On Oct 19, 10:37 pm, tortoise <[email protected]> wrote:
> On Oct 18, 11:29 pm, WhyOSX <[email protected]> wrote:
> >http://cgi.ebay.de/128MB-Apple-Performa-630-Series-72-pin-SIMM_W0QQit...
>
> This looks right but I believe I paid $50 US for both the 64 and the
> 128, from the same people.
> One of these cards was listed as for a nubus powermac but I found a
> reference for the part number somewhere. You just have to find 72pin
> SIMM FPM double sided, but no doubled chips on a single side.
> for the 128, and single side for the 64 (my 64 is in my quadra605)
Assuming you mean 2-bank and single-bank by "double sided" and "single
side"...
All (or almost all) 128 MB 72 pin SIMMs are 2 bank SIMMs. The maximum
amount of RAM that a 72 pin single bank can address is 64 MB, so the
only way to make a 128 MB SIMM is to use two or more banks. There
might be some four bank 128 MB SIMMs out there somewhere, but it seems
unlikely.
Similarly, 64 MB SIMMs are almost always single bank because making a
two bank 64 MB SIMM would require some very oddball memory parts.
Some of those oddball memory parts were manufactured for a while, but
they're pretty rare.
There are other things to avoid, however. There are some 64 MB SIMMs
out there built out of smaller capacity memory chips. So you find
some 64 MB 72 pin SIMMs with thirty-two 4M X 4bit memory chips on
board. These are usually too tall physically and draw too much power
to work well in our Macs. Good 64 MB SIMMs are built out of eight
16M X 4 memory chips (usually all on a single side of the SIMM).
Good 128 MB SIMMs are built out of sixteen 16M X 4 memory chips
(usually eight chips per side of the SIMM).
If the SIMMs have parity, then they'll have the above, plus four 16M X
1 or eight 16M X 1 chips respectively for 64 MB and 128 MB SIMMs.
Even if the 128MB SIMMs meet the above specifications, they still may
not work in the PM6100, because of how the RAS lines are wired. There
are four RAS lines in a SIMM socket. By activating the RAS lines in
pairs, instead of four at a time, a SIMM can be made to operate as if
it is two separate SIMMs, in other words, as if it has two separate
banks of memory. Of course, the chips on the SIMM must be wired to
the RAS lines properly for this to work--half the memory chips wired
to one pair of RAS lines. The other half of the memory chips wired to
the other pair of RAS lines. Some 128MB SIMMs by major
manufactures (HP, IBM, Samsung) pair the RAS lines on the 128 MB SIMM
in the opposite fashion as Apple does on the PM6100. So those 128 MB
SIMMs do not work in the 6100, although changing the RAS pairing on
the SIMM might be as simple as moving two resistors. Strangely, those
128 MB SIMMs work fine in the Quadras which support 128MB SIMMs and
both types of machine work fine with the same two-bank 32 MB SIMMs.
So there's something a little more complex than a simple wiring
difference going on.
One Sunday I sat down with some Mac power supplies, a whole bunch of
Quadras and NuBus PowerMacs, and a stack of 32MB, 64MB and 128MB
SIMMs. Oh, and a notebook, a pencil and a ohmmeter, and a pinout
listing for the JEDEC 72 pin SIMM standard. The above is derived from
that testing.
Jeff Walther
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