64 pin SIMM (Mac IIFX) Specs?
ard at p850ug1.demon.co.uk
Mon Sep 12 17:48:19 CDT 2005
[DIm and DOut]
> Well, darn it, they aren't linked. It turns out that the memory
> controller on the IIfx buffers the writes, so that the CPU can go do
> something else after only 2 cycles instead of 6, unless, of course,
> the next operation is a memory access.
> So the the data lines from the SIMMs are routed to 74F573 D-type
> latches. It appears that the SIMM's DIN comes from the latches'
> outputs and DOUT goes to the latches' inputs. I have not confirmed
> it yet (must pull the PGA 68030 to get access to the data lines) but
> I suspect that the bidirectional databus goes to the input lines of
> the latches.
I would think so. Maybe with more buffers ('F245?) or even an ASIC in
between, but I doubt the latter.
> So, on a read, the data would just come out of the memory straight to
> the bidirectional data bus. On a write the data bus would take data
> to the latches' inputs where it would be buffered, and the memory
> controller presumably has control of the LE (latch enable) and OE
> (output enable) lines to the latches.
If the only thing that drives those DIn lines are the 'F573s, then there
would be no need to ever tri-state the outputs. In which case you may
well find that OE/ is tied to ground.
> So, if I tied DIN and DOUT together, it looks like there are at least
> two potential trouble spots.
> 1) If the memory controller does not switch the latches to High-Z
> (OE inactive) upon completion of Writes, then on a Read, the data
> from the memory would be in contention with the data still held by
> the latches from the previous write.
> 2) While the memory controller is buffering a Write, that data would
> feed back from the latches, along the tied together In and Out to the
> bidirectional data bus.
The outputs of those latches would then be contending with every other
device on the bus. I think it's unlikely those latches are ever
tri-stated in normal operation unless some other device can drive the DIn
of the RAMs -- and what would that device be?
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