64 pin SIMM (Mac IIFX) Specs?

[Jeff Walther]

>Date: Mon, 5 Sep 2005 21:11:02 +0100 (BST)
>From: ard at p850ug1.demon.co.uk (Tony Duell)

>>  >Date: Sat, 3 Sep 2005 00:17:50 +0100 (BST)
>>  >From: ard at p850ug1.demon.co.uk (Tony Duell)
>>
>>  >Most, if not all, of the single-bit-wide DRAM chips (4164, 41256, etc)
>>  >had separate DIN and DOUT pins.

>>  What is the point in using them separately?  Was it for the case
>>  where the destination of a read is different from the source of a
>>  write?

>YEs, that was the idea. It's no use if you want to link the data pins to
>a bidirectional bus.

>I suspect the _real_ reason for it was that there was a spare pin on the
>package and it didn't ever make life more difficult to have separate DIN
>and DOUT pins (oyu could just strap them together externally if you
>wanted a bidirectional data pin).

I'll be getting a IIfx on Thursday and I can confirm (I hope) that 
the DIN and DOUT are common on the mother board at that time.  Your 
advice has given me hope that this is much more doable than I feared.

If they're tied, then I should be able to build a 16MB SIMM using two 
16M X 4 chips.  My concern was that the X4 chips do not have separate 
DIN and DOUT and I wasn't certain if the IIfx actually needed the 
separation.   I think I have some 16M X 1 chips which would work in 
either case, but my supply of 16M X 1 chips is much smaller.

If the DIN and DOUT are common on the MB, and I use 16M X 4 chips, I 
assume that I should either use the DIN pin or the DOUT pin on the 
SIMM but not both?  Using both could result in different length 
datapaths which remerge, possibly causing interference from slightly 
out of phase signals.  The difference in length probably wouldn't 
affect the timing much, but still...

It would still be handy if someone has a mechanical drawing of the 64 
pin SIMMs.  I can derive it from a physical example, but I'm less 
likely to get the actual center measurement and tolerances correct 
that way.  Hmmm.  I'll check around and see if any of the socket 
manufacturers are still selling a 64 pin SIMM socket.  If so, I 
should be able to derive the SIMM dimensions from the socket 
datasheet.

Jeff Walther