June 2000, Issue 107

Low-Cost Software
Bell-202 Modem

AT THE BLOCK LEVEL

The modem design in Figure 1 uses the Scenix SX28AC, which runs at up to 50 MHz and has a 50-MIPS data throughput rate. Although the design accommodates connecting to a PC via an RS-232 connector, it can be changed to some other type of serial interface.

The modem functions can be downloaded from the Scenix web site as virtual peripherals and stored in the on-chip program memory. The virtual peripherals used in this design are:

• two 16-bit timers—one for the power-on LED and one for FSK and DTMF tone duration
• DTMF detection
• FSK detection via on-chip hardware comparator
• DTMF generation
• FSK generation
• UART (1200 bps to 115.2 kbps)

Additional features like caller ID, voice recognition, LCD drive, and numerous types of I/O can be added by downloading the appropriate virtual peripheral into the program memory.

To achieve the lowest implementation cost, the design uses a component (rather than a module) approach for connecting to the external world. For example, the cost of a DAA module is typically about $4. But, a 1200-bps modem doesn’t need the sophisticated DAC, compression and decompression, and other functions performed by a DAA module.

Instead, as you see from Figure 2, I used a configuration based on optoisolators and a transformer to provide coupling to the telephone network, which complies with the Bell-202 standard and makes the cost of the components significantly less than that of a DAA module.