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13;'?';~O
Pr'oce~;~:;o~'
Module
1 :5220-91 OB'l /1 S
Re v
J" AN-· 0 4·_·B2
VIDEO CONTROL SECTION
Ov~~rvi€o~w
The video control
section
generates
the tiMing
signals required
to
fetch character and enhanceMent
data
frOM MeMory and drive the analog
sweep circuitry to display that inforMation on the CRT.
The display is divided
into 26 rows of SO character cells each.
Each
character
cell
is
a
rectangle,
15
dots
vertical
by
nine
dots
horizontal.
Any character to be displayed
is produced by selectively
lighting
the dots
of the character
cell which
shape that character,
leaving the
others blank.
Dots
are left blank on either side and on
the top
and
bottOM of
the character cell to
provide horizontal
and
vertical seperation between
norMal characters.
This is not
true
of
characters which are continuous across the character boundary,
such as
line drawing characters (used to display forMS).
The analog sweep circuitry sweeps the electron beaM
frOM left to right
and frOM top to
bOttOM
across the
display.
As the
beaM
is
swept
horizontally it
is turned on to produce a lighted dot and off to blank
a dot position.
As the beaM reaches the
end of its scan a horizontal
~:;ync:
~:;i(lnal
:i.s
~:;ent
to
the
sweep
c(~using
the
beaM
to
retf'aC(~
horizontally
and begin sweeping
again.
During this tiMe the beaM is
also being swept vertically.
The COMbination of these two produces the
display raster.
As
the beaM reaches
the
bottOM of
the
display
a
vertical
sync
signal is sent to the sweep causing the beaM to retrace
frOM the bOttOM right to the top
left corner.
In this Manner the CRT
display is written 60 tiMes per second
(when configured
at 60 Hz)
or
optionally SO tiMes per second (configured at SO Hz).
HOI~IZONTAL
TIMING
After the 80th character position of a scan line the beaM is turned off
(blanked)
and reMains so as the horizontal
retrace takes place.
The
beaM
is
enabled
again as
it reaches
the
position
for
the
first
character
of
the
next
scan.
This
blanking
interval
is
called
"horizonti:tl blanl<ing".
This
blanking
(~llows
tiM~?
for
th(o?
t.)(~i~M
to
retracB
1
settle at the left side and begin tracing again.
The portion
oft h
f~
~~
can w h e f' (o? the b e (;\ M
i~::,
£~
n <": t b led i
~.)
k now nat:;
.. act :l v
G~
v:l d eo" .
T h
~?
horizontal scan tiMe consists of the 80 character tiMes of active video
plus 3S character
tiMes
of
horizontal bl<":tnking
for
a
total of 115
character tiMes per scan
(1 character tiMe
~
349 ns).
This produces (;\
horizontal scan frequency of 24.9 KHz.
The horizontal sync signal
is activated
16 character tiMes before the
last video character of the scan and is active
for
7 character tiMes.
It is produced in
advance
of the last character to COMpensate for the
d
f.~:I.
a y il"l t h (o?
~:)W(o?(~P
h
0
r i z
0
n tal c en t
fn'
in (1 c:i r cui t .
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