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====ROM disable pins==== | ====ROM disable pins==== | ||
All the previous models of ZX Spectrum have a single ROM chip which could be disabled to facilitate paging in external memory by pulling the {{overline|ROMCS}} line high. The +2A/+3 and +3B however have two ROM chips and brings them out to independent pins on the expansion port. The old {{overline|ROMCS}} pin (Lower pin 25) is not used, and instead Upper pin 4 and Lower pin 15 are used. These pins were both unused on the [[ZX Spectrum 128K edge connector|128K]], however Lower pin 15 was used for composite video out on the [[ZX Spectrum | All the previous models of ZX Spectrum have a single ROM chip which could be disabled to facilitate paging in external memory by pulling the {{overline|ROMCS}} line high. The +2A/+3 and +3B however have two ROM chips and brings them out to independent pins on the expansion port. The old {{overline|ROMCS}} pin (Lower pin 25) is not used, and instead Upper pin 4 and Lower pin 15 are used. These pins were both unused on the [[ZX Spectrum+ 128K edge connector|128K]], however Lower pin 15 was used for composite video out on the [[ZX Spectrum 16K/48K edge connector|16K/48K]]. | ||
====Disc Controller Signals==== | ====Disc Controller Signals==== |