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List:       classiccmp
Subject:    Re: Expansion memory boards (was: Interpreting Vaxstation 3100 memory
From:       David Riley <fraveydank () gmail ! com>
Date:       2011-11-29 0:44:40
Message-ID: BEBCCC7F-1F99-4F04-BDD5-2D2523B879B0 () gmail ! com
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On Nov 28, 2011, at 2:19 PM, Richard wrote:

> > I'm not well versed in memory design, but you might get it to work with
> > some buffer circuitry that kept the memory refreshed properly, even if it's
> > doing nothing for 7 out of 8 cycles.
> 
> Yes, sorry I wasn't explicit about what sort of design I was
> imagining.  I was definitely feeling that there would need to be some
> sort of controller on the memory board that mediates between the host
> and the modern memory parts -- something to match the timings between
> the two worlds.
> 
> I wasn't thinking that just hooking the modern chips up straight to
> the vintage busses was going to do the trick.

Absolutely.  For something newer, like a VAX with the direct memory bus (I haven't \
actually looked at it, so I may well be wrong), it should be fairly simple.  For \
UNIBUS/QBUS boards (including PMI, as far as I know), the drivers for 120-ohm \
open-collector buses are a bit of an issue.  You can hack them together with 74AS641 \
and 74AS760, but I haven't done much testing on either of these to see if they're \
reasonable for actual buses (especially really long ones).  The thresholds on them \
are also not 100% compatible with the DEC standard, though they'll probably do for \
most real-world situations.

One of these days, I need to make up a simple test board to stick in my backplane to \
run some simple bus compatibility tests.  There was a thread either here or on \
comp.sys.dec a while ago about some other possible ICs with built-in series terms \
which looked interesting... anyone remember what they were?


- Dave


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