SIMMs and other memory dongles (was: Common items you passed up

[Jeff Walther]

>Date: Tue, 31 Jan 2006 21:17:49 +0000 (GMT)
>From: ard at p850ug1.demon.co.uk (Tony Duell)

>>  >> It is.  I own some three or four cards which have four 30-pin SIMM
>>  >> sockets and are designed to plug into a 72-pin socket.
>>  > As I look at these, I notice that they're all adapters as opposed to
>>  > something that gangs up multiple SIMMs to create a higher capacity
>>  > "SIMM".  Anyone seen anything like that?
>>
>>  Um, that's exactly what mine do, unless I'm misunderstanding you.  For
>>  example, you can plug four 1M 30-pin SIMMs into one and you get a 4M
>>  72-pin "SIMM".
>
>A 1M 30 pin ZIMM has 1048576 locations each of 9 bits.
>
>A 4M 72 pin SIMM has 1048576 locations each of 36 bits. It's electrically
>much the same a 4 20 pin SIMMs. I suspect the adapter is just connectors
>wired together in the right way.
>
>I think what the first poster wants is a thing that takes 4 30-pin 1M
>SIMMs and looks like a 4M 30 pin SIMM to the rest of the system. That is,
>it appears to have 4194304 locations, each of 9 bits. It's not hard to
>design something like that I think, but it would inovlve a few chips

It would be the same logic one finds on "composite" SIMMs.   These 
are SIMMs of larger capacity built out of many chips of smaller 
capacity.  Typically, there are 32 or 36 (non-parity vs. parity) 
identical memory chips on a composite SIMM, plus one or more chips to 
handle the logic to which Tony refers.  I've seen the logic handled 
in one or two 16V8 PALs in a 20 pin PLCC package.

The composite 30-pin SIMMs I've seen have a total capacity of 16MB 
(16M X 8 or 9) built out of thirty-two or thirty-six 4M X 1 chips.

I have also seen composite 72-pin SIMMs with a capacity of 64MB (16M 
X 36, parity) built out of thirty-six 4M X 4 chips.  However, the 
72-pin composite SIMM had no additional logic.  I haven't looked at 
the spec, but I assume that 72 pin SIMMs support splitting the CAS 
lines from the host somehow into multiple banks on a single SIMM? 
Or maybe there were some surface mount transistors playing with the 
address lines which I overlooked.

I've seen adapters under the commercial name SIMM'verter that do the 
4 30-pin into 1 72 pin conversion.  But I've never seen one that does 
4 30-pin into 1 30-pin.   However, I have seen 2 72-pin into 1 72-pin 
adapters, so there might be a 4:1 30 pin adapter.

Still, a set of four 16MB 30 pin SIMMs on Ebay typically goes for $25 
plus shipping.    So why not just buy the 16MB 30 pin SIMMs and avoid 
the possible reliability issues of converters?  A converter (and/or 
composite SIMM) may overload the current capacity of the address or 
control line drivers of the host machine.

I have been working on (very slowly) a design for some 16MB 30-pin 
SIMMs that use just two 16M X 4 chips.   It is going slowly because 
I'm laying out an 8.5" X 11" panel for economic reasons, and my IIfx 
SIMM design is on the same panel, and my software doesn't copy and 
paste traces, so I'm having to layout duplicates on the panel by hand.

Jeff Walther