I included the datasheets for all the parts in the REF directory, and
even the current today datasheet for that part, specifically the TI one
even, does say:
"NOTE 3: All unused inputs of the device must be held at VCC or GND to
ensure proper device operation. Refer to the TI application report,
Implications of Slow or Floating CMOS Inputs, literature number SCBA004."
I guess that could be interpreted as the data inputs are not even inputs
any more if they are hard-ignored by another control, and so they don't
count as "unused inputs". But if you were using some of them but not
using all four, the unused ones would need to be nailed down.
I don't see any reason not to ground them though. It's no problem
routing the traces, it shouldn't cost any battery life, and may even
help with esd or something like you say. Plus "just because that's why".
Things that are allowed to float can go outside of the safe range too.
---
I think the diode bodge must be preventing a small but unnecessary
battery drain on the host computer.
It prevents the host from ever raising that line to disable the device,
but still allows the host to lower the line to activate the device.
The device has a pullup to it's own battery on this pin, so the device
never needs the host to raise that line, only lower it.
The host holds RAM_RST high while turned off to disable ram, which this
device does not need because of above.
RAM_RST at 5v must leak through R3 to VMEM at only 3.6v or less.
A 3v differential (assume the DATAPAC battery is about to die at 2v)
across a 240k resistor is only 10uA, and a 102 or 200 runs itself all
the way to dead flat both AAs and memory battery in only a number of
months, not years, so I don't think 10uA could be making a noticeable
difference in this particular case, even though it would be a lot on
some other device. The DATAPAC itself draws only 4uA while disconnected
(one with all 256K would draw more), and a mere 80 mah battery would
last almost 3 years. But the 100/102/200 are already way worse than
that. Maybe this user also tinkered with his 102 or 200 and improved it
to where 10uA actually made a difference.
Or maybe the problem they were addressing was something else, like that
5v leakage harming the battery if left connected 24/7 too long, since
it's not being reduced to 4.3v though the diode that VBUS has to go through?
--
bkw
On 8/20/23 18:02, [email protected] wrote:
Brian,
Awesome reverse engineering! Keep us posted on the progress. This is
interesting.
As far as the 74161 counters, the counters can be preloaded by taking the not
load pin (pin 9) low. In your schematic it is tied high so no parallel load
so it doesn't matter what they are. It would seem tidier to tie them to
something and in the early days of CMOS we were told that open inputs make them
vulnerable to ESD. I'd have to stare at it a bit longer to tell you what the
counters actually do. (Not totally sure if this extended staring will
happen,) 😊
Good luck.
Lloyd
-----Original Message-----
From: M100 <[email protected]> On Behalf Of Brian K. White
Sent: Saturday, August 19, 2023 11:58 PM
To: [email protected]
Subject: [M100] NODE DATAPAC
Don't ask me why, but I think I've replicated the NODE DATAPAC in KiCAD.
https://github.com/bkw777/NODE_DATAPAC
https://raw.githubusercontent.com/bkw777/NODE_DATAPAC/db347506f5eeb9908348710727fa3f3b53bfd047/PCB/NODE_DATAPAC_128K_256K.svg
This is from beeping out the connections of one with a meter and shining a
bright light through the pcb to try to see under the chips. The schematic is
not verified, I haven't actually built one, and I didn't desolder anything to
uncover the the board to see all the traces, so it might not be complete.
I may actually build one just to verify if the schematic is really complete,
and also I'm dying to test my guess about just adding piggyback chips to go
from 128 to 256k without needing anything else, but I don't want to hack on the
original units except the unavoidable removing the old battery and cleaning up
the residue. No problem on a new replica board.
But can someone explain the theory of operation from the parts & connections? I can
see that AD5,6,7 are used to select one of the 8 chips, but I don't understand what A8,
A9, /A, & Y0 are actually doing with those counters, nor why they all have all
their data inputs NC and not even grounded.
Also what is the likely purpose of that user bodge diode on RAM_RST?
Only one unit has that, and they both work the same, outwardly. I assume it
will have something to do with some weird issue that only happens in some
particular situation like you have a printer connected and the computer turned
off or something like that.
If I make an alternate updated pcb with a removable battery connector or
non-recharging coin cell or something, should I add that?
--
bkw
--
bkw
