ROM select circuit for ZX Spectrum
My intention with this circuit was to make possible switching diverse ROM's in two ways:
1: With button press - when you are in some program e.g. game.
2: With OUT command from software/command line. With OUT 169,n (n is 0 to 3) the desired ROM will be selected.
Lines EA14 & EA15 lead to
EPROM's address lines A14 & A15. The EPROM type is 27512 -
where it is possible to put 4 versions of Spectrum ROM.
After power on, always ROM section #3 will be selected, so put there your default version.
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