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XT clone overlock -- where can I find 50 ns 18-pin 256K DRAM?

Mike Chambers

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Sep 2, 2006
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EDIT: Mistake in the topic and can't edit, it's 128 KB / 1 Mbit.

In my absurd quest for a 20 MHz overclocked XT clone, my 70 ns RAM is holding me back and I'm stuck at 15 MHz! Going to 16 MHz makes goofy things happen, and beyond that it won't POST. I have a 16 MHZ rated V20 already so it's not the CPU. I had the same issues when I was using a 10 MHz V20, the only difference is it got hot when it was doing it.

Does anyone know if a 50 ns or faster DRAM chip is available that's compatible with this? https://www.datasheets360.com/pdf/6019545828535065751

I've been unable to find anything so far. Hopefully I won't need to make some kind of custom adapter to use a more modern component.
 
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Thanks, I'll get those and see how far they can be pushed. Maybe I can hit 17-18 MHz at best. I might eventually try making an adapter to use some fast modern SRAM.
 
Does this clone you're trying to run this fast automatically clock to a lower speed when you hit the ISA bus? I can imagine a lot of peripherals could be a limiting factor.
 
Wouldn't SRAM just ignore the refresh? I know there are some modern memory cards that are based on a common Alliance 512KB SRAM, and they presumably drop in just fine even though the mainboard was designed for DRAM.

In the worst case, isn't it usually done by using one of the DMA controller channels, so you could deactivate it in software with a few port pokes.
 
If there’s a straightforward way to configure the motherboard to disable its built-in RAM circuitry entirely in favor of memory plugged into the expansion bus then installing a SRAM card like the lo-tech 1MB should give you 55ns RAM. (Or faster, I’m not sure what the fastest grade is for those Alliance chips off the top of my head. You can easily get 15ns or faster in smaller sizes.) I still wonder if this board automatically slows down when hitting the expansion bus or not. Back in the day I used to play with using smaller dividers for the ISA clock on 486-era motherboards, and in my experience most 90’s era ISA cards could handle 11mhz vs. the standard 8mhz, but faster was usually asking for trouble. An ISA video card or whatnot trying to run at 20mhz sounds like a pretty tall order.

Problem with trying to replace the motherboard RAM with SRAM directly is of course the dynamic RAMs have a multiplexed address bus which you’d have to de-multiplex again. Guess that wouldn’t be the end of the world, you could do it with a few latches. Using surface mount packages you might be able to make effectively 20ns-ish 256k SIMMs comparable in size to the original DRAM ones.
 
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