The final piggyback board arrived today; this one eliminates the open-collector 74xx156 with a 74xx155 and a 74xx08 to handle what the pullup resistor used to do.
Everything works well, with one caveat: I never could get reliable operation out of the low-power CMOS 6264 handling the DSR. The old [HN]MOS 6264 that came with the card did fine, but the new Alliance CMOS part kept throwing errors up towards the top of the memory space.
It could be due to the HRD+ build instructions telling us to wire A14 into pin 1 shared by all of the memory chips. This is a non-connected pin on the 6264, but it’s possible that Alliance’s part doesn’t like a fluctuating input.
It could also be due to the funky pullup/pulldown scheme on one of the two chip selects on the 6264. The chip turns on when one is high and the other is low, and turns off when the logic levels are reversed. The HRD+ does that by feeding Vcc through an LED to the non-inverted CS pin, and then grounding it through a resistor. I replaced that with all sorts of schemes (feeding that CS through the signal that handles the actual RAM disk, going to a two-resistor approach, etc) but nothing was stable … and I didn’t have a spare CMOS 6264 to test. My TL866 swears that this 6264 is okay, so fine …
… I have a bunch of low-power Alliance 62256 chips. Standby current draw is roughly the same as the Alliance 6264, the package and pinout is mostly the same, and most importantly it has only one chip select pin. Ground pin 1 (A14) and pin 26 (A13), attach pin 20 (!CS) to Vcc via a pullup resistor to kick it into power-saving mode when the switch is flipped, and suddenly everything worked fine.
I’ll post the piggyback boards (schematics, PCBs, and instructions) when I get a moment over the next few days. The design is validated, so I don’t see any reason not to do a full layout and release that as well. This layout will require a 62256, because I won’t release a variant that I couldn’t get to work myself.
So … we now have a vetted multi-megabyte RAM disk for the TI-99/4A that can be built by regular humans using currently-available parts. Total cost for the chips should be under thirty bucks; the PCB costs however much the fab charges you. You can choose to upgrade an existing HRD+ or build a new one, your choice.
(I’ll also throw in a 32k expansion daughterboard PCB that plugs into the DSR chip socket. That’s totally untested, of course, but it should work … )