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5.2.3 Video Dot Amplifier
The video (dot) amplifier
m
amplifies the
video pulses sent by the attachment. When a dot
is indicated on the CRT, the attachment issues a
negative signal for 444 nanoseconds to the video
(dot) amplifier. The negative pulse is amplified
and sent to the cathode of the CRT. The nega-
tive pulse forces the positive bias on the cathode
to a negative level, sending the electron beam to
the CRT. The attachment holds the video line
at a positive level during horizontal retrace to
block out video.
5.2.4 Wiggle Sweep Circuit
A wiggle sweep circuit
H
controls the height
of the characters on the display screen. The
attachment controls the duration of each wiggle
sweep cycle. The electron beam is deflected (by
the character coil) upward and downward 10
times for each character displayed. Wiggle
sweep pulses and video pulses are synchronized
by the attachment to form the characters. The
dot pattern for an H and the cursor is shown in
the following diagram.
I~
.1
character position
~
I
5.2.5 Horizontal Deflection Control
Horizontal sweep
II
(deflection) of the elec-
tron beam is accomplished by two horizontal
coils (master
iii
and slave
m ).
These deflec-
tion coils work together in a push/pull fashion
as follows.
Assume the initial location of the electron beam
is at the extreme left side of the CRT.
1.
2.
3.
4.
The attachment sends a minus (-) hori-
zontal pulse to the display PC board.
The horizontal deflection amplifier
increases the current to the master coil
and decreases the current to the slave
coil. When the current in the two coils is
approximately equal, the electron beam
is in the center of the CRT. When the
master coil is maximum, the slave coil
current is minimum and the electron beam
is on the extreme right side of the CRT.
The attachment now sends a plus
(+)
hori-
zontal pulse to the display PC board
(retrace command).
The horizontal deflectiom amplifier
decreases the master coil current and
increases the slave coil current. When the
slave coil current is maximum and the
master coil current is minimum, the elec-
tron beam is at the extreme left side of the
CRT screen.
The total time to trace one horizontal line and
return the electron beam to the left (retrace) is
approximately
4
ms. Therefore, the time required
to trace or display six lines is approximately
24
ms.
.
5.2.6 Vertical Line Control
~
The vertical line control controls the vertical
location of the display line on the CRT. Three
vertical input lines control the vertical location
of a line. These input lines from the attachment
are added by the vertical line control circuitry
on the display PC board. The resulting output
is sent to the vertical coil, which moves the
electron beam to the specified location on the
CRT. Below is a chart that identifies the status
Of
the three .,ignal lines for each of the six dis-
pia\, lines on
·~he
display screen.
Display
Line
1
2
3
4
5
6
1
=
Active
0= Inactive
Vertical Input
1
2
4
0
0
0
1
0
0
0
1
0
1
1
0
0
0
1
1
0
1
Display Unit
5-3
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