486 Computer RAM - 30 pin vs 72 pin

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486 Computer RAM - 30 pin vs 72 pin

Postby walterg74 » 2018-10-17 @ 17:14

Hi all, building a 486 machine (or maybe more than one...) on a pretty standard motherboard that has 4 30 pin memory slots and 2 72 pin memory slots.

Reading around seems the general consensus for a 486 is to equip it with either 8MB or 16MB or memory, but regarding the simm type:

- Is there any performance difference in choosing either? (more importantly any NOTABLE difference)
- Would I have to fill up the 4 slots if choosing 30 pin? (I think I read somewhere that these are 8 bit and require all four populated?
- are either harder to get than the other or significantly more expensive?


Thanks!
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby tpowell.ca » 2018-10-17 @ 17:30

You can find faster and cheaper Fast-Page 72pin memory than 30 pin memory.
Look for 2x 16MB 60ns and you should be fine, and future-proof.
Note: You cannot mix 30 and 72 pin memory.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby walterg74 » 2018-10-17 @ 17:57

tpowell.ca wrote:You can find faster and cheaper Fast-Page 72pin memory than 30 pin memory.
Look for 2x 16MB 60ns and you should be fine, and future-proof.
You do not need to populate RAM in banks. 1,2,3 or 4 sticks are OK, but you cannot mix 30 and 72 pin memory.



Thanks, I will double-check one of the motherboards I was given (hd not checked it yet) since it contained both types inserted on the board.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby jesolo » 2018-10-17 @ 18:05

Just want to point out that 30-pin simms each have an 8 bit data width which means you need to populate all 4 banks (slots) to get you to a 32 bit data width on a 486 PC.
With 72-pin simms, you only need to populate one bank (slot)
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby tpowell.ca » 2018-10-17 @ 18:11

jesolo wrote:Just want to point out that 30-pin simms each have an 8 bit data width which means you need to populate all 4 banks (slots) to get you to a 32 bit data width on a 486 PC.
With 72-pin simms, you only need to populate one bank (slot)


Really? My bad. I stand corrected.

Reason more to get 72pin sticks.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby jesolo » 2018-10-17 @ 18:17

tpowell.ca wrote:
jesolo wrote:Just want to point out that 30-pin simms each have an 8 bit data width which means you need to populate all 4 banks (slots) to get you to a 32 bit data width on a 486 PC.
With 72-pin simms, you only need to populate one bank (slot)


Really? My bad. I stand corrected.

Reason more to get 72pin sticks.

If you do get 72-pin simms, rather go for FPM simms and not EDO simms, since most 486 motherboards only supports the former.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby dionb » 2018-10-17 @ 18:38

tpowell.ca wrote:Note: You cannot mix 30 and 72 pin memory.

Oh? Electrically it's exactly the same and I've frequently mixed and matched them in the past. The only limitation is that some boards have 4x 30p wired up in parallel to one of the 72p slots. In that case it's either/or between that 72p slot and those four 30p slots.

However unless you have a good supply of 4MB and 16MB 30p SIMMs, that's entirely academic as a 16MB FPM SIMM is cheaper than a single 4MB 30p SIMM, let alone four of them.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby walterg74 » 2018-10-17 @ 19:53

jesolo wrote:
tpowell.ca wrote:
jesolo wrote:Just want to point out that 30-pin simms each have an 8 bit data width which means you need to populate all 4 banks (slots) to get you to a 32 bit data width on a 486 PC.
With 72-pin simms, you only need to populate one bank (slot)


Really? My bad. I stand corrected.

Reason more to get 72pin sticks.

If you do get 72-pin simms, rather go for FPM simms and not EDO simms, since most 486 motherboards only supports the former.


Even later ones with pci slots?

How can I recognize if a simm module is FPM?
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby walterg74 » 2018-10-17 @ 19:54

jesolo wrote:Just want to point out that 30-pin simms each have an 8 bit data width which means you need to populate all 4 banks (slots) to get you to a 32 bit data width on a 486 PC.
With 72-pin simms, you only need to populate one bank (slot)



Yeah, that’s what I asked in the OP, since I thought I read that somewhere here.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby stamasd » 2018-10-17 @ 20:16

EDO is nothing but a "tweaked" form of FPM. All it does extra compared to regular FPM is allow the overlap of read cycles, if the motherboard/chipset knows how to do it. If not, it functions exactly as FPM. I've ran EDO memory in "FPM-only" motherboards before without any issues. The opposite is not true: if a motherboard requires EDO, then you (most of the times) cannot use FPM memory in it.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby mwdmeyer » 2018-10-17 @ 21:13

I could mix 30/72pin ram on my 486 back in the day.

I had 2x4MB 72pin & 4x1mb 30 pin for 12MB ram.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby dionb » 2018-10-17 @ 21:20

walterg74 wrote:Even later ones with pci slots?

How can I recognize if a simm module is FPM?

Read & decode the chip codes.

Use this guide:
http://www.idhw.com/textual/guide/noin_ram_chip.html
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby jesolo » 2018-10-17 @ 21:22

stamasd wrote:EDO is nothing but a "tweaked" form of FPM. All it does extra compared to regular FPM is allow the overlap of read cycles, if the motherboard/chipset knows how to do it. If not, it functions exactly as FPM. I've ran EDO memory in "FPM-only" motherboards before without any issues. The opposite is not true: if a motherboard requires EDO, then you (most of the times) cannot use FPM memory in it.

Although that might be true in certain cases, if a 486 motherboard doesn't explicitly support EDO RAM (only the later ones did), then I would advise against it.
Personally, I've tested some 486 motherboards where EDO RAM would cause the system not to boot. When I installed FPM RAM, it booted.
Here's one topic where it was discussed: viewtopic.php?t=33850
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby walterg74 » 2018-10-17 @ 21:34

dionb wrote:
walterg74 wrote:Even later ones with pci slots?

How can I recognize if a simm module is FPM?

Read & decode the chip codes.

Use this guide:
http://www.idhw.com/textual/guide/noin_ram_chip.html



Thanks I’ll take a look
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby walterg74 » 2018-10-17 @ 21:36

Guys, one more that just came up…

For one of the motherboards the seller included a cache module what was the benefit of this do I want it how much is it supposed to be in relation to the memory installed?
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby dionb » 2018-10-17 @ 22:01

walterg74 wrote:Guys, one more that just came up…

For one of the motherboards the seller included a cache module what was the benefit of this do I want it how much is it supposed to be in relation to the memory installed?

Depends entirely on exactly what it is, and what is on the board for starters.

How cache works is a topic to read up on, but basically it's a small amount of much faster memory that sits between the CPU and the main memory. Anything that the CPU can fetch from cache is ready to be used much faster than stuff it needs to get from regular RAM. If you have no cache, any cache is a huge improvement. More is almost always better, regardless of amount of RAM. in 486 days you're usually talking about 128kB or 256kB of L2 cache. The difference between none and 128kB is huge, the difference between 128kB and 256kB is measurable, but much, much smaller.

All architectures have a cacheable limit - i.e. the maximum amount of RAM that the cache can cover. Calculating it gets a bit esoteric, but bottom line is that if you have a *LOT* of RAM, on some systems it pays to have a lot of cache too. This gets pretty relevant if you're running Windows 98 on a late Socket 7 system (as the amount of memory you might want to install could easily exceed the cacheable limit), but just as with the 16MB 30p SIMMs - if you hit cacheable limits on a 486 you're building an insane system ;) I'm not aware of any combination of RAM, cache and chipset that has a cacheable limit below 32MB. For any 486 that has 30p SIMMs, anything over 16MB is completely period-incorrect. Consider that, indexed for inflation, 16MB of RAM cost ~EUR 750 to 1000 in 1995, and no normal program would even come close to using that much. So just aim for that sort of memory and forget about cacheable limits until you start building 1998-era socket 7 stuff ;)
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby kixs » 2018-10-17 @ 22:07

stamasd wrote:...If not, it functions exactly as FPM. I've ran EDO memory in "FPM-only" motherboards before without any issues. The opposite is not true: if a motherboard requires EDO, then you (most of the times) cannot use FPM memory in it.


I find it's actually the opposite. Pentium boards will use FPM or EDO. But 99% of 486 won't run on EDO. A few 486 that can take EDO won't be any faster with it over FPM.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby tayyare » 2018-10-18 @ 11:07

tpowell.ca wrote:....Note: You cannot mix 30 and 72 pin memory.


Yes, you can. Maybe some boards are limited like this, but definately there are boards that you can mix and use both sizes together.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby tpowell.ca » 2018-10-18 @ 13:30

tayyare wrote:
tpowell.ca wrote:....Note: You cannot mix 30 and 72 pin memory.


Yes, you can. Maybe some boards are limited like this, but definately there are boards that you can mix and use both sizes together.


I realize I mixed up my motherboard generations.
You typically can't mix 72pin with 168pin DIMM memory.

But I do agree with kixs, none of my 486 boards that didn't state EDO compatibility (namely SiS chipset-based) could boot with EDO.
FPM was no issue on any board, 486 or Pentium as these are backward compatible and could detect FPM.
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Re: 486 Computer RAM - 30 pin vs 72 pin

Postby tayyare » 2018-10-18 @ 13:41

tpowell.ca wrote:
tayyare wrote:
tpowell.ca wrote:....Note: You cannot mix 30 and 72 pin memory.


Yes, you can. Maybe some boards are limited like this, but definately there are boards that you can mix and use both sizes together.


I realize I mixed up my motherboard generations.
You typically can't mix 72pin with 168pin DIMM memory.


Yes you can :lol:

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