SECTION 3
3-1
001-3282-101/102
THEORY OF OPERATION
Figure 3-1 MODEM BOARD BLOCK DIAGRAM
3.1 INTRODUCTION
The function of the modem is to convert digital data into tones that can be transmitted using an RF link. It
also converts these tones back into digital data when the signal is received. A block diagram showing the modem,
the telemetry unit generating or receiving the data, and the RF receiver and transmitter is shown in Figure 3-1.
This modem can be used for full-duplex, half-duplex, or one-way data communications (refer to Section
3.2). The RF receiver/transmitter configuration determines which type of communication is possible. This modem
has an RS-232C compatible port for interfacing with the unit receiving or transmitting the data. A DIP switch on
the modem PC board programs the modem to operate according to one of the Bell standards listed in Table 2-2.
U240
J101
1
SHIELD
2
RS-232 RxD
RS-232 TxD 3
RS-232 RTS 4
RS-232 CTS 5
6
RS-232 DSR
7
RS-232 GND
8
RS-232 DCD
20
RS-232 DTR
22
RS-232 RI
U230
U220
U200
TX AUDIO
RX AUDIO
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
J102
EXTERNAL
CONNECTOR
CONNECTOR
MOBILE RADIO
WB OUT
WB IN
FREQ SELECT
RSSI
+13.6V
PTT OUT
SQUELCH IN
TX DATA
9 GROUND
10 RX DATA
J103
SQUELCH IN
PTT OUT
GROUND
WB OUT
TX DATA
INTERCEPT
TX MUTE OUT
+13.6V
9 +13.6V
10
SUB AUDIO DATA
11 TX/RX
12 ARQ OUT
13 GROUND
14
WB IN
15 RX DATA
+13.6V
+13.6V
WB SEL
TX DATA
WB SEL
+5V
+13.6V
Q250/Q251
Q252
Q253
Q254
Q255
Q256
SCI BUS
CS BUS
ADDRESS BUS
U450D
U401A
U400B/C/D
U401C
U450A
U450A
U401B
U450B/C
IRQ
+5V REGULATOR
RESET
+5V
NORM/LOOP
H/F DUPLEX
BELL 202/103
CONV
SQUELCH INV
OPT 1
OPT 3
OPT 2
S101