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
On 9/11/23 15:47, Ken St. Cyr wrote:
I love the idea, and would be keen to build one for myself if it could
indeed extend the shelf life of my REXCPM memory to at least a few
weeks. I tend to swap between my REXCPM and a standard REX, so even just
having something to keep my REXCPM memory active for a few days while
it’s out of the machine would save some trouble. I was thinking about
doing a quick backup and restore using a TL866, but haven’t gotten
around to tinkering with it.
It doesn’t sound like this would work well with that big cap needing to
be kept charged. It’d be great if there was a way to take it out of
circuit… maybe a jumper that you could remove if you have the REXCPM UPS
hooked up
//Ken
*From: *M100 <m100-boun...@lists.bitchin100.com> on behalf of Brian K.
White <b.kenyo...@gmail.com>
*Date: *Monday, September 11, 2023 at 2:28 AM
*To: *m...@bitchin100.com <m...@bitchin100.com>
*Subject: *[M100] rexcpm battery
I thought I made the slickest thing.
https://github.com/bkw777/REXCPM_UPS <https://github.com/bkw777/REXCPM_UPS>
https://photos.app.goo.gl/i87E4wzimexCR3wL6
<https://photos.app.goo.gl/i87E4wzimexCR3wL6>
I was able to get 60mah of battery onto the system bus interface board
for REXCPM, and theoretically that should be able to keep the sram
memory for at least a year, but it looks like it will only last about 2
possibly 3 days.
So plan B is a separate "keeper". A separate thing with a much larger
battery that you connect to the rexcpm when not in the computer.
But that's only half of what I wanted. What I was really hoping for
though was to have the battery built-in, so that when the 100 batteries
die sitting on the shelf, the on-board battery is still there for at
least another year.
Both of these boards do work as merely ordinary rexcpm bus adapters,
whether the batteries are installed or not.
The 102/200 board also allows one to use a REXCPM on a 102 or 200
without soldering any wires or opening the case, although it does mean
using an external wire which is not exactly robust.
--
bkw
--
bkw
