DataModem

DataModem

Data (RS-232/Serial) to Audio Modem

Usage

Connect the DataModem to your computer's serial port (or USB-to-Serial adapter) and to an audio input/output device (like a cassette recorder or audio interface). Use terminal software to send and receive data through the modem.

Configuration

• Baud Rate: 300 baud (recommended for best reliability)
• Tone Frequency: Approximately 4kHz
• RV1: Adjusts output signal strength to the cassette recorder
• RV2: Adjusts the triggering sensitivity of the Schmitt trigger for playback

How It Works

The DataModem uses a simple one-tone encoding system where the presence of a tone represents a 1 bit and silence represents a 0 bit.

Recording Data: When transmitting, a 555 timer (U2) generates a ~4kHz square wave tone when the UART input is high. The tone is buffered and AC-coupled before being fed to the cassette recorder input. When the UART input is low, the 555 is disabled and produces no tone.

Playing Back Data: During playback, a Schmitt trigger detects the presence of the recorded tone. A retriggerable 555 monostable circuit (U3) converts the rapid tone pulses into a steady output signal. The monostable stays on for 300μs when triggered, and continuous triggers from the 4kHz tone keep it active, producing a clean digital output that matches the original UART signal.

The circuit is designed for 300 baud operation, providing excellent reliability with minimal bit loss when properly calibrated.

Licence

This project is licensed under the CERN Open Hardware Licence Version 2 - Permissive License - see the LICENSE file for details.

Credits

This project is inspired by and based on the following works:

• Datacorder by Mitchell Electronics
• UART Cassette Tape Interface by Robin Mitchell