The Small Computer Central SC126 is a lovely little Z180 machine but the design slightly screwed up the SPI interfaces because it has two independent chip selects, and you can set them both low at once, or indeed at boot both start low.
Thus I designed a little board to use the two chip selects as a single four way select. The first version was just a 138 I had handy as a mux. This second version also copes with the fact that there are a lot of crap SD adapters and the like out there and adds a 153 to switch the MISO line to follow the chip select.
In theory it wouldn't be hard to also steal one of the 8 led indicators as a third CS bit but I didn't need that many devices.
I really should have added a 3v3 regulator and 3v3 pins to the breakout. A lot of SPI stuff is 5v tolerant/3v3 supply and it would have been a sensible place to put it. I guess a new version is needed.
Version one worked but I hadn't realised how much crappy SPI stuff there is that doesn't properly get off the MISO line when not selected.
This version seems to work although I managed to end up with two unconnected power nets and some upside down chip labels. With those fixed and tested with wires I've uploaded the corrected Kicad schematic and PCB files but not yet tested the changes.
The theory is fairly simple. The two original CS lines go into both the 74HCT138 and the 74HCT153. The 74HCT138 in turn sets one of the four CS lines low. Ports 1 and 2 correspond to the original intended ports, whilst port 0 and 3 are extras. There is no 'not connected' so you need a device that doesn't mind being used as a 'parking' chip select on Port 3 (or leave it empty).
The 74HCT153 handles the MISO line the same way but in reverse, it multiplexes the four MISO lines from the devices onto the MISO line to the mainboard.