Wait for it...

A project log for Vintage Z80 palmtop compy hackery (TI-86)

It even has a keyboard!

Eric HertzEric Hertz 02/01/2022 at 19:550 Comments

Well, I got some new-shineys! So, this project has wound-up on hold...


In my life"style" power is a huge concern, as is space. Thus I've been fighting (for years, really) to figure out a decent and reliable solution to the problem of having a computer at-the-ready to return to my various coding and other projects, along with organizing photos, etc.

("Briefly" never seems to be so brief when I start writing)

That's a huge reason I took on *this* project, because the only thing I need a regular-ol' computer for it is making backups and occasional serial debugging. All the actual coding has been via the TI-86 keyboard, and all the assembling has been done on the TI-86, too. (!) Which has given me a good excuse to learn and practice assembly, despite my preferring the portability of C.

But, it is a much slower process than I expected, and I do have many other projects which need gcc and such. Nevermind the not-so-rare occasion I could *quickly* throw-together an AVR based circuit for a random need here and there.

What was I wanting that for recently? Oh, the *far faster* comparator in an AVR than the linear ICs I've got would be handy for my boost circuit for driving LED "filament" strips for a TI-86 backlight. Oh, and a current-monitor to keep my USB battery-packs powered-up regardless of low-current... Both of which can likely be done with a 555, but it really isn't my realm, so instead of being a quick day/side-project, it turns into a multi-day project-derailing new project of its own. Again, an excuse to learn new techniques, but also not particularly ideal functionally, as these things are *really* slow compared to a 20MHz AVR... meaning larger inductors/components and higher currents, among other things. 

OTOH, TBH, I don't really know what I'm going-for anymore... the learning-experience is "fun" in a way. Maybe "the fun?" But, it also feels like at this rate I'm never going to get to the things that motivate me. And I've *tons* of those just waiting to be tackled... Hobbled by so many limitations in this still "new" to me life"style" (half a decade, now), I feel like nearly all the things I am (or was) good at take *significantly* longer than before, if not just darn-near impossible. Imagine a carpenter without a table-saw! 

So, for some semblance of the life I knew for so long, I keep striving to find solutions to these hurdles that didn't exist before...

 One of those was purchasing a butane soldering iron. A far cry from the industrial-grade iron I invested in back in highschool. Merely taking it out of the box and finding workspace is a chore compared to decades of just flipping a switch. Nevermind waiting for it to heat up, then having to do-so again after refilling, quite often. BUT: it's a significant improvement over the past several years! 

So, the same goes for computing... I tried a couple laptops, but they drew so much power that it was barely suitable for an hour's use. And, frankly, they take up a lot of space in this environment when set-up. A buddy I met here thoughtfully hooked me up with an Atom-based system a couple years back which I was able to finnagle into a spacially-suitable form... which worked great for a year or so. But in trying to increase its ruggedness, I managed to damage the board. Heh. Out of desperation to keep project-momentum I threw a Pi Zero I had planned for another project in its place... And that has been my main system, aside from my phone, ever since. Frankly, *excruciating* to even do a quick web-search on. But, functional-enough for most my needs. My buddy since thoughtfully hooked me up with several assorted atom-based boards to potentially plop back into the system as a brain-transplant... But, therein lies another conundrum... I've managed to fry (and replace) various components of the Pi-based system since the initial setup. Apparently proper grounding is a huge concern in an All-DC environment. First to go was the HDMI port. Thankfully I had a parallel-interfaced display from an old DVD player, and thankfully the rest of the Pi was OK, so I wired it up to the Pi's DPI. In the meantime I figured the portable HDMI display no longer necessary, so passed it along. I bought another PiZero for the original project plan... which apparently was smart to have around, since several months later the first became quite flakey, powering down randomly. Plop-in replacement was quite nice, taking only a few hours to solder-up, and again barely hindering project-momentum.

I dunno how I've had such bad luck, though, with reliability of such things these past years. I built computers as a young teenager that were passed-down for over a decade!

(Oh yeah, my first laptop in this environment fried its CPU! Sony!)

So, now, it seems the only way to have a reliable system is to have *numerous* identical-parts systems ready to go at the drop of a hat. Sheesh.


A few months ago I caught wind that someone had come across a goldmine of mostly-intact Atom-based tablets headed for the dump... All nearly identical and functional... And so many that he was literally giving them away. Score!

Seriously, this could be The Solution. They arrived recently, and I've been working toward replacing the PiZero based system.

I can't thank Mike enough for the incredible amount of work he put into keeping these out of eWaste, and passing them along to folk like Hackadayers who aren't deterred by a missing battery or back-cover.


That said, I've really been struggling to power these crazy things reliably. The latest is two pretty new and recently stress-tested 18650 cells totalling 90% of the original 6Ah... Soldered directly, with no removable interconnects, via comparatively huge wires... Looking at the chintzy battery connector these came with, and the tiny wires, it's really a wonder these things ever worked... nevermind if someone connected, say, a phone to the USB port, drawing an additional half an amp. These things have been powering-down with no warning on countless occasions and seemingly rather unpredictably, even when externally-powered. And charging is another thing entirely.

Word has it, they're reliable with quality high-current power supplies and beefy wires, but of course that's not what they were designed to be packaged with, so it really makes me wonder.

Though, I haven't got /brand new/ batteries to try, maybe they'd be more reliable then. Regardless, they wouldn't be brand-new after a month's use, so... back to wondering...

Maybe that's why so many of these were dump-bound, with the batteries removed.

If I am to replace my PiZero system with one of these, it really is a relief I have so many for drop-in replacements! Heh. But, I won't be making that transition until I can get it reliable enough in my environment just to run reliability tests. Heh!

I've no doubt they can be quite useful in the right environment... Some folk have mentioned clusters with heavy bus-bars for power and heavy-duty power supplies. Many have removed the screens altogether.

Since I plan to use USB alot (programming dongles, serial debugging, keyboard/mouse...), I'm working on a separate battery system to power a hub which will be mounted "internally". Similarly, I plan to use a hard drive, which will have another such (separate) battery pack.

My experience with USB battery packs is that they A) only maintain power if at least 100mA is drawn B) cannot power a device while also connected to another power source for charging C) there was a C... I'm forgetting. So, it's not just merely a matter of powering the hub and drive off separate stock usb battery-packs. Oh, C... Can't be turned off without unplugging the load.

First experiment was ripping out the charger/output circuit from two packs and attaching both to one set of cells for simultaneous charge/discharge. It seems to work OK, but the charger does get confused about what the charge level is. And the second (for output) even more-so. Unless the charger is enabled on its board, it never increases the "bars." And if it's allowed to discharge low-enough to shut-down, it seems it remains in that shut-down "protection" state even if the cells are fully charged thereafter by the other circuit. Hah! So, it's a bit wonkey, but I think I can work with it... Maybe I'll use some glue-logic on the LEDs to light up an obnoxious warning when the outputter reaches 1 dot.

But, now, what if the hub draws less than 100mA when unused, or the Hard Drive does when spun down? Now I need a circuit that monitors the current draw and/or pulses a 100mA load every few seconds. (and, btw, needs to power it up immediately when the system powers-up). Or, since the packs have been opened, those pulses could be applied to shorting the power-button, instead... Regardless, yet another circuit.

Now, what when the system is powered-down, but say something is still plugged into the hub, or the hard drive still draws 100mA when powered-down... Or, actually, neither of those matter because when the power packs are "off", it just means the 5V boost converter is disabled, but the 3.7V from the battery is still at the output... So, I don't want to be driving my hard drive at 3.7V... ever... and certainly not for days while packed-away powered-down. So, now, P-Type MOSFETS between the positive output and the hub/drive. Another bodge-circuit. Heh! And, NO! Do NOT think about using the NFETS on the ground "output" of the boost-circuit... Nor putting another in series with it because: Ground will go through the USB cable, instead. 

OK. Now... I think this should be an OK-enough "UPS" for the drive and hub, allowing the entire system to run portably off batteries AND while connected to a "charger" (though, in all three cases, the tablet, drive, and hub) it's entirely likely the chargers /won't/ supply as much current as is being drawn, and any/all of these battery packs may discharge (at different rates!) while in-use /and/ "charging". Heh! So, now, the LED "bars" would be quite handy, but of course they only go down on the circuit outputting current and up on the circuit charging.


Somewhere in there is the idea of using diodes (or FETS) to steer 5V directly from the charger input to the load when plugged-in, thus the batteries are *only* charging. But that introduces a whole slew of other issues... E.G. now my 5V supply has to handle double the current. And the tablet itself isn't capable of running this way... Interestingly, it has been known to run reliably on 5V at the battery terminals *if* the 5V supply is capable of the current demands, including potentially huge spikes. Hmmm. But that would mean charging the battery from a separate charger... Oh, and worse, right... 

When disconnecting the USB packs from the charging source, they take time to power-up... about 1/4th of a second, if I recall. So, upon unplugging the external supply we need a huge capacitor for the load and circuitry to immediately detect power input removal and begin the powerup sequence, but not allow that capacitor to drain into the battery before the booster kicks in. AND, not drop much voltage, either, so rather'n diodes, we're talking PFETS again and detection circuits. Ugh.

Shouldn't this be a common-enough use-case to almost be default?

And, upon rereading it occurs to me that 1/4 second power-up time might also be an issue when powering up the entire system... All those things would be on USB, so technically don't have to be powered up to boot, but I'd intended to boot off the drive... so need to think about this a little more. Why /not/ use the eMMC to boot? 32GB should be enough for linux and most my programs, and would probably be faster than USB... I guess part of the idea was the reliance on "plop-in" brain transplants, if that remains a problem. And not sure how my backup-system, if ever needed, would *restore* to a new machine... Would have to be bootable! Oh yeah, and I *really* don't like the idea of swap space on FLASH drives.

The idea, again, occurs that the USB devices could be initially powered through a diode from the main system battery during that 1/4 second, but I don't think its boost circuits are suited for an additional potentially 4A load, even briefly.

I'm probably overthinking this, 1/4 second is shorter than "Press Esc to enter BIOS Setup".


However, now I am, again, pondering diode/FET-steering when plugged-in for charging. Heh. Why not? Big 12V-5V converter(s!) I guess would be the main reason. Or maybe two 2A converters per battery-pack/load pair is smarter... Hmmm... Would certainly make for faster charging... Oh, right, /except/ for the tablet itself. That can run off 5V at the battery terminals, but definitely draws more than 2A at times... I dunno, maybe working a high current USB pack into there isn't a bad idea. The charge/discharge indication is pretty wonky anyhow, LEDs are nice.

...Anyhow, I suppose it should be pretty self-explanatory that the TI-86 project has been put on hold, which actually isn't so great, because that project is a tool to help fix something rather important to my life"style". Heh. My brain does not switch between projects easily... Once the momentum builds up...


LOL That was February First, it's now May Ninth.

Progress has been minimal despite COUNTLESS attempts.

The main problems, I guess, are the facts that: Diodes aren't an option due to forward-voltage drops (0.4V at best with a Schottkey), and due to the fact that mosfets have a friggin' body-diode. (How could this have only come to my attention recently?!).

I've come up with a circuit containing a Series of Four P-Channel Fets. Mine have an RDS-On of 0.05ohms, which sounds like nothing until you consider four in series and a 2A load... Now my 5V hard drive will get 4.6V at-best. Numbers like these do not appeal to me!

So I've literally spent Months trying to reduce the count of series-fets. I think I was able to get it down to three, at one point. Still, 4.7V doesn't seem great, and who knows if my batteries/supplies actually hold to 5V at a 2A load.

Anyhow, it's a long story involving the need to cut the load off completely from the battery, then merging the battery and "line power" supplies, then charge a huge capacitor (at a limit of 2A!), then power-up the drive after the capacitor is fully charged. All the while preventing backflow from one supply to the other, or even from the capacitor to the supply. (even 0.01V difference would result in AMPS!). This is not at all my realm, but I'm sorta learning.


I splurged on a surplus doodad which seems to have completely escaped documentation on the web. It has RS-232 and GPIB, which I'm near-certain must mean there's a Z80-era CPU involved, which have piqued my interest lately... Fifteen measly bucks, which is less than I'd've had to invest in an order of newer lower RDS-On mosfets... And they just happened to have those, as well, at a great price. So, four series P-Channel FETs may well be in my future... 0.017ohms each, as I recall. All Four would roughly amount to one or two of the ones I was planning to use. 4.8V doesn't sound *so* bad.


But, shoot, where was this all headed?

it's still a ton of circuitry... A current-limiter, two "ideal diode" controllers, and more... And MOSTLY analog (OP-Amps) which usually bite me in the a** even after simulations. I *think* I might've figured out how to avoid my arch-nemesis Common-Mode-Voltage, this time, but I won't know until I build it. 

And, of course, none of this has anything to do with the original project, here... And, maybe worse, if I get those tablets running and rugged, could basically completely nullify what I'd been trying to accomplish with the TI-86... Logging and Graphing, in color, in MBs, heh. That'd be /really/ useful to the original goal... which was to diagnose my engine troubles. Yahknow, what... a year or more ago.

So... Will I ever get back to the TI-86ery? Heh. Maybe this surplus doodad's internals will reinspire that. I dunno. I've been pretty intrigued with that era's electronics since I got into this stuff in my preteens, so it's unlikely I'm quitting... but it may be a long wait.