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Midland SYN-TECH 70-050 - Page 12

Midland SYN-TECH 70-050
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THEORY
OF
OPERATION
70-050/055
Manual
Channel
Selection
Activation
of
the
Up-Down
channel
selector
switch
is
sensed
by
the
microcomputer,
the
receive
audio
is
muted
and
incrementing
or
decrementing
of
the
channel
display
is
begun.
Upon
release
of
the
channel
selector
switch,
E/PROM
data
corresponding
to
the
new
channel
is
strobed
to
the
programmable
divider.
If
the
synthesizer
lock
signal
is
not
detected
after
a
channel
change,
receiver
and
transmitter
are
inhibited
and
the
channel
indicator
displays
the
error
code
95.
When
the
PTT
is
depressed,
the
microcomputer
IC901
pin
13
output
switches
the
voltage
regulator
IC401
to
the
transmit
condition,
outputting
8
V
DC
to
the
transmitter
and
disabling
the
receive
8
VDC
output.
The
microcomputer
then
outputs
E/PROM
address
data
corresponding
to
the
selected
transmit
channel,
which
results
in
the
programmable
c.ivider
IC701
being
reprogrammed
for
the
correct
transmit
frequency.
Scan
Operation
for
the
When
scan
operation
is
activated
by
selection
of
either
of
the
front
panel
pushbuttons,
the
transmit
and
receive
addresses
of
the
displayed
channel
are
stored
in
microcomputer
memory
as
the
priority
channel.
The
address
data
corresponding
to
the
first
scan
channel
is
then
strobed
to
the
latch
IC952,
resulting
in
the
generation
of
the
correct
local
oscillator
frequency
as
previously
described.
After
synthesizer
lock
is
achieved,
a
25
millisecond
period
(Interval
A)
is
allotted
operation
of
the
noise
squelch
circuitry.
At.
the
end
of
Interval.
A
microcomputer,
based
on
the
noise
squelch
output
and
E/PROM
program
either
a)
causes
the
SCAN
sequence
to
resume
(noise
squelch
closed),
b)
opens
the
audio
squelch
gate
(noise
squelch
open
and
noise
squelch
scan
mode
programmed
or
noise
squelch
open,
CTCSS
scan
mode
prog;rammed
but
no
CTCSS
tone
programmed
for
the
specific
receive
channel);
or
c)
activates
the
Interval
B
timer
(noise
squelch
open,
C1CSS
scan
mode
programmed
and
CTCSS
tone
programmed
for
the
specific
receive
channel).
Interval
B
is
a
time
period
(programmed
to
300
ms
for
CTCSS
operation)
allotted
for
signalling
system
decoder
operation.
At.
the
end
of
Interval
B
scanning
resumes
(no
decoder
output,
indicated
by
a
high
level
on
the
TSQM0N
line
at.
IC901-5),
or
the
audio
squelch
gate
is
opened
(successful
decode
indicated
by
low
input
to
IC901
—
->).
If
the
receive
channel
corresponds
to
the
operator-selected
priority
channel,
a
two-beep
signal
is
generated
when
the
squelch
gate
opens.
After
squelch
closure,
scan
resumption
is
delayed
by
the
amount-
programmable
delay
tinmer
(RX
or
TX).
If
the
priority-sampling;
mode
is
selected,
the
microcomputer
automatically
channel
after
every
(programmable).
While
in
channel.
the
priority
(programmable).
the
normal
scan
checks
the
fourth
or
eighth
non-priority
a
hold
condition
on
a
non-priority
is
sampled
every
second
or
two
The
priority
channel
sampling
sequence
is
procedure
described
above.
of
the
scan
priority
channel
channel
,
seconds
identical
to
[f
the
priority
sampling
scan
mode
is
selected,
sensed
by
a
high
level
input
to
IC901-30
causes
the
transmit
address
to
be
latched
in
IC952
and
immediately
on
that
channel.
PTT
activation
priority
channel
transmit
occurs
12

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