Spectranext is built for the ZX Spectrum user port (48K, 128K “toastrack”, +2, +3). The cartridge has now been exercised on a +3 end-to-end; earlier revisions did not behave there even though 48K machines looked fine.

On the 48K edge connector, several user-port lines are unused or tied to VIDEO. On +3, the same physical lines drive on-board ROM chip selects: the cartridge’s ROM-disable outputs sit in that path. Both ROM CS lines are asserted together when the Spectranet ROM is paged in—electrically consistent, but the +3’s own ROM paging and reset timing are different from a rubber-key 48K.