OpenSpand is a single expansion board for the 1981 Sinclair ZX81 that replaces nearly every add-on the platform ever had. Built by adam.klotblixt and hosted on Codeberg, it runs on an RP2350 microcontroller powerful enough to disable and fully emulate the ZX81's original RAM and ROM while the Z80 CPU keeps running.
The board delivers configurable RAM across the full 64K address space minus ROM, SD card storage hooked into Sinclair BASIC's native LOAD and SAVE commands, high-resolution graphics, emulated sound chips, a joystick port, serial ports, selectable ROMs, and composite video output. The original machine shipped with 1K of RAM and nothing else. The only period-correct feature missing is a printer interface.
The full project is worth reading for how the RP2350 handles bus arbitration with a 45-year-old CPU, and how BASIC command hooks become a filesystem interface without modifying the host machine. The implementation details expose exactly how much overhead a Z80 leaves on the table.
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