Vintage computing enthusiast Anders Nielsen has released design files for a "Simple Universal Modem," designed to allow data to be saved from almost any computing device to almost any medium capable of recording audio — and then loaded back into the computer again.
"This little board has everything you need to save data from a retrocomputer, a PC, a microcontroller, or just about anything else you can think of," Nielsen explains of the project. "On the other side of things it'll save data to pretty much any media that can record and play audio."
Measuring just 50×15mm (around 1.97×0.59") and using a minimum of components, the Simple Universal Modem features a five-stage resistor-capacitor (RC) filter and emitter-follower buffer transistor, which takes the square wave of a digital computer's binary logic and modulates it to a sine-like wave ready for recording. Once recorded, the data can be played back and loaded into the original source device — or any other.
"Caveats include that you still have to generate a square wave and not just a serial signal," Nielsen admits, "but the positive side to that is that you are less locked in to a certain baud rate (considering the TX [Transmit] RC filter can also be changed with new values), adding to the universality of the board."
As well as being able to save and load data of your own, Nielsen suggests that modem could prove handy for those performing a little digital archaeology or data recovery. "If you have some old tapes lying around with invaluable data from the 70s or 80s," he explains, "the Simple Universal Modem, will make it super easy to read the data with a modern MCU [Microcontroller], Arduino, [or] Raspberry Pi, since it has an encoding agnostic demodulation interface."
While the modem is designed to be universally applicable to almost any device capable of loading software, Nielsen built it primarily for use with his ABN6502 single-board computer (SBC) — the latest revision of which we looked at two months ago.
Full details are available on the project's Hackaday.io page, with Gerbers, KiCad project files, and schematics available on GitHub under the Creative Commons Attribution-NonCommercial 4.0 International license.