OpenSpand Is a Feature-Packed Expansion Board for Your Sinclair ZX81, Timex Sinclair 1000, or Clone
Sound support, joystick compatibility, high-res graphics, and up to 56kB of RAM — where was this in the 1980s?
Maker and vintage computing enthusiast Adam Klotblixt has designed a Raspberry Pi RP2350-powered add-on for Sinclair's classic ZX81 microcomputer — delivering additional RAM, SD card storage, high-resolution graphics, sound output, joystick connectivity, and more: the OpenSpand.
"The OpenSpand is an all-in-one expansion solution for Sinclair ZX81 computers and clones. Functionally very similar to ZXpand+, with more and extended functions," Klotblixt explains. "Since the hardware design and software are open source, everything can be changed and expanded by others, making this a future-proof ZX81 expansion. It is relatively cheap and easy to build. Others will hopefully find it easy to expand the functionality."
Sinclair's ZX81, also known as the Timex Sinclair 1000, launched in 1981 as an ultra-affordable home computer in kit or fully assembled forms. Built around the Zilog Z80 processor and featuring a copy of BASIC on ROM, the machine used a Ferranti uncommitted logic array (ULA) to dramatically cut down on the number of chips required in comparison to its predecessor the ZX80. With just 1KB of RAM, though, it was definitely built to a budget — which made RAM expansion cartridges a must-have for anything but the most simple of uses.
Today, modern equivalents of these expansion cartridges add not only more RAM but a wealth of other features — and the OpenSpand is no exception. Powered by the Raspberry Pi RP2350B microcontroller, the board includes a choice of 1kB to 56kB of additional RAM, SD card storage, pseudo-high resolution and true-high resolution graphics, user-defined characters, emulated sound chip support, stereo sampled audio capabilities, a joystick connector, serial interface, hardware reset, a battery-backed real-time clock (RTC), and even the ability to switch between different ROM versions.
If that wasn't enough of a feature list, the expansion cartridge also includes a composite video output with backporch fix — designed to work around an issue with the original hardware's video output, which works fine with contemporaneous TV sets and VCRs of the early '80s but is near-impossible for modern displays to lock on to.
More information on the project is available along with design files and software source code, under the Creative Commons Attribution ShareAlike 4.0 International license and the GNU General Public License 3 respectively, on the OpenSpand Codeberg repository. Additional details can be found on Klotblixt's Hackaday.io page.