Yes very common with Dram arrays. Similar to write enable on Chipselect for Sram arrays.
Allison On Sat, Mar 14, 2020 at 4:20 PM Chris Zach via cctalk <cctalk@classiccmp.org> wrote: > 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 > > >
