THEORY OF OPERATION
3-3
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3.3 DATA TONE BANDWIDTH
3.3.1 INTRODUCTION
The baud rate and modulating frequencies utilized by each of the programmable operating formats are
shown in Table 2-2. As indicated, 300 baud full- or half-duplex data rates are available with the Bell 103 mode and
1200 baud full- or half-duplex data rates are available with the Bell 202 mode. However these standards are
intended primarily for use with a two-wire phone line which provides simultaneous two-way communication.
Operation with an RF link is different because a single RF channel can provide communication in only one
direction at a time.
3.3.2 PHONE LINE AND RF LINK BANDWIDTH
The two tones used by this modem to transmit and receive data are in the audio range of approximately
300-3000 Hz. With a 2-wire phone line, full-duplex data transmission is achieved by frequency multiplexing two
300-baud channels into the 3000 Hz bandwidth of the phone line (refer to Figure 3-2). Each of these 300 baud
channels use two different frequencies. With half-duplex operation or pseudo full-duplex operation utilizing a back
channel (refer to next section), only one main data channel is transmitted at a time. The occupied bandwidth can be
greater and data can be transmitted at higher baud rates.
With an RF link, each RF channel also has a bandwidth of 3000 Hz. In half-duplex applications which use
a single RF channel, 300/1200 baud rates are available. For full-duplex operation, two RF channels are required
(similar to a 4-wire phone line). Since two 3000 Hz bandwidth channels are then available, full-duplex 1200 baud
data transmission is possible (versus 300 baud with a 2-wire phone line).
3.4 GENERAL AND LOOPBACK MODES
3.4.1 GENERAL MODES
When a General operating mode is selected, the receive and transmit filters in modem IC U230 are
configured for different frequencies. If the selected mode is a full-duplex mode, the transmit filter is set for the
transmit frequency and the receive filter is set for the receive frequency shown in Table 2-2. If the selected mode is
a half-duplex mode, the configuration of the receive and transmit filters is controlled by the RTS input of the RS-
232 port. If RTS is high (indicating data is to be transmitted), the transmit filter is set to the transmit frequency and
the receive filter is set to the back channel (refer to Section 3.5.3). If the RTS input is low, the receive filter is set to
the receive frequency and the transmit channel is set to the back channel.