THEORY OF OPERATION
3-4
001-3282-101/102
3.4.2 LOOPBACK MODES
When a Loopback mode is selected, both the filters in U230 are configured for the same frequency. Half-
duplex operation in the Loopback modes is the same as it would be in the General modes because the back channel
is not used.
If a full-duplex originate mode is selected, the filters are set to the transmit frequency; if a full-duplex
answer mode is selected, they are set to the receive frequency.
3.5 TRANSMITTING DATA
NOTE: The levels in the following information are the levels at the RS-232 port. The levels on the pins of U230 are
the inverse of these levels because of the inversion produced by U240.
3.5.1 INTRODUCTION
The transmit data signal originates from the data equipment. This data signal is applied to the TD
(Transmit Data) input of the RS-232 port, inverted and converted to TTL by U240, and then applied to the TxD
input of U230 and converted to the MARK and SPACE tones. These tones are fed out of U230 on the TxA
(Transmit Audio) pin. They are then routed to a separate RF transmitter module where they frequency modulate
the RF signal.
3.5.2 DTR SIGNAL
This signal indicates to the modem that the data equipment connected to the RS-232 port is ready to
transmit or receive data.
3.5.3 RTS SIGNAL
When the equipment connected to the RS-232 port is ready to send data, it brings the RTS (Request-To-
Send) input high. This signal must remain high for the duration of the data transmission. If the DTR input is low,
bringing the RTS input high has no effect. It is common protocol that a SPACE (logic 0 or low) be presented to the
Transmit Data (TxD) input just before the RTS input is made high. When the data transmission is finished, the
RTS input is brought back low which disables the transmitter section of U230. In the half-duplex General modes,
the RTS input also controls the transmit and receive filters in U230 as described in Section 3.4.1.