Well, I'm cooking up this board at the moment. I have all the hardware figured out to run like the original CPU boards.
I need to do a little more testing of the hardware, then I'll proceed to building the boards.
The Memory, I/O, UARTs and SPI interface to the SD card appear to all be working.
8K chunks of memory can become either ROM, RAM or mapped off the board.
UARTS: 2x 8250
RAM: 2x 32K
I/O: 8 input, 6 output pins
SPI to SD Card interface
ALTAIR & IMSAI Front Panel Connectors.
Where ROM is selected in a block it overlaps the RAM on reads, but writes go through to RAM.
The ROM can then be disabled and reads will come from the RAM at that location.
The firmware is copying itself to RAM then switching off the ROM.
The CPU can be deselected with jumpers and the board will work as RAM/ROM and I/O for another CPU on the bus.
Pictures attached. Yellow-Data, Brown-Address, Blue-I/O, Green-Control/Status & everything else.