dionb wrote on 2020-03-07, 15:28:
Whoa there! Maybe it's different where you are located, but here in EU I find it more difficult/expensive to find 32MB FP 72p SIMMs than 4MB 30p SIMMs, in fact I was able to find 16MB 30p SIMMs for less than 32MB FP 72p.
They're not common here either, but I've got at least 4 sticks spare at the moment. No need to panic though, I'm only planning on desoldering the chips for the purpose of testing the design. Regardless of whether it works, or if I blow up the chips trying, I'll be ordering chips to replace them, and probably a bunch of extras in case I need any more in the future.
xtgold wrote on 2020-03-07, 16:52:The original xt pc had all it's memory on a multifunction board.
I was thinking about that approach,an isa or eisa card with ext […]
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The original xt pc had all it's memory on a multifunction board.
I was thinking about that approach,an isa or eisa card with extra memory.
I once had an eisa prototype board from radio shack, but both are long gone.
A pic of a 286 mainboard with no simm sockets,all memory was discrete chips.
Try to troubleshoot memory problems with those !
The only way to add more memory is with a board, which probably never existed.
The missing chips are 20pin shadow ram,probably why the board doesn't boot.
IBM and other clone makers did make ISA memory expansion cards which were designed for those sorts of systems. I have a couple of these cards from AST. One takes up to 2MB chip memory, and can be configured to map that pretty much anywhere, base/expanded/extended memory areas (or some combination). The other takes 30 pin SIMMs, but I'm not sure if it can only be used for extended memory or base/expanded memory too.
Unfortunately I haven't been able to test these because the 286 motherboard I've got isn't working. Possibly because there was a faulty power cable connected to the monitor I was using to test it, which was capacitively coupling around 120V to the ground of the monitor, which was connected to the motherboard via the VGA cable and card. I originally wanted the SIMMs for a 386 motherboard, but I only found the faulty power cable last weekend, so it's probably also dead. I did also buy a 286 clone machine a while back, but that went straight into storage since I ran out of room in my tiny apartment. I haven't fully tested both those boards though, so there may be an incorrect jumper or something I missed. In any case, I can always just take a bunch of stuff to my storage space (which I need to do anyway), that should free up enough space to bring back the 286 machine so I can test the memory using the AST ISA card.
In any case, I'm still not sure about the 2K vs 4K refresh thing, so if anyone has any knowledge or experience they can share about this, it would be greatly appreciated.