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
I think the big cap is only a good thing. It's not draining the
batteries, it's only helping them. It's providing the first 15 minutes
of life for free, and then not hurting after that.
It also provides up to 15 minutes of battery-change grace period,
depending on the starting level. If you briefly power-on the main
computer before changing the batteries, you'll get the full 15 minutes
at least (a few more before the ram actually loses data, but 15 is
safe). If you start with almost dead batteries and simply remove them,
you still may get a whole minute or more depending on just how dead they
were. Even 15 seconds is a lot if you prepare and have the new batteries
unpackaged and ready to go first.
The excess drain might possibly be because of the unintended ground path
backfeeding though what are supposed to only be signal handling input or
output gates in other components, instead of having an actual GND
connection from the REXCPM to the battery negative. It could even be
causing some ICs to no be disabled by their active-low /CE pins maybe.
Or it might be the REXCPM is keeping more stuff alive than the sram, or
maybe there are pullups or pulldowns that have lower than necessary
values burning current at all times. Or, maybe my replacement sram chip
burns more than the original*.
These questions don't really have to be mysteries, they can all be
answered by just looking and testing, I just haven't done that yet.
* When I got my REXCPM, Steve didn't have any 4M chips, so I got a 2M
unit and replaced the chip myself, and I don't remember off the top of
my head exactly what it's quiescent current was. The "over 1 year"
estimate is just based on the typical value for almost any sram since
the early 90's of 5 uA or less.
It just turned me on to get removable batteries into the available space
right on the board and still be able to close the compartment cover.
It does require flush-cutting all the solder posts and the battery
holder tabs. But to me that is more straightforward, convenient, and
repeatable than finnicky hack or otherwise specialized pin construction.
Normally for "fake DIP legs" on a pcb intended to go into a dip socket,
you want the pins to be as thin as possible to mimick an actual dip leg,
so as not to stretch out flat leaf-style sockets. And often also want
the pcb to be as low profile as possible with little or no
shoulder/insulator between the contact leg and the pcb, to allow the
most room for the components on top. But in this case the socket has
machined round pin holes, and commodity machined round pin headers are
perfectly fine in them, and the huge 4mm shoulder height is ok if all
the parts can fit on the bottom of the pcb and leave the top clean and flat.
A separate keeper board will be trivial so I'll add that to the repo
shortly. So, even if we have to just live with the inefficient sleeping
instead of solving it, a couple AA's or something will be cheap, easy,
and last a long time. That will be better than nothing and simple to do.
//Ken
*From: *M100 <[email protected]> on behalf of Brian K.
White <[email protected]>
*Date: *Monday, September 11, 2023 at 2:28 AM
*To: *[email protected] <[email protected]>
*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
