OUTPUTS
0r
0a 0
t"n"ut
@
ROW
ADDRESS
000
001
010
011
100
t0l
tI0
IIl
OW O
IS
ALWAYS
BLANK
PICTORIAL
4.5
Since the electron beam
in
the
CRT
scans only
one
line at a time, the
dots
that make
up the characters
must be
shifted out serially, one row at a time. To
do
this, the
character
generator puts
out up to
five
dots in
a row for
each character.
Pictorial
4-5 shows the row
outputs for the character H. The
S-bit dot data
is
always followed by three
spaces
(the
absence of dots)
before the
dots
for
the next character are shifted out.
Since the CRT scanning
process
requires the dots data
to
be applied to the
video
circuits in a
serial
format,
the s-bit dot data is loaded into a shift
register
and
shifted eight times. This shifts
out
the five bits
of
dot
data
followed by three
spaces.
The three spaces are
formed
when a blanking signal is applied to the video
circuits
during
the fifth through seventh
shift
sequences. They are also necessary because the
character location on the screen has been defined as
being eight rows wide. The spaces
provide
space
between characters.
During the fifth through the
seventh
shift sequence,
the
shift
register is loaded with new dot data for the
next
character.
The RAM
address counter
is also
incremented
to
its next
address.
This
process
of
addressing
succeeding
RAM location and
outputting
dot data continues until a horizontal sync
pulse
is
generated
at the
end of
a scan line. The horizontal
sync
pulse
increments the row counter and causes the
electron beam to begin a new scan line.
The
character
generator
now
outputs
dot data
for
the
next row in the
dot matrix, This
process
continues until all seven
rows
of
dot data have been outputted.
Horizontal
scan
lines 8 through 15 are normally
blanked. The cursor,
will appear on lines 12 and
13. A new character line
(16
scan
lines) begins after scan
line 15 is
completed.
The
cursor
is
displayed on scan lines 12 and 13 when
an unblanking signal is applied
to
the video
circuits.
This happens when
the cursor
location,
which
is
stored in the cursor latch, is
coincident with the
address coming from the
RAM address counter.
TECHNICAL
DESCRIPTION
The master
clock
of
the
entire
Terminal
is
located
on
the
character
generator
circuit
board and it is com-
posed
of
IC210B,
crystal Y2O'1., and transistor
Q201,
The clock frequency is
12.395 MHz. The true and the
complemented
outputs are
provided
for
the dot clock
and the
other system timing. lC's 214,27OC,227D,
277C, and 207,4'
generate
timing signals A, E, and F
from the master
clock signal. Refer
to timing diagram
#7
for
these waveforms.
Keep in mind
as
you
study
the waveforms,
that the trailing
edge
(the
negative
transition)
of the clock
pulse
always
preceeds
any
logic level
transitions
of
the
other
IC's
by approxi-
mately 30 nanoseconds.
At
the start of the character
generation
cycle, the RAM
counter
(on
the RAM
and counter circuit board)
is
told by
the trailing edge of waveform F to count up.
After
565 nS, the RAM data is latched into
IC2O2
and
IC203
and then
transferred to the input of the
charac-
ter
generator
by
the next leading
edge of waveform F.
Eighty
nS later, the
trailing edge
of
waveform F
tells
the RAM
to count up again. In
the
meantime,
the
character
generator
is
starting to access its internal
dot information
and, after approximately 450
nS, the
five bits
of
dot
data appear at the
output of
the
charac-
ter
generator.
Waveform A is used as a load
pulse
for
shift registerlC2O4.
The dots
are
latched
into the shift
register
by the trailing
edge of the dot clock before the
load
pulse
goes
back to zero. Actually,
the shift
regis-
ter is loaded three times
(at
t6, t7, and t0), but only the
load at t0 is
used.
The
next five cycles of the dot clock
shift dots out serially as
video
information, which,
in
turn,
is
applied
to the CRT,
At the end of the
fifth
pulse,
waveform
A is used as a
blanking signal to
provide
inter-character blanking,
Besides latching
RAM data
for
the character
generator,
waveform
F is
used
in
increment the
character countet,
which is
composed of
IC's 215 and 220. After 80 characters
have been accessed and shifted out,
the D output of
ICz15
provides
a
retrace blanking signal.
This
signal
blanks the
video
during
the retrace
time and it also
inhibits further RAM count-ups.
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