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ZX Spectrum edge connector: Difference between revisions
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→ROM disable pins
(Created page with "Category:Edge Connectors Category:Hardware Category:Spectrum <!-- end of category list --> {{Template:Spectrum edge connector|A15|A13|D7|[[#ROM disable pins|{{overlin...") |
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The Spectrum 16/48k 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 Spectrum 16/48k 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 {{overline|OE}} signal routed to the expansion port. These two {{overline|OE}} 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|16/48k]]. Lower pin 25 ({{overline|ROMCS}}) is not connected on the +3/+2A/+2B and | The +3 and its derivatives however have two physical ROM chips and each have their {{overline|OE}} signal routed to the expansion port. These two {{overline|OE}} 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|16/48k]]. Lower pin 25 ({{overline|ROMCS}}) is not connected on the +3/+2A/+2B and lower pin 15 is unused on the 128k. | ||
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 16/48k. | 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 16/48k. | ||
Applying ROM paging signals to the 16/48k composite out pin does no permanent damage, but causes burst of patterning on the display. | Applying ROM paging signals to the 16/48k composite out pin does no permanent damage, but causes burst of patterning on the display. | ||
