Being nostalgic for the birth of laptop computers we recently acquired an Olivetti M10. The M10, like many of the KC-85 derivatives like the Tandy TRS-80 Model 100 and the NEC PC8201A, uses custom modules for their system RAM. These modules contained four 2Kx8 SMD SRAMs mounted on a ceramic substrate with a non standard DIP footprint. Needless to say, these modules have become increasingly hard to find. M10MEM is a modern replacement for the memory modules using a single 8Kx8 SRAM.
The schematics is quite simple. The four 2KBytes memories were replaced by a single 8KBytes SRAM. Most signals are passed to the memory unaffected. Extensive signal swapping has been performed to simplify the layout.
The only active part of the adapter is the encoding of the 4 chip selects into a single chip select and a 2-bit address. Since the CS1-CS4 are the output of a 2-to-4 decoder only there are only 5 valid rows in the truth table. The main change from the older A00 version is the use of CS2 instead of CS1# for the RAM chip select to keep CS1# free to use with the system RAMRST. This signal is used to put the RAM in low power mode during system sleep.
The logic can be implemented using four 2-input XOR gates contained in a single 74HC86 IC.