486 Computer RAM - 30 pin vs 72 pin

Discussion about old PC hardware.

Re: 486 Computer RAM - 30 pin vs 72 pin

Postby tpowell.ca » 2018-10-18 @ 14:45

tayyare wrote:Yes you can :lol:

PC Chips TXPro M560

PS: Maybe this board is not typical but sorry, I couldn't resist :blush:


I am getting 0wn3d.
In my defense, that appears to be a feature of the TXpro chipset and maybe a few others.

The danger here is that SIMMs typically run at 5v and DIMMs run at 3.3v. Some claim that mixing memory types can cause you to overvolt the DIMMs. Not sure if that's true.
Last edited by tpowell.ca on 2018-10-18 @ 15:11, edited 1 time in total.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby Koltoroc » 2018-10-18 @ 15:06

tpowell.ca wrote:I am getting 0wn3d.
In my defense, that appears to be a feature of the TXpro chipset and maybe a few others.

The danger here is that SIMMs typically run at 5v and DIMMs run at 3v. Some claim that mixing memory types can cause you to overvolt the DIMMs. Not sure if thats true.


FPM and EDO SIMMs are 5v, EDO DIMMs can be either 5v or 3.3v and SD-RAM is 3.3v

Mixing SD-RAM with EDO RAM does not work at all because of how different they work Mixing EDO DIMMs with EDO SIMMS would only work with 5V DIMMs. Disallowing mixing makes sense for safety reasons, but I am not surprised some PC Chips Bargain bin board allow it. I mean PC Chips isn't exactly known to give a shit about reliability.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby dionb » 2018-10-18 @ 15:40

This has very little to do with PC Chips and more with the basic functionalities of a chipset. "TXPro" is PC Chips-speak for the ALi Aladdin IV+ chipset, which is an interesting and very fast (clock for clock the best memory performance of any So7 chipset, just beating the i430TX, but with far more features). Never knew about this one though...
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby allCAPPS » 2019-3-09 @ 11:49

I installed 4x 2MB 30-pin SIMMs on a 486 board, but the BIOS only sees 4MB total (1MB each). Is there some limitation I don't know about?
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby rasz_pl » 2019-3-09 @ 12:47

allCAPPS wrote:I installed 4x 2MB 30-pin SIMMs on a 486 board, but the BIOS only sees 4MB total (1MB each). Is there some limitation I don't know about?


2MB 30-pin SIMM? are really sure
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby Tiido » 2019-3-09 @ 12:57

30pin SIMMs only come in 256KB, 1MB, 4MB and 16MB varieties. Single sided 72pin ones are 1MB, 4MB, 16MB, 64MB (and 256MB) and double sided ones are twice the single sided ones, filling the inbetweens.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby rmay635703 » 2019-3-09 @ 16:42

Tiido wrote:30pin SIMMs only come in 256KB, 1MB, 4MB and 16MB varieties. Single sided 72pin ones are 1MB, 4MB, 16MB, 64MB (and 256MB) and double sided ones are twice the single sided ones, filling the inbetweens.


I don’t know if the above is fully accurate in terms of 30pin sizes

https://stason.org/TULARC/pc/io-cards/I ... Y-EXP.html

My old pcchips manual listed 64kb,128kb,256kb,512kb,1mb,2mb,4mb simms in the manual

Generally the “ODD” sized Simms are always listed as 32pin Simms are far as I can tell,
But historically when I bought 32 pin Simms they fit in my 286 just as well as the 30 pin ones so not sure if it was a typo or something to do with having 9 bit parity
Many 30pin vram modules were 512kb and Macintosh documentation is always mentioning 512kb Simms

Maybe they are rare or maybe they meant to make them and never did, no idea.

Oh and I do have a pair of 64kb Sipps from a strange XT that had 30 pin Simms so those are a thing too.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby allCAPPS » 2019-3-09 @ 18:23

These are the exact same sticks I have, except four of them. I assumed the "2M" meant 2 megabytes. So either 1) "2M" is just part of the model number or something OR 2) they're intended for another system that can see the full capacity?

https://classiccomputershop.eu/Webwinkel-Product-212096159/Set-2x-L.A.-Components-MT-Q2M-SI-2-MB-2MB-4-MB-4MB-kit-80-ns-80ns-30-pin-SIMM-parity-RAM-memory-modules-vintage-retro-90s.html
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby rasz_pl » 2019-3-09 @ 20:34

Those sure are 2MB per simm. Hmm what is a pld doing on those modules? and how are they parity without additional parity chip? is this some sort of faux parity emulated by the pld? Wonder what they were meant to work in, most likely not a standard PC.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby Tiido » 2019-3-09 @ 22:59

Each address line on a DRAM quadruples the size. Only way 30pin chip can have 512KB 2MB or 8MB is when some sort of assymmetric addressing is used, i.e CAS and RAS have unequal size in bits but I don't think there are many motherboards that support such a method. I haven't seen a DRAM chip to actually do such a thing on its own (outside SDRAMs).
PLD on the modules in the link is likely choosing which 2 chips out of the 4 are enabled according to top address line used, thus creating assymetric addressing.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby torindkflt » 2019-3-10 @ 02:50

I know for a fact that 2MB 30pin SIMMs did exist, because the 486 my family owned when I was a kid had them. Specifically, it had four of them mounted on a special adapter that then allowed you to plug them into a single 72pin slot, totaling 8MB for that slot. It then had a second regular 8MB 72pin SIMM in addition to that for 16MB total. The only reason I haven't gone that way with my recreation is because I simply couldn't find any 2MB 30pin SIMMs, so I presume they were very rare.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby allCAPPS » 2019-3-10 @ 08:05

Tiido wrote:Each address line on a DRAM quadruples the size. Only way 30pin chip can have 512KB 2MB or 8MB is when some sort of assymmetric addressing is used, i.e CAS and RAS have unequal size in bits but I don't think there are many motherboards that support such a method. I haven't seen a DRAM chip to actually do such a thing on its own (outside SDRAMs).
PLD on the modules in the link is likely choosing which 2 chips out of the 4 are enabled according to top address line used, thus creating assymetric addressing.



The BIOS has options for CAS and RAS, but I guess it doesn't support assymetric addressing. Interesting. I have two other motherboards I'll have to try them in. They all take two 72-pin modules in addition so I'm not hurting for memory. I just like to see all the slots completely filled. :lol:

torindkflt wrote: The only reason I haven't gone that way with my recreation is because I simply couldn't find any 2MB 30pin SIMMs, so I presume they were very rare.


If you want these, I might be willing to part with them for a nominal price.
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