64 pin SIMM (Mac IIFX) Specs?

[Tony Duell]

> My main concern is that the pinout lists a separate data pin for data 
> in and data out.   So even though they are eight bit SIMMs, there is 
> a Data In [0:7] and Data Out [0:7] in the pin out.  It's possible, 
> perhaps likely, that these are simply tied together, but it would be 
> nice to see some confirmation.  The chips I've identified on IIfx 
> SIMMs don't appear to be dual ported and I don't see any extra 
> circuitry on the SIMM that could be making an adaptation, but I'm 
> working from JPEGs so far.   If this looks feasible I'll actually 
> spring for a IIfx and a few SIMMs to work from.


Most, if not all, of the single-bit-wide DRAM chips (4164, 41256, etc) 
had separate DIN and DOUT pins. They were not dual-ported, though, you 
only had one address bus for both reading and writing.

You could link these pins together externally, or you could use them 
separately. IIRC on the IBM PC, the DIN and DOUT pins are linked on the 
RAMs storing data bits, but wired separately on the RAMs storting the 
parity bits.

I think SIMMs brought the pins out separately so that you could use them 
separately if you wanted to, or just link them together on the SIMM socket.

If you have the machines that are supposed to use these 64 pin SIMMs, can 
you not just check to see if DIN and DOUT are linked on the mainboard 
with an ohmmeter?

-tony