John Calhoun's Adam74 Is a Teensy-Powered Text-Based Terminal for the Modern Age

Engineer and retro computing enthusiast John Calhoun has put together a modern take on the text-based terminal, inspired by the Lear Siegler ADM-3A and the Apple I "ASCII bus:" the Adam74.

"Something that struck me reading about the history of the Apple I computer was the fairly simple 'ASCII bus' that it used for input and output," Calhoun explains. "As I understand it a hobbyist back in the day could get an ASCII keyboard that quite literally presented as an interface a header giving 7-bits of ASCII and an additional strobe pin that indicated that a key had been pressed. In short, ASCII comes in from a keyboard, ASCII goes out to the TV buffer."

Calhoun fancied recreating something similar using rather more components, with a view to building the display portion of a text-based terminal like the Lear Siegler ADM-3A — naming it the Adam74 in homage to the device and the era in which it would have been used. "The Adam74 is a little project that tries to act as the latter half of the above 'ASCII bus,'" Calhoun explains. "It is in short a little terminal that a hobbyist might enjoy using for a small eight-bit computer they’re experimenting with."

Built around a Teensy 4.0 board connected to an ILI9341-based 320×240 LCD panel and level-shifter, the Adam74 acts as a 40-column 24-line display — but accepts ASCII text, rather than bitmap graphics, automatically handling issues like buffering, cursor handling, scrolling, and line-wrap. There's even a buzzer, capable of responding to a terminal bell control code — just like a traditional terminal would.

"Initially my Adam74 had a small LED to indicate the power was on but I ultimately removed it because I feel there are just too many LEDs on devices that indicate the power is on. Besides, you hear the beep sound when the Adam74 boots up and you should have a blinking cursor prompt after," Calhoun adds.

"So that you could interface the Adam74 across a wider voltage range, I added a level-shifter chip. Also, once I accidentally swapped the power and ground wires and let out the magic smoke from the LCD panel. After that mishap I added a small diode to hopefully protect the display should it happen again."

More details on the project are available on Calhoun's website, Engineers Need Art, while the source code, PCB production files, and a template for an optional laser-cut acrylic stand have been published to GitHub under an unspecified open license.