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Theory of
Operatlon
-
4g4A/494Ap
Service, Vot.
1
Flgure
7-21.
Character
timing dlagram.
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lnstrument
Bus
Interface
The
microcomputer
controls
the crt
readout
and
fre-
quency
marker
dot
over
the
instrument
bus through
the
following
ports.
Decoder
U3051
asserts
5F when it sees
a value
of
5
on
the upper
four
bits of
the instrument
bus address
lines, and
2F when
it
sees
a value
of
2. The
decoder
must
be
enabled
by DATA
VALID high
on
the
instrument
bus. The false
transition
of
DATA VALID
causes
the
7-56
addressed
port
to
latch the data
on
the
instrument
bus.
Conlrol
Port
Control
address
port
U3034 turns the
readout on
or
off, steers
data
sent
to the
address/data
port,
controls
the
mode
of
the
frequency marker
dot,
and contains
two bits
of the
RAM
address.
The bits
are
defined
in Table 7-9. Bit
numbering
on
the
instrument
bus
starts al zero. However, the
D and
Q
pins
of
U3034
(and
some other
lCs)
are
numbered
starting at
one,
following th€ manufacturers
data.
Bit 0 turns the crt readout
display on
(1)
or off
(0).
When
set, this
bit
releases CLEAR
from
the GEN
RUN-
NING flip-flop and allows the
off
timer, U1055,
to
set
+GENRUNNIN
CoLUMN
to
1 2345
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ourpurs
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COLUMNS
ROWS | |
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(7)---.
441S120