Character Sets; Standard Characters - NCR PC4I Technical Reference

VIDEOTEX
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Figure
10.9 Videotex
Controller - Internal Architecture
Prior to the Videotex controller taking over the
system bus, it issues a bus request signal (BR/). The
period of this advanced warning is a programmable
feature (see below). A corresponding signal (DTACK/)
is issued to denote completion of a data transfer.
CHARACTER SETS
Standard Characters
The Videotex Controller contains in its own read only
memory the patterns of 512 alphanumeric and graphic
characters in four separate tables of 128 characters
each (see Figures
10.10 - 10.13),
The ROM character set is divided into four tables
because only seven bits are available for the code of
each character in video memory. The most significant
bit of the character code byte is used to instruct
the Videotex Controller to refer to a set of
attributes for that character (see "Character
Attributes"). One of these attributes is selection of
the table to which the character code applies.
10-12