64 pin SIMM (Mac IIFX) Specs?

[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. 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.

YEs, that was the idea. It's no use if you want to link the data pins to 
a bidirectional bus. I've seen DRAM boards with separate input and output 
buffers, converting the bidirectioanl off-board bus into seprate input 
and output buses to the DRAM chips themselves. It's also useful when 
you're using the DRAMa in a video display circuit and want to feed the 
DOUT lines to the shift register.

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).

-tony