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ZX Spectrum +2A/2B, +3/3B

2,188 bytes added, 15:44, 14 July 2014
lots of stuff about motherboards
* Cassette interface daughterboard.
* "Datacorder" cassette mechanism.
==Models and Motherboards==
When designing the new Spectrum models Amstrad made the decision to produce a single common motherboard for both the +2A and +3 (the Z70830 motherboard). This board was designed in such a way that by omitting certain components they could be assembled as either a +2A or a +3 computer. There are two versions of the Z70830 board (ISSUE 1 & 2, both ©1987).
The three disc controller signals generated by the gate array are placed on the expansion port so that an external floppy disc controller could be connected to a +2A to provide the functionality of the omitted internal components. Amstrad planned to release this as the ''SI-1'' but it never went on sale.
=====+2B & +3B=====
The +2A/+3 circuit contains a design flaw in the audio output circuitry which causes serious distortion to AY-3-8912 music. A new design was developed in 1988 with redesigned audio circuits and manufactured as the +2B and +3B. Unlike the +2A/+3, Amstrad did not create one common motherboard for these computers. Instead the +2B board is stepped on the right hand side to allow two boards to be panellized side by side and use a much smaller area (hence cheaper to produce) than two +2A/+3 boards.
The identification of models is regularly confused because Amstrad sold large numbers of computers with a Z70833 motherboard (+2B) in cases that were moulded for the +2A. The change to shipping +2B motherboards instead of +2A boards appears to have happened fairly early on because most +2A cases encountered contain a +2B motherboard. Z70830 motherboards populated as a +2A are consequently quite rare.
The +2B motherboard is the Z70833 ISSUE 1 & 2 ©1988. There is also an ISSUE 4 ©1990 which has a different FM modulator circuit for the audio (no example of an ISSUE 3 board has surfaced so it's possible none were ever produced/sold).
The +3B motherboard is the Z70835 ISSUE 1 ©1988. Unlike the Z70830 it has no provision for connecting a datacorder but the connections for an external floppy controller remain on the expansion port. Unlike the +2A and +2B all +3 cases are the same. i.e there is no +3B case moulding.
==Timings and Contention==
The contention and ram timings on the +3 and Black +2 differ significantly from earlier models due to the redesigned [[The Amstrad ASIC|gate array]]. Timing patterns and memory contention are described in detail in the [[Contended_memory#.2B2A_.2F_.2B3|contended memory]] article.

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