ZX Interface 2

Posted on Feb 6, 2014

The circuitry of the ZXInterface2 is very simple and is comprised of a single custom made IC which performs address decoding for the twin joystick interface. The ROM cartridge socket simply exposes signals already available from the Spectrum`s edge connector. As can be seen from the P. C. B. photographs, at least two issues of the board were produce

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d. However, the Service Manual indicates that no modifications were made to the ZXInterface2 design and hence it is likely that only issue 2 versions were sold. The pinout of the ROM cartridge slot is shown below. Note that the contacts of the socket, and of each ROM cartridge, are gold plated to achieve a more reliable electrical connection. When a ROM cartridge is plugged in, it connects the /ROMCS (ROM Chip Select) line directly to the 5V line. This disables the Spectrum`s ROM thereby allowing the cartridge`s ROM to appear in the 0 - 16384 memory map area. The cartridge`s ROM must only output to the data bus when the lower 16K of memory is addressed. When an access to this memory area occurs, the address lines A14 and A15 and the /MREQ (Memory REQuest) line will all be at logic 0. These three lines are therefore used inside the cartridge to enable its ROM. Address lines A0 - A13 specify a location within the ROM and the data from that location is output on lines D0 - D7. Note that the /RD (ReaD) and /WR (WRite) signals exposed at the edge connector of the Spectrum are not available at the ROM cartridge slot. As a result, both read and write accesses to the ROM will be treated as a memory read. This is not a problem as, by its very nature, a ROM is a read-only device and hence would not be written to. This approach has the advantage that it reduces the size of the ROM cartridge socket but the main reason why it was...

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