MSV11-Q info and interesting observation

[Chris Zach]

Noel, you're incredible! Thanks for fuzzing this out, I've been working 
on chiming clocks as of late and put this board on the back burner, but 
with this swapping out the bad chip should be a piece of cake.

Thank you again!
CZ

On 3/14/2020 4:12 PM, Noel Chiappa via cctalk wrote:
> So, a while back someone had a broken MSV11-Q QBUS memory card, and needed
> info on them. I said I'd provide same, but then got distracted. Well, I
> finally got to it, and it's been added to the CHW page for them:
> 
>    https://gunkies.org/wiki/MSV11-Q_QBUS_memory
> 
> It includes a table which says which chip each bit in the memory is stored
> in (which is what one needs to fix one which is basically working, but has
> some bad bits).
> 
> 
> While working out that table, I ran into a hitch, which is a good part of why
> it took so long. The hitch, when solved, revealed something mildly interesting.
> 
> The hitch was in my process for finding out which bit was stored in which chip.
> I whipped up a simple loop to store a word with a single '1' bit, and the
> rest 0's; I set that running, and used a 'scope probe on the DIn pins to find
> out which column of chips held bit 0, etc. So far, so good. I then looked on the
> -Wr pin, to find out which row of chips held which banks.
> 
> Not so good! There were pulses on -Wr for _all_ the banks, no matter which
> address I tried to write to.
> 
> Eventually I worked out what was going on: when writing data, the MSV11-Q
> sends a 'write' signal to _all_ the banks, and selects the one to _actually_
> use by use of the RAS signal. I'm not certain why DEC did this, but since
> there is no explicit 'read' signal on the DRAM chip, and likely the data
> outputs from all the banks are wire-OR'd together, use of RAS to select the
> desired bank works for read, and also for write.
> 
> Has anyone else seen this trick used anywhere else?
> 
> 	Noel
>