FROM
+70V
SOURCE
R27
+12V
+12.0V
2.2K
TO
VERT.
CIRCUITRY +
C11
R26
R25
27K
1/2W
v
370V
PP V
'
'"'
'
c1:
('--1o_o
___
®JRN
Q4
B2V ,
YEL
VERT.
OUTPUT VERT.
Figure
11.
Vertical
Output
Circuit
VERTICAL
OUTPUT
(Refer
to
Figure 11.)
The positive-going
sawtooth
waveform from
the
emitter
of
Q3
is
applied
to
the
base
of
vertical
output
stage,
04,
which
conducts
only
during
the
ramp,
or
rise time,
of
the
sawtooth
waveform. The inverted
ramp
output
(collector
current)
is
the
vertical trace period, which
is
AC coupled
(via
C14)
to
the
vertical deflection yoKe winding. The
same collector
current
output
is
also applied
to
L 1, which
builds
up
a large electromagnetic field. This field will
collapse very rapidly when
04
turns
off
during
the
retrace
time
of
the
waveform applied
to
the
base
of
04.
The
back
EMF
is
in
the
form
of
a high voltage positive pulse, whose
duration
represents
the
vertical retrace period.
To
limit this
pulse
to
a safe value, a varistor
is
connected across L 1, with
R26
providing damping.
Except for
the
vertical
output
stage,
04,
the
vertical cir-
cuitry
operates from a +12 volt source, which
is
derived
from
the
+70
volt source. Resistor R27
drops
the
+70
volt
DEFL.
YOKE,
L2
source
to
the
required +12V, Zener
diode
03
holds
the
+12V
constant
while C11 provides additional filtering.
STEPSCAN FUNCTION
With existing logic,
the
number
of
characters
that
can be
displayed
is
limited by logic speed. Anything
that
increases
the
speed
at
which
the
logic must
work,
will allow an in-
crease in
the
number
of
characters displayed.
When
Cl
video
monitor
is
used
as
a
data
display,
the
system
bandwidth
required (and logic speed) increases
in
direct
proportion
to
the
number
of
scan lines displayed. Since no
data
is
written
in
the
blank spaces between
character
rows,
a
method
is
required
to
speed up vertical deflection
in
the
blank spaces
to
decrease bandwidth requirements. This
makes
the
blank space height less
dependent
on
scan time,
allowing time
to
display more characters. This
is
accom-
plished by
"stepping"
the
reference
sawtooth
between char-
acter rows so
that
a row
to
row space
of
from 3
to
5
horizontal lines equivalent height can be displayed
in
the
time it takes
to
deflect
one
horizontal line. This
is
illus-
trated
in
Figure 12.
STEPSCAN CIRCUIT
(Refer
to
Figure 10.)
This circuit requires an external (approximately
4.0
volts
P-P)
TTL
positive-going pulse. These pulses are applied
to
the
emitter
of
the
StepScan Amplifier,
02,
which
is
turned
on when
the
emitter- voltage exceeds
the
base voltage.
The +12 volts stored
on
C5
is
applied
through
02,
R11
and
R14,
to
the
sawtooth
forming capacitors C6 and C7. This
momentarily increases
the
charge rate
of
C6 and C7, and
the resultant
action
produces
the
stepping
sawtooth
shown
as
waveform
D.
The rate
at
which
the
vertical oscillator
steps
is
determined
by
the
repetition
rate
of
the
incoming
StepScan pulses. The slope (charge rate)
of
the
stepped
portion
of
the
sawtooth
is
adjustable with
the
STEP ADJ.
control,
R11, which varies
the
spacing
from
3
to
5 horizon-
tal scan lines. With
the
vertical sawtooth
thus
modified,
the
collector
current
of
04
and,
therefore,
the
yoke
vertical
deflection
current
will be
"stepped"
during
the
line be-
tween character rows chosen.
(PARTIAL
VERTICAL
FRAME SHOWN)
HORIZONTAL
SCAN LINES - -
-{1-0
Q Q 0
0-0
0 0 0 0
(NUMBERED)
2-0
0-----------
FIRST
3-0
.
0-----------
CHARACTER
4-0
0
0---0
0
0----------
ROW
5-0
0-----------
6-0
0-----------
7
-0
0 0 0
0-0
0 0 2
_0-_-_-_---~~-
WITH
STEPSCAN, THE
8
--
- - - - - - - - - } TIME REQUIRED FOR
g STEPSCAN TRACE 3
HORIZ.
SCAN LINES
START OF
10
___
-
IS
SAVED
(lWO-THIROS
SECOND . - - - - - - - - - - - - LESS TIME REQUIRED)
CHARACTER
_,,
- 0-
(j
U
(J
iJ
ROW
12-0----------------
13-0----------------
AEPC-00116
Figure
12.
Displaying Characters on a
Monitor
with
StepScan
4-26
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