Some years ago, I read from a MSX project to use SIMM modules to replace/extend the RAM in these machines. Some posts on the speccy.org forum about damaged Spectrums led me to reconsider that hack.

A SIMM module is a 8-bit DRAM in a easy to plug package. I don't know a single IC 8-bit DRAM chip. SIMM's have two, or even eight DRAM chips, being the former 4-bit DRAM's, and the later, 1-bit DRAM's.

A SIMM has the very same signals that you can find in a 8-bit DRAM IC (if such memory exists): RAS, CAS, WE, ROW/COLUMN lines, and bi-directional data lines. Lower RAM in the ZX Spectrum are configured the same way. Sinclair used eight IC's to have an 8-bit data bus. RAS,CAS, WE, and ROW/COL lines are shared among the eight chips. Only data bits are routed directly from each chip to the corresponding data bus line.

Interestingly enough is that, although data buses are bidirectional, 4116 DRAMs feature a separate data bus for reading and for writting. To the date, I know of only one chip that takes advantage of this configuration: the TMS9918A/9929 video controller chip. The Spectrum design simply ties data-in and data-out pins on each memory IC, so the resulting pin is a bi-directional data pin.

SIMMs modules are not common today, but at least, they are easier to find than 4116 chips, and, above all, they don't suffer the inconveniences of the former ones.

This diagram depicts the correspondence between pads at a SIMM module and the corresponding solder points on the ZX Spectrum PCB. It's like soldering a modchip! :D