I might need some help from smart people who Actually Know Electronics™ soon. I want to build new memory modules for the 3B2 computer, since you can't find them for love or money. Unfortunately, a reliable source for 5V asynchronous DRAM ICs is unobtainium now, so I'd like to design a replacement around SRAM. Latching the RAS and CAS lines with the right timings sounds trickier than anything I've done to this point. Help?
@twylo You could make a complex board with an onboard clock generator, but it probably isn't worth the effort.
Is there a reference for the timings seen on the 3B2? Do you have part numbers for the DRAMs it uses?
A DRAM-ish wrapper around an SRAM shouldn't be too difficult. The key is that you need latches, which are clocked by RAS and CAS: they are strobes, which are clocks by another name.
@EdS Thanks for these pointers! A colleague of mine has a board design using a 72 pin SIMM adapter. The major advantage there seems to be that 72 pin SIMMs are relatively cheap on the used market, and are basically a drop-in with no extra glue logic needed. A kludge, perhaps? But maybe the best idea.
@twylo how fast is the cycle time? The custom SDRAM controller in my SPI flash emulator cheats on a few of the timings to keep up with the tight requirements of the SPI read command, and might be reusable for your application.
@qrs Ooo, thanks, I'll take a look at your implementation. Is it documented online somewhere?
@twylo I had not heard of a 3B2 computer before. After some searching, I came across this http://www.hartetechnologies.com/manuals/3b2/AT%26T%203B2%20%28300-400%29%20RAM%20Expansion%20Manual.pdf
Are you creating your own plug in memory boards with more modern tech?
Full list of 3B2 manuals http://www.hartetechnologies.com/manuals/3b2/
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