30 pin simms come in the following "standard" capacities: 256kb, 1mb, 4mb, and sometimes 16mb. that is each simm.

30 pinns simms are 8 bits wide. (without parity)
72 pin simms are 32 bits wide (without Parity)
So you need sets of 4 on the simm extender

AFAIK you''ll need four 2MB 30-pin SIMMs to equate to a 8MB 72-pin SIMM

torindkflt wrote:

My quest to recreate a childhood 486 has me continuing to search for information to better help me rebuild this system. Now, I a […]
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My quest to recreate a childhood 486 has me continuing to search for information to better help me rebuild this system. Now, I am looking for information regarding the RAM.

In another thread, I mentioned that the original 486 from my childhood had a total of 16MB RAM in a 2x8 configuration. What I failed to mention was that one of those slots was occupied with a "SimmXtender" adapter that took four 30pin SIMMs and allowed them to plug into a single 72pin slot. I recall that all four of the slots on the SimmXtender were occupied, but I do not recall what capacity those modules were, just that they were all identical.

Now, I don't need help finding a SimmXtender, because someone is actually selling them on Amazon. 🤣 Rather, what I need is information regarding the 30pin SIMMs themselves. Specifically, in order to make the combined whole (SimmXtender adapter with four 30pin modules) equal one 8MB 72pin SIMM, would I need four 2MB 30pin SIMMs, four 4MB 30pin SIMMs or four 8MB 30pin SIMMs? I ask because I have a very slight fuzzy memory in my head about 30pin SIMMs having a narrower bus width than 72pin SIMMs, thus requiring them to be installed in identical multiples to match the bus width of 72pin. I just do not recall if this is true or not, or if so what that multiple would be.

Am I remembering correctly?

72pin SIMM modules are 32bit modules, so on a 386 or 486 board, these modules can be used individually. 30 pin SIMMs on the other hand, are 8bit modules and should be used in groups of four on a 386 or 486 boards, and should be used in pair for a 286 or 386SX board.

In short, a SimmXtender should have all its four slots occupied with 30 pin SIMMs, for it to work.

But what you said about your old SIMM's presumed capacity is something I never heard before. I really don't think 2MB 30 pin SIMMs ever existed. They came with 256 KB, 1M, 4MB, and 16MB capacities, and nothing else, AFAIK. So, in practice, your fully occupied SimmXtender will be a 4MB (1MB x 4 modules) or 16MB (4MB x 4 modules) 72pin SIMM.

Indeed, I too find that strange. But, I am 100% certain that the populated SimmXtender used in our old 486 totaled 8MB. I have already attempted to find 2MB 30pin SIMMs, and supposedly they DID exist, but were extremely rare and primarily used in old Macintosh systems. My oldest brother is the one who gave that computer to us, and whether this unusual upgrade was done by him or the person/store that built the computer for him, I do not know, and neither does he remember.

I think 2MB and 8MB 30 pin SIMMs did exist, but I have never seen them. I have also never seen baby-AT motherboard that claimed to support them. I think they may have been for Apples and PS/2s.

it is a LOT more likely you had 4mb in the extender and another 4mb on the motherboard. Maybe 2 extenders?

Anonymous Coward wrote:

I think 2MB and 8MB 30 pin SIMMs did exist, but I have never seen them. I have also never seen baby-AT motherboard that claimed to support them. I think they may have been for Apples and PS/2s.

go ibm! I have a whole pile of 512kb simms too! but the ONLY work in like 2 different ibm systems.

No, there is no onboard RAM on the motherboard (QDI V4P895GRN/SMT), and I know for certain that there was only one extender, and only two of the memory slots on the motherboard were occupied, one of them by the extender. Could it be possible that my brother or whoever built the system for him took some Mac 2MB SIMMs and stuck them in? Granted I can't speak for 30pin SIMMs, but I do remember taking a 72pin SIMM from a Mac once and having it work in a PC just fine.

HIGHLY unlikely. those 2mb simms are special, i'd honestly be shocked if somehow they would work in anything else.

2 MB SIMM30s do exist for the PC as well, although they are extremely rare. I have a set of 4 for a future 386 build. Never came across any other, though.

Seems it was quite expensive as well, around 100 USD in 1995

EDIT: wrong link removed (thanks tayyare)

Granted, this does re-raise the question I posted on my other RAM-related thread: would there be a performance difference between using the extender populated with 8MB and using a true 8MB 72pin SIMM, assuming the memory speed and type is otherwise identical between the two? Logic suggests the physically longer traces on the extender would cause a tiny increase in latency, but would that be noticeable to any appreciable degree? Or, is the 33MHz bus speed slow enough to wait out any potentially added delay from using the extender?

AFAIK there is no difference. Whatever delay a tiny increase in trace length will add should be well within what bus speed can tolerate. Also, I don't think a 386 can even automatically add wait states to compensate for delays like that.

Jolaes76 wrote:

2 MB SIMM30s do exist for the PC as well, although they are extremely rare. I have a set of 4 for a future 386 build. Never cam […]
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2 MB SIMM30s do exist for the PC as well, although they are extremely rare. I have a set of 4 for a future 386 build. Never came across any other, though.

Seems it was quite expensive as well, around 100 USD in 1995>

BE WARNED, IT IS A LARGE SCANNED MAGAZINE:

https://books.google.hu/books?id=L8IEyhjiSk8C … 0%202mb&f=false

Most probably any mobo supporting them would be even rarer. By the way, I cannot see any 30 pin 2MB modules in that page you bookmarked. The ones that I saw all 72pin, but of course this could be my mistake.

Wrong link, you are right. (I was looking for another article, I remember I found the thing by looking for prices). Given the time, I will make a shot of my modules instead.

In the meantime I found other traces on vintage comp forums, but IIRC mine look to be "PC type", no additional chips

http://www.vintage-computer.com/vcforum/showt … MB-30-pin-SIMMs

It makes sense that most motherboards that do support 30-pin SIMMs don't work with the 2MB SIMMs individually simply from their rarity. But, the fact that they're being installed on a SimmXtender four at a time means that, theoretically, they would appear to the motherboard as a single 8MB 72-pin SIMM, would they not? Thus, in my particular situation I would presume they'd work just fine, and the history of the original 486 from my childhood seems to uphold this thinking.

I can't see if anyone has mentioned, and i'm sure you're probably already aware, that when using 30 pin simms... you need to use a multiple of 4 modules, of the same size i.e. 4 x 256Kb, 4 x 1Mb, 4 x 4Mb. This is due to the bandwidth of each module being 8 bits (without parity) when a 386 / 486 has a 32 bit bus. This is in contrast with the adapter which you only need one of as it can be addressed using 32 bits alone.

That is correct, and I already knew this. The SimmXtender adapter I had on the original 486 required four 30-pin SIMMs to work. It ganged them together so they would plug into a single 72-pin slot. It was identical to this same one that's being sold on Amazon. http://www.amazon.com/White-SIMM-Adapter-30-7 … s/dp/B0081SD1V8