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Identifying 72-pin SIMMs

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Tiberian Fiend

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When I mentioned needing 72-pin memory for my 486, a gentleman was kind enough to give me a bag full of SIMMs. Problem is, most of them are unmarked, so I don't know what they are. How would I go about identifying them? I'm not sure if I could take clear photos, but I can relay any numbers or markings that might be helpful.

I installed the only two that were marked (8 MB), and elicited the Memory Beep of Doom. I don't remember anything about memory from that era, so I have to ask: what compatibility issues might I be facing (this is a 486 DX4-100, ca. 1995)?
 
There are two main compatibility issues you'll find: one is that many 486 systems require FPM and don't like EDO memory, and a good number of 72-pin SIMMs are EDO. There's also the issue of parity vs. non-parity, and good number of systems will only work with one or the other.

Unfortunately, I don't know of a reliable way of telling the difference between EDO and FPM just by looking at the sticks... you can sometimes look up the part numbers on the chips, but I've found quite a few cases where the chips are so long obsolete that there's not much information to be found on them. About the best you can expect to do with the part numbers is figure out the capacity of the stick.

As far as parity vs. non-parity, that's pretty easy. You can generally just count the chips... non-parity will have an even number, while parity will have an odd number. (The exception is on sticks with 12 chips, which are usually, if not always, parity.)
 
8MB SIMMS on a 486 were a problem for me. Multiple incompatible standards were added while keeping the connectors the same. The system I am looking at right now (Alaris 486) supports 1MB, 4MB, and 16MB SIMMs but not 8MB SIMMs and only parity SIMMs. Check a reference for your motherboard. Also, most 486s were able to accept SIMMs one at a time; if you can, only install and test with one SIMM.

The chips on each SIMM should indicate its capacity though it has been years since I tried to decipher that.
 
I do got a hardware book from the 72-pin SIMM RAM eera that tells that the most imporiant you have to keep in mind is to NOT mix RAM speeds. If you absolutely HAVE to, place the slowest RAM in the first slot.

You can often read the RAM speed as the last diggit of the long number on the IC's. For 72-pin SIMMs, it's in most causes "6" (for 60ns), but it may be "7" (for 70ns, whitch is slower than 60ns).

For the sizes, I don't think that really matters. EDO and FPM wise, it shouldn't have too much to say. EDO should behave as FPM if installed in a FPM-based motherboard, but of course you allways have oddballs. I don't suggest mixing FPM and EDO.

Pairity can problably be disabled with the CMOS setup program, if not, there should be a jumper somewhere on the motherboard. If not, it may be autodetect, or you may be out of luck.

So what I say is just try out anything. If it doesn't work, try another combination or just move around. Eventually you'll reach a combination that works.
 
Look at the pins on the chips themselves.

If they are in 4 groups of 5 pins each then they are 1Mb x 4 and 8 of them will give you 4Mb of DRAM. If the SIMM is double sided with 16 chips then it's 8Mb.

If the pins are in 4 groups of 6 pins each then they are 4Mb x 4 and 8 of them will give you 16Mb of DRAM. If the SIMM is double sided with 16 chips then it's 32Mb.

8/16 chips = non parity
9/12 chips = parity

If the part # for the chip ends in 0 or 1 then it's probably a standard DRAM. If it ends in a 3, 5, or 7 then it's probably (but not always!) EDO. Don't confuse that with the number after the dash which is the speed rating. (-6 = 60ns, -7=70ns, etc...)
 
Tiberian Fiend said:
I managed to find my motherboard on statson.org. What does "mode 1" and "mode 2" mean? Does this page tell me anything else about what type of memory my motherboard requires?
Click to expand...

Mode 1 and Mode 2 seems to be how memory is managed. It seems like mode 1 is for dealing particulary with 30-pin SIMM memory and mode 2 is for dealing particulary with 72-pin SIMM memory. The reason they have it like this may be because 30-pin SIMM memory ofthen has another speed than 72-pin SIMM memory. You must use mode 2 if you don't got bank 0 and bank 1 filled with 30-pin SIMM memory, in fact, you really have no choice if you want a working system.
 
That would explain why the memory wasn't working. According to the writing on my motherboard, mode two swtiches the 72-pin banks from 2 and 3 to 0 and 1.
 
Tiberian Fiend said:
That would explain why the memory wasn't working. According to the writing on my motherboard, mode two swtiches the 72-pin banks from 2 and 3 to 0 and 1.
Click to expand...

Then it's obivous.

It simply starts searching for memory in bank 0, then keeps going through the rest of the banks untill it don't find any more. If bank 0 is empty, it won't find any memory at all.

---

Do you got it working now?
 
I know it's sad, but I've finally gotten back to this project a year-and-a-half later. I swapped the memory between this system and my Unisys CWD, and the CWD expirenced the same problems as this computer, and this computer worked fine. I've tried them one-by-one, and it seems every one of the SIMMs in this bag causes the same problem except a lone 4MB one (which is also the only one that didn't come in a pair). I can only guess these chips were fried by the plastic bag they came in (though it seems odd that the guy I got them from would put them in a non-anti-static bag).
 
Tiberian Fiend said:
I know it's sad, but I've finally gotten back to this project a year-and-a-half later. I swapped the memory between this system and my Unisys CWD, and the CWD expirenced the same problems as this computer, and this computer worked fine. I've tried them one-by-one, and it seems every one of the SIMMs in this bag causes the same problem except a lone 4MB one (which is also the only one that didn't come in a pair). I can only guess these chips were fried by the plastic bag they came in (though it seems odd that the guy I got them from would put them in a non-anti-static bag).
Click to expand...
Maybe he gave you a bag of memory modules he had problems with?
 
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