All three forms of VSCII keep the 95 printable characters of
ASCII unmodified.
VSCII-3, also known as TCVN 5712-3, VN3 or simply TCVN3, includes the fewest assignments. It is an
extended ASCII, because it keeps all 128 codes of ASCII unmodified. It does not reassign any of the
C0 and C1 control codes. Compared to
ASCII, it adds 75 characters: • 67 lowercase characters, allowing full lowercase support. • 7 uppercase characters, allowing uppercase support for the
29 base letters without tone marks. • The
non-breaking space. Tone marks on uppercase vowels is accomplished in TCVN3 by switching to an all-capital font.
VSCII-2, also known as TCVN 5712-2 and VN2, is a superset of VSCII-3. It is an
extended ASCII, because it keeps all 128 codes of ASCII unmodified. It does not reassign any of the
C0 and C1 control codes, making it conformant with
ISO 2022 as a 96-set. Compared to VSCII-3, it adds (for a total of 96 non-ASCII characters): • 16 more uppercase characters with pre-composed tone marks (for a total of 23 non-ASCII uppercase characters) • 5
combining diacritics for
tone marks, allowing other combinations of uppercase letters and tone marks to be represented. Combining marks follow the base letter as in
VNI (rather than preceding them as in
ANSEL).
VSCII-1, also known as TCVN 5712-1 and VN1, is an extension of VSCII-2, and is a modified ASCII, since it replaces 12 of the 33
control characters with precomposed characters. Compared to VSCII-2, it (for a total of 140 non-ASCII characters): • Adds 44 more pre-composed uppercase letters, bringing them to the same count as the lowercase • Does this by replacing 12 ASCII control characters and allocating 32 graphical characters to the C1 control area, breaking ISO 2022 compatibility Conversion from VSCII-3 to VSCII-2 or VSCII-1 and conversion from VSCII-2 to VSCII-1 are not necessary, but can result in smaller files. Conversion from VSCII-1 to VSCII-2 or VSCII-3 and conversion from VSCII-2 to VSCII-3 require expansion of some pre-composed characters. ==Character set==