESC)7	ESC*7	ESC+7	None
DEC French Canadian	ESC(9	ESC)9	ESC*9	ESC+9	None
DEC Swiss	ESC(=	ESC)=	ESC*=	ESC+=	None