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When
it
is not
being used, the
DTMF
generator is
in
the quiescent state and draws
very
little
current.
The
COPE
sends a message
on
the bus
to
"awaken"
the
DTMF
generator,
causing
the
generator
to start its
3.579545 MHz crystal oscillator, Y501. The DTMF
generator listens
for
messages,
which
turn
tone pairs
on or off.
These
tone pairs are sent
from pin 15
through resistor
R526
to
the main radio board.
Resistor R526
is part of
a voltage divider that
attenuates the .9-volt peak-to-peak signal (by about 30
dB)
to
a
level that is
in
the
same range as
the micro-
phone output level.
The
other resistor
in
the divider is
R9
on the main radio board.
R526 also serves
to
iso-
late the
DTMF
generator
from
the microphone circuit
when transmitting voice.
C503
filters out high-frequen-
cy clock
noise that might corrupt the
DTMF
signal.
The DTMF signal shares
the
same
line
as
the
microphone
on
the front
shield. During DTMF
sequences,
the
COPE
mutes the microphone by inter-
rupting its
bias current via Q506.
m.
SECURENET
Module (U900)
(SECURENET
radios only)
The
SECURENET
module,
U900, uses pins
4, 5,
7, and 16 for
keyloading.
If
the encryption key is lost
or
destroyed,
the module will indicate this by
sending
a
logic
low level from pin 16
whenever the radio's
PTT
switch is pressed and, periodically, when the radio
is
not transmitting or
receiving.
16
When the
radio is transmitting, the
SECURENET
module is put
into the
appropriate mode (coded
or
clear) by
its microcomputer, which
gets
this information
from
U400 via the DATA
line
(U900,
pin 11).
In
the
coded mode, the audio signal
from
the audio filter IC
(U101,
pin
22) enters the module (U900
pin
17), where
it
is
converted
to
a 12-kilobit/second digital format.
Within
U900,
the
signal
is then
encoded, filtered, and
returned,
via
pin 1, to
U101, pins
15 and 16.
In
the
clear
mode, the audio enters U900 on
pin
17 and the mod-
ule's microcomputer switches
it
back
out on pin
1.
When the radio
is
receiving, the
SECURENET
module continuously monitors (at U900,
pin
2) the out-
put of the
discriminator (U100,
pin
31). The module
determines
if
the
code
(or
key)
is
correct.
If
the
received code
is
correct, the
SECURENET
module
decrypts the signal and sends
it
(U900,
pin
3)
to
the
audio
filter
IC (U101,
pin
7). At the
same
time,
U900,
via
the
DATA
line (pin 11), tells the radio's
microcom-
puter
(U400, pins 22
and 27) that the received signal
is
encrypted.
The
radio's microcomputer then
sends
appropriate data
to
the audio filter IC,
U101, so that
it
will
process
the decrypted audio
as
it
is routed out of
the
SECURENET
module
(U900,
pin
3).
The easiest way
to
determine
if
the
SECURENET
module, U900, is faulty is
to
replace
it
with the
SECURENET
bypass
module (Motorola part no.
NTN4720A). By
replacing this module,
all functional
tests (receiver quieting,
transmitter audio,
etc.) can be
checked
with the radio
in
the
clear
mode.
When
it
is not
being used, the
DTMF
generator is
in
the quiescent state and draws
very
little
current.
The
COPE
sends a message
on
the bus
to
"awaken"
the
DTMF
generator,
causing
the
generator
to start its
3.579545 MHz crystal oscillator, Y501. The DTMF
generator listens
for
messages,
which
turn
tone pairs
on or off.
These
tone pairs are sent
from pin 15
through resistor
R526
to
the main radio board.
Resistor R526
is part of
a voltage divider that
attenuates the .9-volt peak-to-peak signal (by about 30
dB)
to
a
level that is
in
the
same range as
the micro-
phone output level.
The
other resistor
in
the divider is
R9
on the main radio board.
R526 also serves
to
iso-
late the
DTMF
generator
from
the microphone circuit
when transmitting voice.
C503
filters out high-frequen-
cy clock
noise that might corrupt the
DTMF
signal.
The DTMF signal shares
the
same
line
as
the
microphone
on
the front
shield. During DTMF
sequences,
the
COPE
mutes the microphone by inter-
rupting its
bias current via Q506.
m.
SECURENET
Module (U900)
(SECURENET
radios only)
The
SECURENET
module,
U900, uses pins
4, 5,
7, and 16 for
keyloading.
If
the encryption key is lost
or
destroyed,
the module will indicate this by
sending
a
logic
low level from pin 16
whenever the radio's
PTT
switch is pressed and, periodically, when the radio
is
not transmitting or
receiving.
16
When the
radio is transmitting, the
SECURENET
module is put
into the
appropriate mode (coded
or
clear) by
its microcomputer, which
gets
this information
from
U400 via the DATA
line
(U900,
pin 11).
In
the
coded mode, the audio signal
from
the audio filter IC
(U101,
pin
22) enters the module (U900
pin
17), where
it
is
converted
to
a 12-kilobit/second digital format.
Within
U900,
the
signal
is then
encoded, filtered, and
returned,
via
pin 1, to
U101, pins
15 and 16.
In
the
clear
mode, the audio enters U900 on
pin
17 and the mod-
ule's microcomputer switches
it
back
out on pin
1.
When the radio
is
receiving, the
SECURENET
module continuously monitors (at U900,
pin
2) the out-
put of the
discriminator (U100,
pin
31). The module
determines
if
the
code
(or
key)
is
correct.
If
the
received code
is
correct, the
SECURENET
module
decrypts the signal and sends
it
(U900,
pin
3)
to
the
audio
filter
IC (U101,
pin
7). At the
same
time,
U900,
via
the
DATA
line (pin 11), tells the radio's
microcom-
puter
(U400, pins 22
and 27) that the received signal
is
encrypted.
The
radio's microcomputer then
sends
appropriate data
to
the audio filter IC,
U101, so that
it
will
process
the decrypted audio
as
it
is routed out of
the
SECURENET
module
(U900,
pin
3).
The easiest way
to
determine
if
the
SECURENET
module, U900, is faulty is
to
replace
it
with the
SECURENET
bypass
module (Motorola part no.
NTN4720A). By
replacing this module,
all functional
tests (receiver quieting,
transmitter audio,
etc.) can be
checked
with the radio
in
the
clear
mode.
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