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HP 9020 User Manual

HP 9020
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Theory
of
Operation
2-39
Graphics/Digital Video Board
The graphics/digital video board has
an
attribute ROM
and
an
English
or
Katakana character ROM
(Figure 2-22).
The
board
receives the alpha signals from the alpha/interface board, generates
appropriate characters
and
attributes (blinking, underline,
and
inverse),
and
produces the red,
green,
and
blue analog video streams for the video board. Alpha characters have
I-of
-8 colors.
The graphics/digital video board has a graphics display controller which receives graphics data from
the alpha/interface board
and
shifts
it
into graphics memory.
At
the appropriate time, data stored
in
the
RAM
is
read
out
and
sent to the color map. The color
map
has
16
locations of 12 bits each, four
bits per gun. Therefore, at any given time, the color map can store
16
of a possible
4096
colors (16
red by
16
green by
16
blue).
The graphics/digital video board also has a 51
MHz
crystal oscillator which provides timing signals
to both the alpha/interface
and
graphics/digital video boards. The board has a sync generator circuit
which provides the vertical
and
horizontal sync signals
and
the vertical blanking signal to the video
board.
Three digital-to-analog converters
on
the graphics/digital video board convert the 12-bit digital
output of the grahics color map,
and
the red, green,
and
blue alpha video streams which are not
colormapped, into red, green,
and
blue analog signals suitable for the video board.
LD
o/From T
Alph
a!lnterface
Board
~
a
Alph
Timin
g
-+-
ha Alp
Controller
Graphics
Graphics Latches
8
Color
Display
Lsr->
Buffer
~
16
) and
Map
Controller
Shift Registers
t
f'I'
~
J
C>
8
Graphics
Graphics Timing
Multiplexer Memory
State Machine
12
12SK Bytes
+
I
V
Character and
Shift Register
Alpha!
~
)
~
)
Attribute ROMs
S
and
S
Graphics
Attributes Mixer
Mixer
i
J
Crystal
Sync
Oscillator
Generator
51MHz
Circuit
Figure 2-22. Graphics/Digital Video Board Block
Diagram
Red
Green
Blue
Hsync
Vsync
Vblank
To Video
Board

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HP 9020 Specifications

General IconGeneral
BrandHP
Model9020
CategoryDesktop
LanguageEnglish

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