Tektronix 067-0892-00, Инструкция по эксплуатации

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care of
that. Remember
that
the
counter
only
requires 260
microseconds
to
force
the
Y2 output
of U1030 low.
When
we
had
a
2400
Hz input,
the
PULSE
line
went
high
before
Y2
went low.
With
a
1200
Hz
input,
counter
U1020
will
reach the "1010"
count
before
the
next PULSE.
When
Y2
goes
low,
then
high again at
“1011”, the
RXC
input to
U2040
is toggled.
It's
toggled again
when
PULSE
does
go
high.
In
effect, the incoming frequency
has been
doubled.
When
the
incoming signal
is
only
1200
Hz,
the counter
injects
a
signal
between
each
rising
edge
of
PULSE,
thus
creating
a
4800
Hz
baud
clock.
If you placed an oscilloscope probe on
the
RXC
input to
U2040,
you
wouldn’t
see a symmetrical
square wave, nor
would
it
seem
very stable.
Because
this
clock
is
actually
generated
by
the
signal from
a
tape
recorder,
all
of
the
recorder’s
wow
and
flutter
is
passed through to
the
RXC
input. However,
the
signal is
stable enough
to
clock
the
ACIA.
Now
let's
talk about how
the
incoming
signals are
transformed
to
data.
The thing to
remember
in
this discussion
is
that when
a
2400
Hz
signal
is
being received,
we
need
to
present
a1”
to
the
RXD input
of
U2040 when U2040 receives
a
clock
pulse. And,
we need
a
“0”
when 1200
Hzis being
received.
At
the
bottom
of
schematic
3, you can
see that the
PULSE
line
is
tied to one
input
of
U1050A.
U1050A
acts
as
an
inverter.
When
PULSE
is
low,
the
output
of
U1050A
will
be
high,
and when
PULSE
is
high,
the
output
of
U1050A
will
be
low.
This signal and
PULSE
are
connected
to
the
positive-edge
clock
inputs
of
U1040A
and
U1040B.
As
you
can see,
U1040A
and
B
are
set
up
much
like
the Cassette
Modulator
J-K
pair.
Notice
that the set
input
of
U1040B
is
connected
back
to
the
Y3
output
of
U1030. We'll
discuss
this
connection
later.
Let’s
assume that
we’re receiving
a
2400
Hz signal, and
that
PULSE
goes
high
at
“time 0”.
Because
J
and
Kare
tied
low,
that
high
causes
the
Q
output
of
U1040B
to
go
low,
as
shown
in
Fig.
5-7.
Five microseconds
later,
when the
clock
input to
U1040B
goes
high
(remember, this
is
the
inverse
of
the
PULSE
line),
the
Q
output
of
U1040A goes high,
indicating
a1”
data toU2040.
And as
longas
the
incoming
frequency
is
2400
Hz,
the states
of U1040B
and
U1040A
will remain
this
way. Each
time
U2040 receives
a
clock
pulse,
it
will
read
a
“1” on
its
RXD
input.
Now
let's
look
at
what
happens
if
the
incoming
signal
is
1200
Hz.
Refer to
Fig.
5-8 while you read
the
following
paragraphs.
@
Theory of Operation—MicroLab
|
Instruction
PULSE
U1040B-10
PULSE
U1040A-7
J
2827-17
Fig.
5-7. 2400
Hz
("1")
Timing.
As
before,
counter
U1020
comes
into
use
when
reading
1200
Hz
data. Notice
in Fig.
5-8
that
it
takes one
complete
PULSE
transition
to
set
up
the
RXD
line.
The
next clock
(RXC)
pulse
will be generated
by
counter U1020. Recall
that the
Y2 output of
U1030 goes
low
every
260
microseconds, and that
the
Y3
output
follows
by
26
microseconds. The
Y3 output
is
connected
back
to
the set
input
of U1040B. Look
at
Fig.
5-8,
and
you'll see
that the
timing
of
the
set pulse
forces
the
output
of
U1040A
low
just
before
the
RXC
clock
created
by
the
counter
(the
second
RXC
pulse
in
Fig.
5-8)
goes
high. The next time
PULSE
goes
high,
the
whole
thing
starts
over again.
PULSE
RXC
J
U1040B-11
l
U1040B-10
PULSE
J
U1040A-7/RXD
2827-18
Fig.
5-8. 1200
Hz
(“0”)
Timing.
5-13