Apologies for the radio silence to anybody still following progress on this project. The last few months I've been rather busy with other projects, and the progress on ZX Prism has mainly been on-paper; as per the previous post, I've redesigned ZX Prism to base it around a real Z80 instead of implementing the Z80 on the FPGA.
The design's been completed and I've started realising it in hardware. I'm using a cheap FPGA mini dev board which basically contains a Cyclone II FPGA, clock source, serial EPROM and breaks out all the IO to pin headers. They cost around $20 (about 10 quid). I chose this board as there's several vendors on eBay selling them cheaply and hopefully if all works out, that'll mean the project's more accessable for others.
The prototype so far - Z80 (top right), 29c040 512K Flash rom, 3 x 628512 (1 Meg RAM, 512K non volatile RAM), 2 x 61256 (64K video memory), AY 3 8912A sound chip, Cylone II FPGA dev board, 7 x 74245 level converters to convert 5v TTL signals to 3.3v as used by the FPGA and vice-versa.
Still a long way to go, but definitely getting somewhere!