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ZX Spectrum 128: Difference between revisions
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Due to a bug either in the 128's HAL10H8 chip or in the PCB, memory banks 1, 3, 5 and 7 are contended (and the rest uncontended) as opposed to 4, 5, 6 and 7 as documented in the service manual. Whereas the +2 (with grey case) shares this behaviour, the +2A and +3 apply contention to pages 4–7 instead. The paging scheme documented in the manual would have been implemented as was (presumably) originally intended had the B0 and B2 inputs to the HAL10H8 been reversed, either on the Spectrum 128 PCB or in the logic of the HAL itself. | Due to a bug either in the 128's HAL10H8 chip or in the PCB, memory banks 1, 3, 5 and 7 are contended (and the rest uncontended) as opposed to 4, 5, 6 and 7 as documented in the service manual. Whereas the +2 (with grey case) shares this behaviour, the +2A and +3 apply contention to pages 4–7 instead. The paging scheme documented in the manual would have been implemented as was (presumably) originally intended had the B0 and B2 inputs to the HAL10H8 been reversed, either on the Spectrum 128 PCB or in the logic of the HAL itself. | ||
Later grey +2s were shipped with an updated HAL chip which corrects there issue whereby reads of port 0x7ffd would crash the machine[ | Later grey +2s were shipped with an updated HAL chip which corrects there issue whereby reads of port 0x7ffd would crash the machine[https://www.worldofspectrum.org/forums/discussion/35523]. It is not known whether this also updated the contention scheme to contend on pages 4–7. | ||
The Spectrum 128 suffers from a "rain" effect when the Z80's I register is set to a memory address within the region of 0x4000–0x7fff. | The Spectrum 128 suffers from a "rain" effect when the Z80's I register is set to a memory address within the region of 0x4000–0x7fff. [http://velesoft.speccy.cz/zx/umbrella/umbrella.htm Velesoft's "Umbrella" GAL] corrects this rain effect as well as the crash that results when reading from port 0x7ffd, although it does not change the set of pages that are contended. For the rain effect to be corrected, a flying lead must be connected between the Z80's {{overline|RFSH}} signal and the GAL. | ||
== Keypad == | == Keypad == | ||
The keypad released for Investrónica's Spanish 128 has been found to be incompatible with at least some UK machines. Issue 6K boards provide the wrong voltage to the keypad. This is fixed in Issue 6U by replacing R137. | The keypad released for Investrónica's Spanish 128 has been found to be incompatible with at least some UK machines. Issue 6K boards provide the wrong voltage to the keypad. This is fixed in Issue 6U by replacing R137. | ||