Actually,
the
D
output of
IC215
does
not turn
on
the
blanking directly.
It is delayed
by
two character
times
in IC211 to allow time for the
last
character
accessed
to be shifted out.
The
outputs
of IC211A
generate
blanking and sync
pulses
and count
up
IC2 13
,
the row
counter,
which tells the character
generator
to
access
the next
row
of dot
information. When the
retrace
blanking
period
is
over,
the
process
is
repeated until
all
eight
scan lines of
information for the same
80
characters have been shifted out,
and a complete
row
of characters
is displayed
on
the screen. During
the
next
eight scan
lines, the
entire
process
is
repeated
and the same
80 characters are shifted out once
again,
but the
video is blanked
to
provide
spacing
between
character lines.lC222C
decodes
the
row counter out-
put
of
IC273 so that
the cursor
information can
only
appear
in scan
rows 12 and
13
of
the character
row
cycle.
At the end of the
16th scan
line, the D output of
IC2 13
(the
row counter)
is used
to count up
the
+12
(divide-by-12)
counter in the
RAM to access
the next
row
of 80
characters.
It is also used
to increment the
sync or
frame
counter,
1C21.6,
which keeps track of
the
number of
rows
of
characters that are displayed
on
the
screen.
At the end of
the 16th scan
line
of
the tZth
character
row, the C and
D
outputs
of
IC216
generate
the vertical blanking signal.
IC's
209C and 221,{,in
conjunction with
the vertical blanking
signal,
gener-
ate the vertical sync
pulse
to start
vertical
retrace.
Refer to waveforms
1 through 22 on timing
diagram
#2
for a description of
the scan and
frame cycles.
Cursor
information from the RAM and TPU
is
latched
into
IC202 along with the ASCII data by waveform
F.
This
cursor signal
is
delayed
by IC208A for the
duration of the character
generator
access
time and it
appears
as
video
information at
inputs 1 and 4
of
IC278 in the video chain.
If
the ASCII character latched in IC's 2O2 and 203
is
a
control character, bits o and
7
will
be
0
and the
Q
outputs
of
the latches will both be high causing the
output
of
IC2O1A to
go
to a logic 0. This logic 0 is
delayed by IC?O7B for the duration of the character
generator
access time and it is used to blank the dot
information
for that control character.
The
preset
input
of
IC207B is
driven
by
a4Hz clock
pulse,
which
will unblank the control character dots
4
times a
second,
causing
the
character to appear
to blink
on
the screen.
In
the
plot
mode, the first 128 scan lines are used to
display
information that
corresponds to the binary
value
of a
7-bit
ASCII word.
In
this mode, the first row
of characters that would
normally be seen
on
the
screen is repeated 4 times,
starting
with the 129th
scan
line
(in
the 9th, 10th, 11th,
and
12th
character
lines). In the space directly above
each
character, the
beam
is turned
on
to
display
a lighted dash in the scan
line
that corresponds
to the binary value of that
character.
The 128
scan
lines
are arranged
so that the
128th
line
(line
127) is
at the top of the screen
and
the
first line
(line
0)
is at the bottom
of
the
8th character
line
(128
lines
from the top).
For
example,
if the
character
was a
rubout, which
is all 1's the dash
would
appear at the
very top of the screen.
If
the
character
was a K
(binary
1001011), its
value would
be 75 and it would
appear on
plot
line 75. Refer
to
Pictorials 1.-2
(Page
6)
and 1-3
(Illustration
Booklet,
Page
2).
The
four
outputs
of
IC213
and the A,
B, and
C outputs
of
IC216,
count the
scan lines from
0 at
the top to'1.27
at the
end of the 8th
character row. The
seven
outputs
are inverted
by
1C272
and IC219A
so that the
count
will
be 727
at the top
and 0
on the 127th row.
The
outputs
of the inverters
are
applied
to the inputs
of
exclusive NOR gates (IC206
and
IC209A,
B, and D).
The
other inputs
to the
NOR
gates
are connected
to
corresponding
bits
from the
outputs
of
latches
lC2O2
and IC203.
When
the seven
data bits at the
latches are
exactly
coincident
with the
outputs
of the
inverters,
a
logic 1
will appear
at the
outputs
of
IC206
and IC209.
If
the
plot
function
has been
selected by
the Plot
button
on the keyboard,
this
coincidence
signal is
gated
through IC201B
to
the input
of
ICz08A.
After a
delay
of one character time, the
plot
coincidence sig-
nal will
appear at
the output of
IC20BA
as a
plot
video
signal that
will then be
displayed on the screen as a
short dash. During the
plot
time, the cursor informa-
tion is blanked by
the output of IC201D and
the
ASCII
dot information is
blanked by the
output of
IC221,8.
The video
and blanking channels can be broken down
into
three
video
channels and two basic blanking
channels. The
dot
information video
channel consists
of
IC210D, ICz19D,
and
IC218C.
If there are no dot
blanking signals
present,
the
dots
that shift
out of
IC2O4
will appear directly at
pin
8 of
IC218C.
The
dot
video
will be blanked by the output
of
IC207B if the
dots represent
a control character by
the 9-to-15
line
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