datasheets for 82S21 - Signetics 32x2 SRAM?

William Donzelli wrote: Concerning transistors of the 60s (even Germanium-type PNPs), I wouldn't have a problem to use modern types as replacements if the go or no-go of an old system would depend on it. Discrete electronic components are rarely a problem, even coils and transformers could be replaced. More difficult are certain kinds of integrated circuits. The mentioned 7480 is maybe hard to locate even as a datasheet (good idea to keep old TI TTL databook, IIRC also at bitsavers), and even more difficult to locate the chip itself, but this is a function that would easily fit into a GAL. A real problem are species what can't be simply replaced by some GAL, namely open collector components or "extendable" gates like the 7450. Replacing a single chip of such a kind usually means replacing its environment as well, e.g. that 7450 would then require to pull the extension gate (was it 7460, my memory is failing) as well. In the area of unobtainium, however, you usually don't find SSI or MSI circuits, but rather nasty special analog circuits and early LSI MOS circuits. Often a modern uC could emulate the latter, but this often needs significant tinkering for each problem, and an many cases also rewiring its environment. Surely the question original replacement vs. reengineered functionality has some religious aspect, but if it is the question of whether an old system should be left inoperable in a museum in contrast to actually switching its blinkinligths I favor reengineering. Regards Holger