I just had the best idea of the last days, in the most unlikely place, but I'm not surprised anymore because that's the way progress happens : with cross-polination of ideas :-)
Everything is there ! This is the solution ! Just replace the coils with capacitors ! Diodes to the rescue !!!
When the previous DRAM array circuit is rotated 90°, the circuit saves the large duplicated MUX that connects the diodes to 0V. After all, only one of them will conduct current.
The revised DRAM circuit is there :
The added diodes to the left increase the voltage drop but it's not significant. Their leakage though is critical.
The increase in diodes count is not significant but the saving in relays is great ! Less relays means less power consumption, fewer assembly hassles and less expensive...
So the number in the log Backplane routing considerations must be corrected.
- For 256 words: 2 MUX16=32 relays, or 576 relays for 18 boards
- For 512 words: MUX16+MUX32= 48 relays, or 864 relays
512 words is in the realm of possible but I'll start with 256 so I need "scalability"!
Wait wait wait !
There's something else with the flip dot arrays : all the lines are driven simultaneously and only one column is active ! Could this be applied to our capacitor array ? If it is possible (without the gotchas that I was pointed to in the first iteration of the DRAM array) then I can save AGAIN a whole lot of relays and use only one MUX for all the columns ! I must test this ! If this holds true then
- For 256 words: 1MUX16=16 relays, plus another MUX16= 304 relays for 18 boards
- For 512 words: 1MUX16=16 relays, plus one MUX32=320 relays only !
Following @roelh's comment, here are the (expected) current flows through the MUX and array:
I hope it's clearer now :-)
Re-update: falstad simulation shows @roelh's concerns were justified... See https://hackaday.io/project/20864-ygrec-15-bis/log/56753-first-dram-array-prototype#discussion-list