Hardware · ZXS 128k modification

Universal memory expansion for ZX Spectrum 16/48k (all issues) and Didaktik Gama (all revisions), designed by Pavel Cimbal (aka Zilog). This is third revision. It's fully compatible with original 128k, even 2nd video RAM is working as expected. There are also FASTIO (ports speed up) and UNRAIN (solves ULA raining bug) options - you can enable them with switch.

The expansion uses undocumented behavior of 48k ULA - when CPU accesses the RAM, /MREQ is active, but during video access ULA stops the CPU clock, and /MREQ is inactive. Logic in GAL prepares the combinations of A14 and A15 for ULA, and checks the paging register. Then it forces pages 2 or 5 to the right place, depending on state of paging register, A14 and/or A15.

Part list:

Phase 1:

Phase 2:

WARNING! If you are modifying ZX Spectrum Issue2, Issue3 or Issue 4s board, change C64 from 100pF to 180pF, otherwise you will end up with memory errors as with the 100pF the /CAS signal is active too early and the addresses are not stable.