To take the Sideways RAM cartridges to their final design (I shouldn't say final as the RAM size can grow quite considerably) I have added battery backup to the cartridges. On v3.5 (upgrade to v3) only U1 is battery backed, slot 0/1 or 2/3 or 4/5.

PCB layouts.

Boards assembled and under test.

On first power on both boards did not show the SRAM present but when I put my logic probe on pin 6 of the DS1210 (CE) they kicked into life and have been stable through loading images, power cycling and unplugging & inserting the cartridges. I am wondering if this effect is something all DS1210 exibit or just the batch I have. These chips I bought a few years ago so I have ordered some 'original Dallas' stock to compare.

Second board built with the 'new old stock' DS1210 and it worked first time so it's worth paying a little extra for the original part.

Battery Backed RAM cartridge with three ROM images loaded, banks 4, 5 and 13.

If you would like to build one for yourself, everything you need including the Gerbers, BOM and Circuit Diagram are here - v3.5 Electron 2+1x16K Battery Backed SRAM and v2.5 Electron 2x16K Battery Backed SRAM.