blanking signal
from the D output of
IC213, or by
the
plot
blanking
signal
from
pin
1O of
IC221.C which
blanks the first 128 scan
lines.
The
cursor
video channel consists of
IC201C,
IC208A,
and IC218B.
In
the
absence
of
cursor blanking signals,
the cursor
video signal from
pin
2
of
lC2O2 will ap-
pear
(after
the
proper
delay)
at
pin
6
of
IC21BB.
The
cursor can be blanked
by a
cursor
blanking signal
from the TPU or by a
plot
signal,
both
of
which appear
at
pin
10 of IC201C.
The
cursor
is also normally
blanked by
the
output
of ICz19B, except during
the
12th
and 1.3th scan
rows. This blanking signal ap-
pears
at
pin
5 of
IC21BB.
The
plot
video
channel
consists of
IC201B, ICz08A,
and IC218A. The
plot
video
signal that appears at the
outputs of
IC206
and IC209 will
appear after the
proper
delay at the output
of
IC218A.
This video
signal can be blanked by the
plot
blanking
signal that
comes from
the output of lC22LC.
The
dot, cursor, and
plot
video
signals are logic 1's in
the absence of
any
video information.
They drive the
KEYBOARD
The keyboard
contains a
52-key ASCII keyboard
and
encoder, a
special ASCII
character
generator,
four
cursor control
latches.
and twelve mode
control
switches.
The ASCII
keyboard is
a matrix
of single-pole,
normally-open
switches that
are scanned
by
IC
02,
the keyboard
encoder,.The
X2 through
XS
outputs are
scanned with
a series
of
pulses.
When
a key is
pressed,
one ofthese
pulses
appears at
one ofthe
eight
Y
inputs.
After
an appropriate
debounce
period,
the
encoder recognizes
the
pulse
as
a
valid
key
depression,
decodes the X
and Y
pulse
data into
the
appropriate
7-bit
ASCII
word,
latches the
word in
buffer IC403,
and
puts
a logic
1 on
the data strobe
output to indicate
that there is
valid
data
present
in
the buffer.
The
negative
true logic
output
of the
keyboard
encoder lC4O2
is inverted
fto
positive
true
logic) by
inverter/buffer
IC403
to
provide
positive
true
logic for the bus.
The table in Pictorial
a-6
(Illust-
ration
Booklet,
Page
9)
shows the
7-bit
ASCII codes,
inputs
of
a 3-inputNAND
gate,
IC222B,and anyvideo
information
(0's)
will appear at the output of
lC222B
as logic 1's. This is the composite
video
output.
The vertical blanking information
appears
as a
negative logic
signal at
pin
3 of
IC21.7 A. The
blanking
signal will be developed
for
the
vertical retrace
period
and for the duration of a
refresh
interrupt signal from
the TPU circuit board.
Horizontal
blanking information appears as a
negative logic
signal at the output of
IC2178.
This
signal is developed during the horizontal
retrace
time
and during the three dot spaces between characters.
The
vertical
and
horizontal blanking and the
video
signals
drive the inputs of
NAND
gale
lC222A. The
output
appears as negative
logic video information.
That is, a logic
0
will turn on the electron
beam in the
CRT.
The
A write
pulse,
which is used in the screen
erase
cycle, is
generated
by
lC221,D from the
A and D out-
puts
of the master
clock divider,lC274.
CIRCUIT
BOARD
generated
by the keyboard encoder, as they will ap-
pear
at the
output of
inverter/buffer IC403. It also
shows the
key
entries that are needed to
produce
these
outputs. When the output enable
line
(OE)
is
brought
to a logic 0, the 3-state buffers are turned on
and
the ASCII data is
put
on the bus. A logic 0 on the
output enable
line
also
resets
the data strobe output of
IC402
to
indicate
that the data
in
the buffer has been
read.
IC401
is a
+
1.6 counter
that
generates
four repeat rates
from
the 60.5 Hz vertical frequency. The
frequencies
set the rate
of
the
repeat
(REPT)
function.
A
special
character
generator
composed
of IC404,
IC405,
and IC406
is also
connected to the ASCII
bus.
IC406
decodes inputs A1
and A2 into four
states; 1) a
3-state
OFF,
2)
and ASCII
carriage return, 3) an ASCII
line feed
,
4J
an ASCII
space.
When both inputs
are
low,
the 3-state
outputs
of
IC404
and
IC405
are turned
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