ZX Spectrum +2A/2B, +3/3B edge connector: Difference between revisions

{{Template:Spectrum edge connector|A15|A13|D7|[[#ROM disable pins|NOE]]|[[#Key_Slot|SLOT]]|D0|D1|D2|D6|D5|D3|D4|NINT|NNMI|NHALT|NMREQ|NIORQ|NRD|NWR|[[#Power|NC]]|NWAIT|[[#Power|+12{{smallcaps|V}}]]|[[#Power|−12{{smallcaps|V}}]]|NM1|NRFS|A8|A10|[[#RESET|RESET]]|A14|A12|[[#Power|+5{{smallcaps|V}}]]|[[#Power|NC]]|[[#Key_Slot|SLOT]]|0{{smallcaps|V}}|0{{smallcaps|V}}|[[#CPU_Clock|CKEXT]]|A0|A1|A2|A3|NC|0{{smallcaps|V}}|[[#ROM disable pins|NOE]]|[[#Disc Controller Signals|NDRD]]|[[#Disc Controller Signals|NDWR]]|[[#Disc Controller Signals|NMTR]]|NBUSRQ|NRESET|A7|A6|A5|A4|[[#ROM disable pins|NC]]|NBUSACK|A9|A11}}

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 !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 !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]].

The RESET signal is an inverted version of the NRST signal, and is produced by one of the inverters in IC15. The purpose of this signal is unclear since any peripheral could derive it by inverting the NRST line iself. In the +3 it is connected via a 10k resistor (R76) to the floppy drive select line. This however seems to be an error in the circuit design though it has no adverse effects.