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ZX Spectrum edge connector

14 bytes added, 10:04, 9 May 2017
ROM disable pins
The Spectrum 16K/48K and Spectrum+ 128K both provide {{overline|ROMCS}} on lower pin 25. By holding this pin high an external peripheral can prevent the Spectrum's ROM from driving the data bus, and place its own ROM or RAM within the first 16K of the 64K memory space.
The +3 and its derivatives however have two physical ROM chips and each have their output enable routed to the expansion port. These two {{overline|OE}} (Output Enable) pins are upper pin 4, and lower pin 15. Upper pin 4 was not connected on any of the earlier models, but lower pin 15 was used for composite video out on the [[ZX Spectrum 16K/48K edge connector#Video Outputs|16K/48K]]. Lower pin 25 ({{overline|ROMCS}}) is not connected on the +3/+2A/+2B and lower pin 15 is unused on the 128KSpectrum 128 or +2.
This means that when designing an interface which will page external memory in place of the Spectrum ROM a configuration jumper or similar is required to connect the ROM disable signal to lower pin 15 on a +3, but disconnect it on a 16K/48K.

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