SVI-728 Keyboard Reset Mod

The Spectravideo SVI-728 is an MSX1 computer with no reset button. This mod adds one to the keyboard: hold GRAPH + CODE + ESC for about a third of a second and the whole machine resets, exactly like at power-on. The cartridge slot resets too, so a cartridge such as a Carnivore2 starts over with the machine.

An ATtiny85 reads the key combination from the machine's own keyboard scan. When the running software does not scan the rows it needs, the ATtiny strobes those rows itself. The combination therefore works whatever the software does with the keyboard: a game with its own scan routine, a program that never reads the keyboard, or even a machine that has crashed with interrupts off. No MSX software is modified and the keyboard works normally.

To turn the mod off without opening the machine, hold ESC alone for about ten seconds while the machine is running normally, for example at the BASIC prompt. The machine resets once and then behaves as unmodified until the next power-on.

Building it

The mod is nine through-hole parts: the ATtiny85 in a socket, one capacitor and seven resistors, plus seven thin wires to solder points inside the machine. You can order the small two-layer board from any PCB house with the ready Gerber files, or build the circuit on a scrap of perfboard. The ATtiny is programmed with an Arduino as the programmer before it goes in the socket.

Get it

The firmware, the KiCad project with Gerbers, and the full build and installation guide are on GitHub: github.com/arttu76/svi728keyboardreset.