Let's give THIS a try...
https://photos.app.goo.gl/JaSofoK2VotHzVco9
The connector is a standard JST-SH, and cables are available pre-made in
both 2" for the 100 and 12" for the 102 or 200.
There's even an off-the-shelf cable that has dupont sockets on one end,
so you could connect to an unmodified rexcpm.
So now we have
120 mah
gnd connection
polarized plug
I think we're looking at at least 2 years of shelf life now.
--
bkw
On 9/12/23 15:05, Brian K. White wrote:
I've done a little more testing.
The GND path is backfeeding only via the RAM line. Nothing on /WR.
(the only possible connections)
I am measuring only 6.6uA on the RAM_RST line between the two boards
with the boards outside of the 100.
That is a little high but actually not too bad. I would hope for under
5uA ideally but 7uA is in the same class.
At that rate, nominally the batteries should last over 11 months. And
at that low current there shouldn't be much voltage drop across the
diode on RAM_RST, and indeed there isn't. I only lose about 0.1v from
the battery to RAM_RST.
The bulk of the voltage drop is happening along the GND path, and
things change a lot when the boards are installed in the 100 and the
100 provides a real GND connection between the two boards.
With both boards outside of the 100, I currently measure
(the batteries are draining as time goes on so these numbers all
change a little each time I check something, so I'll keep saying the
battery voltage along with whatever else I'm talking about)
2.99v between BATT- and BATT+
2.91v between BATT- and RAM_RST (so practically no drop through the
diode on the bus board)
2.76v between BATT- and CAP+ (it's not labelled CAP+, I just mean the
positive side of the big cap) on the REXCPM (after giving it time to
drain away the 5v from the 100)
2.26v between CAP- and CAP+
So there is not actually much drop from BATT+ along RAM_RST to REXCPM
VCC, but a 0.5v loss along the GND-via-RAM path.
Next, when I provide an actual GND connection between the boards by
plugging them in to the 100 with the memory power switch turned off,
the current draw actually increases slightly to 7.2uA (after an
initial spike probably charging the cap), but the voltage drop
decreases. The battery is currently at 2.99v and I get 2.72v at the
cap instead of 2.26v
While writing this the current along RAM_RST very gradually dropped
further to 6.9uA. Maybe it will continue creeping down to settle at
the same 6.6-6.7 eventually?
This suggests that maybe the batteries would provide a worthwhile
shelf-life extension after all, at least while installed in the
machine which provides a real GND connection.
60mAh at 8uA works out to just over 10 months. (according to
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life)
That's worth doing I think. Especially when added to the 100's own
shelf-life, that should work out to maybe 1.5 years total.
I still have to check the actual current drain at the battery. My
current measurements above were just on the RAM_RST line from the bus
board to the rexcpm, not at the battery itself. It's possible the
battery is seeing more total current than just that through additional
paths like reverse leakage through the schottkey from BATT+ to VBUS,
and through RAM_RST into the 100.
Anyway it looks a lot more promising than I thought at first.
As for getting that same long shelf-life outside of the 100, one idea
might be to mod the REXCPM to use a 4-pin JST-SH connector and use the
same connector on the bus board, and a pre-made cable assembly.
Those are very small connectors and would be difficult to crimp the
female connectors on the cable by hand, but they sell those pre-made
in suitable lengths. And the male connectors are only 2.95mm tall and
7mm wide (really only 6mm but the flanges on the cable connector are
7mm) and that fits easily on both the rexcpm and the bus board.
On the rexcpm side it would have to be bodge wired to the 3 existing
pins and the 4th pin to the negative side of the big cap, then secured
with glue or mounting foam tape.
On the bus board it's nothing. The connector footprint fits right
where the existing 3-pin header is now.
https://mou.sr/3K9FwRe
https://mou.sr/44Pcako
or
https://www.digikey.com/short/7hb4cm4q
https://www.digikey.com/short/wzj8ppjb
And as an added bonus, the new cable is fully polarity keyed on both
ends too.
--
bkw
