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How to build your own ZX80/ZX81 (2019)
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CASSETTE INTERFACE Ear socket takes a signal, the capacitor blocks the DC content and the resistor limits the current when it is forced outside the normal TTL voltage levels. Six ICs, a small number of other components and a ZX81 8K ROM is all that is needed to convert the ZX80 into a fully functional ZX81 which supports both SLOW and FAST modes using the same timings and synchronisation as a real ZX81. Corrections to the foil for 1.1 can he seen here Changes for 1.2 Update to use round pads on the upper IC pins instead of oval to avoid touching nearby tracks, and some small graphic touch-ups.
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