Using TCM3105 FSK modem chip from Texas Instruments, you can easily build a Bell 202 or CCITT V23 compatible modem. This modem circuit is capable transmitting the data at 75, 150, 600, and 1200 baud, and receive the data at 5, 75, 150, 600, and 1200 baud. This circuit operate at 5V supply and has low power consumption feature. Here is the schematic diagram of the circuit:
An external 4.4336 MHz crystal connected between OSC1 and OSC2 pin is needed to run the internal oscillator. A clock signal equal to 16 times the highest selected bit rate (transmit or receive) is available at CLK output pin. This bit rate, or more exactly the baud rate, and the various operation mode are selected accoding to table 1( a logic high is a mark and a logic low is a space). All analog inputs and outputs for this modem interface must be ac coupled. [Circuit's schematic diagram source: Texas Instruments Application Notes]
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