User Manual

120 10503-201 PRELIMINARY T-96S Technical Manual
15
(U12), operates in DRCMSK
1
mode at
4800, 9600 and 19200 bits/sec. It incor-
porates a 7-bit hardware scrambler and
uses Differential (NRZI) encoding in
DRCMSK mode to minimize data pat-
tern-sensitivity. Electronic potentiometer
U9B (E-Pot), controlled by CPU U5, is
used to set the transmitter deviation by
amplitude adjustment of the baseband
signal.
4.3 RECEIVE DATA
Received signals are applied to the RXA
pin on P1-13 and amplified by U7A,
whose gain is set by the electronic po-
tentiometer U9A, then filtered by U12.
The same filter circuit is used for trans-
mission and reception: two analog mul-
tiplexer/demultiplexer gates (U11 A and
B) controlled by TX_EN line are used
for sharing. The filter U12 cut-off fre-
quency is programmable by the CPU,
based on the data rate. The analog signal
is then fed to Peak Detectors U7C and
U7D, to the slicer circuit U8C via U7B
and U8B. The resulting synchronous bit
stream is converted to asynchronous at
U2 and shifted to RS-232 levels by U1.
4.4 SYNTHESIZER
PROGRAMMING
The CPU programs the RF module syn-
thesizer serially on each Tx/Rx transi-
tion. Logic of U1 and U11A/B switch
the receive and transmit data path from
the modem to the radio and/or the exter-
nal serial port, under CPU control. The
CPU also controls the sync/async con-
1
DRCMSK = Differential Raise Cosine Minimum
Shift Keying
verter U2, the filter cut-off frequency,
the serial port handshake lines, and the
LED indicators via Q1-3.
Three channel select inputs, ESD pro-
tected by D1,2,3, in parallel with the on-
board DIP switches, are read by the CPU
to select the active channel. The fourth
DIP switch puts the unit in TEST mode,
which sends a test, tone (data rate/4).
4.5 POWER SUPPLIES
The main DC power (13.3V nom.)
coming from J1.5,10 is filtered out for
tranzient and then sent directly to power-
up the radio module at P1.2. Voltage
Regulator U14 (AVCC) provides 5 V
for the receiver RX_5V and analog mo-
dem circuitry, while U13 (DVCC) pro-
vides 5V for the CPU and other digital
logic.
4.6 MISCELLANEOUS
FUNCTIONS
U4 generates a power-on reset for the
CPU and U3 is a temperature sensor
used by the firmware to compensate for
variations in RSSI.
The RF module’s RSSI output, P1-12, is
read by an analog input on the CPU,
which implements a squelch threshold in
software. Other analog inputs are used
to read ambient temperature (used to
correct RSSI variations) and various in-
ternal voltages (B+_LVL, AVCC, etc.).
The DTR_PGM input, J2-10, puts the
CPU in programming mode in which the
CPU accepts commands and setup data
from the Radio Service Software.