First post, by cosam
So I took a punt on some cheap eBay cache chips, fully expecting they might not be up to spec. Of the ten, nine checked out OK in a simple USB programmer/memory tester, which agrees with the commonly quoted 10% DOA rate but also confirms they are at least RAM of some description. They however don't want to work in a 486 motherboard so I suspected they might not quite have the 15ns access time they're labeled with. They don't look glaringly fake but they do have a bit of a texture to them, possibly suggesting the top of the package has been skimmed off. If 1901 is a date code that would seem pretty late for this chip in a DIP package, or not?
In an attempt to measure the access time I hooked up a test rig and scope. Top trace is /OE and the bottom one is a high bit on the data bus. To be honest I'm not sure what to make of the results although they are hard to distinguish from those of a 55ns SRAM I had lying around. The input isn't very square but you do see the output starting to react pretty immediately when /OE drops, although it does take a while to get to something you might deem as stable.
Unfortunately I don't have any known good 15ns SRAM to compare this with and I'm not brimming with confidence that this test is even accurate enough to mean anything useful. Anyone know if this either confirms or refutes my hypothesis that these chips are on the slow side?
Edit: added photo of underside of chip.