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Theory
of
Operation
-
4g4Ll4g4Ap
Service,
Vot.
l
Figure
7-18.
Character
on/of
timlng.
Character
counters
synchronize
the
horizontal
and
vertical
scan
with
the
Z-Axis
signal
from
the
character
g€nerator
lC
to
draw
the character.
These
counters,
U2022,
U2018,
U2026,
and
U2014,
divide
by
g
for
the
columns
within
a
character
(columns
A,
B,
Ci,
divide
by
8 for
the rows
within
a
chaiacter
lrows
A,
e,
C1,
OiuiO.
by
40 for
the characters
within
a
iine
lcharacters
A, B,
C, D,
E, F),
and
divide
by 16
lor
the
lines
within
a
display.
The
counters_
are
enabled
only
when
the
gen-
erator
has control
of
the
crt
beam (GEil
RUNNING-line
asserted)
and
INCR
{increment)
ls
htgh (when
INCR
ls
low,
the crt
beam
is
stopped
to write
JOoi
on
the crt).
The
SKIP
line from
V2OS2
permits
software
control
of
the
allowable
states
of
line
counter
U2014. By
plac-
ing
a one
in
this bit
of a
character,
the line
count€r
is
allowed
to count
up
to the next
state.
This will continue
until a character
is €ncountered
with
ths
skip
bit
set
to
zero. This
allows
the
addition
of
a
third
line
to the nor-
mal
two-line
readout
for status
messages,
by operating
with
the circuit
normaily
in
the
16-line
mode
(all
but the
bottom and
top
lines
start
with a
readout
character
of
40
hex,
which
has
the
SKIP line
set
high).
Thus, all
but
th€
bottom
and
top lines
are
skipped.
When
large
mes-
sages
are
to be displayed,
the
SKlp line
is set low
for
all
characters
and
16
lines are
displayed.
oFF-T'MER
U1055
cEN
ON F/F
U1041A
GEN
ON
R/O
OFF
FREO
DOT
I
zo-co
u" I
lF-€,|
GENERATOR
RUNN!NG
u1041
u1(MrB
oFFTME
u1055-d
44t6-Lt7
7-53