64 pin SIMM (Mac IIFX) Specs?

[Jeff Walther]

>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. They were not dual-ported, though, you
>only had one address bus for both reading and writing.

Thank you.  I have since noted this on X 1 DRAM datasheets.  I had 
never noticed that before.

>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.

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?  I could see where that could happen in parity checking.

For the data bus though, the source and destination would be the 
same.  For a little while I thought it might be for some odd timing 
advantage.  But since the write data isn't required on the bus until 
just before the CAS anyway, I don't think the timing idea could be 
valid.  Besides separate IN and Out could only confer a speed 
advantage in the cases where a Read followed a Write or vice versa.

>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?

I don't have a IIfx or the SIMMs yet.   I was exploring the 
feasibility of the project first.  However, I posted a WTB to the 
austin.forsale news group and got lucky with a fellow who wants to 
give a IIfx away.   So next week I should have hardware to explore. 
It's nice to avoid shipping when shipping would be greater than the 
value of a machine.

Still, I would like to get my hands on the Apple Hardware Developer 
Note for the IIfx.  It should at least mention whether they were 
using separate data_in and data_out for some timing related purpose. 
Of course, if an ohmmeter reveals that In and Out are tied together 
on the motherboard, then I'll have my answer.

Jeff Walther