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

The [[ZX Spectrum +3/2A/2B|ZX Spectrum +2A/+3 and +2B]] [[:category:Edge Connectors|expansion connector]] is a double sided card edge connector with a 0.1 inch spacing. The two rows of conductors are numbered from right to left looking into the rear of the computer.

The CKEXT signal is available on Lower Pin 8. The CPU clock signal is generated by the [[The Amstrad gate array|gate array]] (IC1) and is interrupted during [[contended memory]] access. This clock signal is fed into the Z80 via a series resistor. The CKEXT signal is inverted in relation to the CPU clock as it has been passed through a NOT gate.

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 {{overline|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 {{overline|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]].

Unlike the +3, the +2A and +2B have no floppy disc controller. Amstrad's original intention was to produce an external floppy controller addon which would have connected to the expansion port on these computers. Since the [[The Amstrad ASIC|ASIC]] is the same on all three machines, all the decoding logic is already present to generate the disk read/write and motor control signals. These three signals are therefore connected through to the expansion port.