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defines the configuration of the display on the CRT.
There are 30 lines, 6 of which are used for vertical
retrace and 24 of which are used to display charac
ters . The 30 lines are composed of nine scans each.
Each scan being composed of 96 character times,
80 for display and 16 for horizontal retrace. Each
character block is composed of 7 x 9 dot matrix
field which contains a 5 x 7 character matrix for
the displayed character.
Oscillator The oscillator is crystal controlled with
a frequency of 10.8864 MHz. Two 74H04's are
connected in series by a lOOpf capacitor. Each
74H04 has a 1 Kohm feedback resistor around it.
A 10.8864 MHz crystal is connected from the
input of the first 74H04 to the output of the
second. The output of the oscillator is buffered,
inverted and fed to the Dot Position Counter.
Dot Position Counter This divide-by-seven coun
ter defines each of the seven dots required to
compose one character. The outputs of this four
stage counter are labeled DPC1, DPC2, DPC4,
and DPC8. The Counter actually presets to a
count of 10, counts up through the overflow
point at 15 to a count of zero, and presets then
back to a count of 10. The Dot Position coun
ter output, DPC8, drives the Character Position
Counter.
Character Position Counter The Character
Position Counter is composed of two binary-
type counters that define 96 character times,
each being seven dots wide. The output of the
Character Position Counter drives the Charac
ter Height Counter.
Character Height Counter The Character Height
Counter is a standard counter that defines 9
scans of 96 characters each, with each charac
ter being seven-dots wide. The output of the
Character Height Counter drives the Character
Line Counter.
Character Line Counter The Character Line
Counter is a binary counter that starts at a
count of zero and counts to a maximum of 29
for a total of 30 character lines. The final out
put of this counter runs at the vertical refresh
rate.
Horizontal and Vertical Drive The Horizontal
4-7
Drive is started when the Character Position Coun
ter leaves the video area of the scan and is active
for the following 40 character times. The high
active output of this flip-flop is sent to the moni
tor on pin 9 of connector J1.
The vertical drive is generated during the time that
the Character Line Counter is decoding 26.
4.3.8
Cursor Location Counter
The Cursor Location Counter identifies the loca
tion of the cursor. This is a count made from the
Cursor Line Counter, called CURL, and the Cur
sor Position Counter^ called CURP. These two
counters, in conjunction with the ROLL counter,
are used to address the memory to determine the
entry point of the next character. The cursor loca
tion counters are compared with the next character.
The cursor location counters are compared with
the Character Position Counter and the Character
Line Counter to generate the signal called CNTR
CURSOR. This signal is used to generate the cur
sor displayed on the CRT. Also associated with the
cursor location counters is the appropriate circuitry
to move the cursor up, down, right, left, home,etc.
A LINE FEED code causes the Cursor Line Counter
to increment by one. A CARRIAGE RETURN code
clears the Cursor Position Counter.
With the terminal operating in FORMAT MODE,
when the cursor is incremented off the bottom
line, the cursor automatically wraps around to the
top of the display, i.e., the Cursor Line Counter
is reset to zero. However, if the terminal is not in
FORMAT MODE, the display scrolls whenever the
cursor increments from a count of 23.
A scroll is initiated by any of 3 functions if act
ivated when the cursor is on the last line of the
display and the terminal is not in FORMAT MODE:
a.
LINE FEED or CTRL J
b.
CURSOR DOWNorESCB
c.
If the cursor is on the last position
of the last line.
1. Cursor right
2.
Any displayable character
3. Space
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