CML Semiconductor Products
1
AN/Data/469/2 November 1997
Asynchronous Modem Circuitry
This application note, used with current FX469 product information, outlines the construction of a low-cost
asynchronous modem for the transmission of RS-232 data in the form of Fast Frequency Shift Keying (FFSK,
or Minimum Shift Keying (MSK), between terminals by a radio or line medium.
The modem circuitry shown in Figure 1 accepts asynchronous data while transmitting synchronous data using
the FX469 series modem. The FX469 may be used to transmit data at either 1200 or 2400 bps, depending on
the setting of Switch 1 shown in Figure 1. Details of the devices used in this application, are outlined below.
Function Requirement Device
RS-232 Driver/Receiver Level Translator Maxim MAX-232
Async to Sync conversion MICRONAS MAS7838
Sync to Async Conversion MICRONAS MAS7838
Data Carrier Detection CML FX469
Controlled RTS/CTS Delay 74HC04 Delay Element
Generation of FFSK/MSK Signals CML FX469
Reception of FFSK/MSK Signals CML FX469
Interface into Radio System CML FX469
RS-232 Handshake Misc. Circuitry
The signals required for an RS-232 handshake for asynchronous data are as follows. A complete definition
of each is given at the end of this application note.
DTR Data Terminal Ready
DSR Data Set Ready
RTS Request to Send
CTS Clear to Send
TXD Transmit Data
RXD Receive Data
DCD Data Carrier Detect
The Maxim MAX-232 converts the TTL/CMOS input/output levels to the nominal ±10V RS-232 input/output
levels. On power-up, the data set ready (DSR) signal is set by the MAX-232’s DC to DC converter. The
incoming data terminal ready (DTR) signal enables the transmitter keying signal.
The request to send (RTS) and clear to send (CTS) signals are level shifted from the RS-232 interface to
TTL/CMOS signal levels. When the modem (DCE) receives an RTS, the RF transmitter and the MSK tone
are keyed immediately and a 30 ms to 100 ms timer is started. At the completion of the timing, a CTS signal
is generated for the data terminal (DTE) to allow serial data flow.
Continued on next page ......
Using an FX469 FFSK Synchronous Modem
with an Asynchronous Data I/O