Graphics/Digital Video Board Block Diagram - HP 9020 Service Manual

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