VOGONS


Let's make some memories (SIMMs)

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First post, by UpLateGeek

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Larger capacity 30 pin SIMMs (e.g. 4MB) are pretty hard to come by where I live, so I've decided to make my own!

I've thrown together the following schematic based on datasheets I've found with this great new search engine called Google.

The attachment 30P SIMM.png is no longer available

There are various models of allegedly new old stock DRAM chips available on the eBay; I'm basing the design on two 4M x 4 chips, plus an optional 4M x 1 chip for parity, simply because it's going to be cheaper and easier to assemble than eight or nine 4M x 1 chips.

Anyway, feel free to check out my schematic and let me know what I've got wrong! (:

Reply 2 of 29, by eisapc

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Anybody has a PCB Layout for 4 or 16 MB 30 pin Simms at Hand?
The question recently popped up in the german VzEkC board.

Reply 3 of 29, by UpLateGeek

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Tiido wrote on 2020-03-05, 10:12:

This should work fine. I recently made my own 4MB SIMMs too

Awesome! Thanks for the feedback!

eisapc wrote on 2020-03-05, 11:01:

Anybody has a PCB Layout for 4 or 16 MB 30 pin Simms at Hand?
The question recently popped up in the german VzEkC board.

Eagle has a blank 30 pin SIMM part in the con-amp library named "SIMBRD30". Should be able to get it routed in a few hours (or a few minutes using the autorouter) over the weekend, but I wouldn't go handing out a PCB design without having assembled and tested it myself. (In other words it'd be a lot quicker for them to copy my schematic and press autoroute.)

Reply 4 of 29, by UpLateGeek

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Just a quick update, the boards have been routed, and an order placed with a certain popular Chinese PCB manufacturer for the first 5 prototypes (technically I only need 2 or 4, but the minimum order quantity is 5).

I've got some old 32MB 72 pin FPM SIMMs with 4M x 4 chips that I can sacrifice for the purpose of testing the design. I've also got some 4MB 30 pin SIMMs with parity that use 4M x 1 chips, so I can safely steal one chip to test if the parity bit works.

However the only working machines I've got that use 30 pin SIMMs are 68K Macs, which I'm pretty sure don't use the parity bits, so I might have to find someone local who could test them properly for me.

Reply 5 of 29, by UpLateGeek

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Ok, so I've been looking for stock of suitable chips, and going by the below datasheet, my SIMMs will only be compatible with 2K refresh chips, not 4K refresh ones. However, what I'm not sure about is whether that means they'll be more or less compatible with different machines or chipsets. 4K refresh chips seem to be more commonly available, so I have no idea if I should be making 4K or 2K refresh SIMMs!

The attachment MCM517400C.pdf is no longer available

Reply 6 of 29, by keropi

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yes is it a problem in EU region to get good priced 4MB 30pin simms
I just got some from Tiido for my needs , his price is really good.
Not trying to discourage anyone though! I expect to see pics of the new modules 😀

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Reply 7 of 29, by dionb

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UpLateGeek wrote on 2020-03-07, 04:00:

[...]

I've got some old 32MB 72 pin FPM SIMMs with 4M x 4 chips that I can sacrifice for the purpose of testing the design. I've also got some 4MB 30 pin SIMMs with parity that use 4M x 1 chips, so I can safely steal one chip to test if the parity bit works.

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.

Reply 8 of 29, by xtgold

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1 meg simm kit from the 1980's
I would strip the 1meg x1 dram chips from defective 286 motherboards

Reply 9 of 29, by xtgold

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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.

Reply 10 of 29, by UpLateGeek

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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.

Reply 11 of 29, by dionb

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UpLateGeek wrote on 2020-03-10, 13:34:

[...]

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.

Murphy's law in reverse: just after I typed that reply somebody turned up on a local site offloading a big pile of 32MB FP parity SIMMs. Four are now headed my way for a very reasonable amount, so I can hardly complain about your activities any more 😉

Reply 12 of 29, by UpLateGeek

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PCBs finally arrived today, but I'm so busy with a project at work that I'm way to exhausted to work on them. ): Hopefully things will have calmed down by the weekend.

Reply 13 of 29, by Jed118

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xtgold wrote on 2020-03-07, 16:29:

1 meg simm kit from the 1980's
I would strip the 1meg x1 dram chips from defective 286 motherboards

I would usually keep these (I still do if the 286 board has SIPP or SIMM sockets) and put them into video cards if they were at least 80ns.

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Reply 14 of 29, by jcarvalho

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Hi! Sorry to ask, but can I use this PCB in an 386 motherboard? or it is only for mac?
Thanks!

Reply 15 of 29, by Doornkaat

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jcarvalho wrote on 2020-10-29, 22:43:

Hi! Sorry to ask, but can I use this PCB in an 386 motherboard? or it is only for mac?
Thanks!

You should be able to use them a 386 board unless it's a non-standard board or the board/chipset only supports smaller capacity modules.

Reply 16 of 29, by jcarvalho

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Doornkaat wrote on 2020-10-29, 22:49:
jcarvalho wrote on 2020-10-29, 22:43:

Hi! Sorry to ask, but can I use this PCB in an 386 motherboard? or it is only for mac?
Thanks!

You should be able to use them a 386 board unless it's a non-standard board or the board/chipset only supports smaller capacity modules.

Thank you so much for the tip!
Regards!
Jorge

Reply 17 of 29, by kwyjibo

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This is a bit offtopic, but can anyone confirm if 24 bit address are passed to memory modules in different clock cycles to allow modules of more than 4 MB?

Reply 18 of 29, by mkarcher

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kwyjibo wrote on 2020-10-31, 09:05:

This is a bit offtopic, but can anyone confirm if 24 bit address are passed to memory modules in different clock cycles to allow modules of more than 4 MB?

Are you talking about 30-pin SIMM modules? They don't have "clock cycles", but a row address cycle and a column address cycle. Both the row and the column address can be up to 12 bits to allow for 16MB SIMMs, although these modules and devices supporting them are uncommon. A well-known wide-spread device supporting 16MB SIMMs is the SB AWE32 (and all of its variants) with 30-pin SIMM slots. Most 386DX and 486 mainboards only use 11 address bits for module sizes up to 4MB, as you need 4 of them anyway to fill a bank, so a single bank of 16MB SIMMs would have been 64MB. By the time 64 MB became affordable, 72-pin PS/2 SIMM were already established as the standard mainboard memory format.

Last edited by mkarcher on 2020-11-01, 08:52. Edited 1 time in total.

Reply 19 of 29, by kwyjibo

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Thanks for the explanation 😀