Baud And Clock
Generator
<>
EE
The
Baud
and
Clock
Generator
starts
with
the
crystal-
controlled oscillator
at the
top
left of
schematic
3.
The
oscillator
frequency
of
2.4576
MHz
is
fed
to the
clock
input
of
U1010,
where
it’s
divided
by 2
to
create
a
1.2288
MHz
clock.
The
1.2288
MHz
clock
is
then
fed
to
U2080. U2080
is
a
12-stage
ripple
counter that
divides
the
input
frequency
into
six
different
frequencies.
Each
output frequency
is
equal
to 16
times
the
corresponding
baud
rate. For
example,
the 1200
baud
clock
output
from
pin
2
of
U2080
runs
ata
frequency
of
19.36
KHz
(16
X
1200).
This division
is
necessary
because
each
of
the
asynchronous
communications interface adapters
(ACIAs)
is
set
up to
accept
a
clock
frequency
16
times
that
of
the
selected
baud
rate.
Notice
that
the 1200
baud
clock
is
also
fed to U3090.
U3090
is
a divide-by-11
counter
that
produces
the
required
frequency
to
operate
at
110
baud (110
baud
x
11 x
16
equals 19.36
KHz).
U2080
is
a
CMOS
device,
and each
output
line
is
buffered
to
prevent
static
damage
when
a
rate
jumper
is
moved.
Terminal Port
®
fH
The
Terminal
Port
is
made
up
of
U9020
and
buffer U9010.
U9020
is
an
ACIA
that takes
serial data from
connector
J101 and
converts
it
to
parallel
data, and
vice
versa.
You'll
notice on
schematic
3
that the
incoming
portion
of J101
is
shown
on
the
left
side
of
the
page,
while
the
output
portion
of
J101
is
shown
on
the
right side
of
the
page.
U9020
is
enabled
by
the
TSEL
line coming from
the
1/0
Decoding
logic
on
schematic
4.
(We'll
discuss schematic 4
later.) The receive
and transmit
clocks
(RXC
and
TXC)
come
from
the
Baud and
Clock
Generator.
The
position
of
jumper
J1080
selects the operating
frequency for U9020.
Data
from
J101
is
accepted through
pin
2
of
U9020
when
the
DCD
(data-carrier-detect)
input
is
low.
The
data
is
then
read
from
BDO-BD7
by
the
personality
card.
When
the
personality
card
wants
to
transmit
data
to the
terminal, U9020
forces
its
RTS
(ready-to-send) line
low,
then waits
for
the
CTS
(clear-to-send)
input from
the
terminal.
When
CTS
is
received,
U9020
begins
to
transmit
the data,
in
serial form, from
BDO-BD7
to
J101.
U9020 automatically
adds any
parity or stop bits
when
transmitting data, and
checks
for
parity
and stop
bits
when
receiving
data.
@
Theory
of
Operation—MicroLab
|
Instruction
MODEM
Port
EBA
The
MODEM
Port works
the same
way
as
the
Terminal
Port.
The
only
difference
is
that connector
J102
is
designed
to
plug into
a MODEM.
U7020
is
enabled
by
the
SSEL
line
from
the
I/O Decode
logic
on
schematic
4.
Es
The
MicroLab
|
can
store data
on,
and
retrieve
data
from,
a
cassette
tape
recorder. Since
cassette
recorders are
basically audio devices,
the
MicroLab!
Cassette
Port
converts
serial
data
into
audio
tones
in
a
format
called
the
“Kansas
City
Standard.”
Parallel
data
is converted
to
serial
data
by U2040.
The
serial
data
is
then
modulated
to
create
the
1200/2400
Hz
tones,
and
sent
to the
tape
recorder.
When
data
is
received from the
recorder,
it’s
demodulated,
then
converted back
to
parallel data and placed
on
BDO-BD7.
The
Kansas
City
Standard
involves
turning
1’s
and
O's
into
2400
Hz
and 1200
Hz
tones.
A
logic
"1"
is
represented
bya
2400
Hz
tone.
A
logic
0”
is
represented
by
a
1200
Hz
tone.
Figure 5-6
shows a
representative
timing diagram
of
the
way
in
which
the Kansas
City
Standard
works.
Cassette
Port
The
Modulator.
Notice
in
the
upper right corner
of
schematic
3
that there are
two
J-K
flip-flops,
U7090A and
U7090B.
These
two
flip-flops
make up
the Cassette
Port
modulator. Notice also
that each
flip-flop’s clock
input
comes
from
the
300
baud
clock
line
(4800
Hz).
U7090B’s
J
and
K
inputs
are
connected,
through
an
inverter,
tothe
TXD
(transmit
data)
output
of
U2040.
When
the
TXD
output
of
U2040
is
high,
indicating
a
"1"
state, the
J-K
flip-flop pair
act
as
a
divide-by-two
counter.
Because
the
1”
holds the
set
input
of
U7090B
high,
a
high
appears
on
the
J-K
inputs
of
U7090A,
causing
it
to act
as
a
divide-by-two
counter. The
4800
Hz
signal from the
Baud
and
Clock
Generator
is
divided
by
two,
and
the output
of
U7090A
will
be
a
2400
Hz
signal.
Data
Out
2400
Hz
=
1200
Hz
=
A"1”
State
A"0”
State
Fig.
5-6.
Timing Diagram
For The
“Kansas
City
Standard”.
A
logic
“0”
is
represented
by
a
1200
Hz
tone,
and
a
logic
1"
by
a
2400
Hz
tone.
Summary of Contents for 067-0892-00
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