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General Description
INPUT/OUTPUT MODULES
1-9
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Also a part of the logic system are several terminal input/
output modules: the keyboard interface PCA, the data
communications PCA, the eight-bit duplex register PCA
(used for the HP 9866A and HP 9871A Printer interface),
and the serial printer interface PCA. These never request
control of the bus, but all must respond to commands from
the microcomputer and its programs.
The basic
110
commands output data or control from the
microprocessor and input data or status from the interface
module. Each of the
110
cards has different data and con-
trol formats, but all are controlled by the microcomputer.
Each
110
module has a rear edge connector for the attach-
ment of a connector hood and cable assembly to carry the
signals out the back of the terminal.
The display subsystem has two functions that use the bus.
Cursor control is an output function and the DMA refresh
is a bus requestor for memory read operations.
CRT MONITOR
The CRT monitor section contains sweep and high voltage
circuits, the high-resolution, low-profile cathode-ray tube,
and fan.
THE RASTER
The terminal uses raster scan deflection method, similar
to that used in television sets. In a raster scan display, the
electron beam traverses the screen in a series of closely
spaced horizontal lines, starting from the top. Characters
are formed from line segments and dots produced by turn-
ing the beam intensity on and off at appropriate times.
The terminal uses a low-profile CRT to keep overall height
to a minimum while maintaining a screen capacity of 1920
characters, partitioned into 24 rows of 80 characters each.
All of the character positions are fundamentally rectan-
gles 7 dots wide by 9 scan lines high. Four additional scan
lines beneath the 7x 9 matrix are used for the descender
areas oflower-case characters, for underlining, and for the
blinking underscore cursor. One other dobs used on either
side for character-to-character spacing, and one scan line
is reserved at the top and bottom for row-to-row spacing.
This results in a character cell of 9 dots by 15 scan lines
replicated over the entire screen area (see figure 1-2).
ALPHANUMERIC
AND
MICROVECTOR
CHARACTER SETS
by use of the half-shift. With this type of set the
character-to-character spacing of two dots is hardwired.
This prevents the design of characters that would form
continuous horizontal lines. However, all 15 scan lines of
the row are available so that vertically contiguous symbol
segments can be'designed. An example of this is the
three-row-high integral sign found in the math symbol
set.
Microvector sets use the entire 9-dot-by-15-scan-line
character cell without the half-shift. This allows charac-
ters to be designed with both horizontal and vertical con-
tinuity. This type of set finds its greatest application
where a minimal set of graphic kernels is needed to repre-
sent more complex pictorial information. The figure on
page 3-6 illustrates the use of the line drawing set in
representing a form.
DISPLAY FEATURES AND ALTERNATE
CHARACTER SETS
The basic 2641A terminal has a repertoire of 64 al-
phanumeric characters (128 optional), 128 APL charac-
ters, 64 overstrike characters, inverse video fields (black
characters on white background), half-bright, underline,
and blinking fields. One additional 128-character set can
,be stored in the terminaL,
The basic 2645A/N/S Terminal has a repertoire of 64 al-
phanumeric characters (128 optional) and one display fea-
ture, inverse video fields (black characters on white
backgrounds). With the addition of the display enhance-
ments board, up to three additional 128-character sets can
be stored within the terminal. Three display features are
also added: half-bright, underline, and blinking fields.
Two types of character sets can be stored within the ter-
minal: alphanumeric sets and microvector sets. Al-
phanumeric sets support the primary use of the terminal,
displaying textual and numeric information. Characters
are designed around a basic 7x 9 dot matrix with provision
for lower-case descenders. The characters are embellished
All sixteen possible combinations of the four display fea-
tures can be applied to any character or characters on the
screen. No displayable character positions are required to
start, stop, or modify either the features or the character
sets. Therefore, consecutive characters on the screen may
be from different sets or have different display features.
1-4
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