Michael Wessel Turns a Raspberry Pi Pico Into an Add-On for the Four-Bit Busch 2090 Microtronic SBC

Developer and vintage computing enthusiast Michael Wessel has turned a $4 Raspberry Pi Pico into a high-speed memory expansion for his Busch 2090 Microtronic Computer System, a four-bit single-board computer released by a model-maker back in 1981.

"Wouldn't it be nice if I could load and save programs, or even better — full memory dumps! — into my beloved Microtronic four-bit computer system from 1981 in a fraction of a second rather than in minutes with the cassette interface," Wessel asks, rhetorically, in support of his creation. "This project aims at realizing a RAM-image based mass storage solution, and maybe some other extensions for the original Microtronic along the way (display, speech, and a banked memory expansion)."

Released in 1981, the Busch 2090 Microtronic Computer System is unusual for reasons beyond its four-bit word length: it comes from Busch Modellbau, a German company founded in 1955 as a fireworks manufacturer before pivoting to plastic model kits and model railway accessories. In 1976, the company took its toy division into the educational market with the launch of electronics experimentation kits — and in 1891 the Microtronic was released as the logical next step in the range.

The machine was based on a Texas Instruments TMS1000-series four-bit processor running at a sedate 500kHz and with just 576 bytes of RAM and 4kB of ROM. Programs were loaded and saved using an optional cassette tape system, the Busch 2095, which plods along at 14 baud — which is why Wessel was so keen to speed things up. The solution: the Raspberry Pi Pico, which with two Arm Cortex-M0+ cores running at 133MHz, 264kB of RAM, and 2MB of flash storage is considerably more powerful than the Microtronic.

"This emulates the 2114 SRAM [Static RAM] found in the Busch 2090 Microtronic Computer System — it's a fully compatible 'drop in' replacement," Wessel explains of his creation, which connects the 3.3V RP2040 microcontroller on the Raspberry Pi Pico directly to the 5V Microtronic. "I thought [the breakout board] already had level converters on board, but this turns out to be not the case," the maker explains. "So you should still add level converters; this setup is probably not entirely safe and might damage your Pico in the long run."

Wessel's project is already functional as an SRAM replacement, but he has grander visions too: the ability to save and load SRAM contents to and from the Raspberry Pi Pico's flash memory or a microSD card, to make it easier and considerably faster to load and save programs; and to add bank switching to the Microtronic, expanding its available program memory fifteenfold.

The full project write-up, including wiring instructions and source code, is available on Wessel's GitHub repository; information on the Microtronic itself, including the original documentation in German, can be found on the Busch website.