It's time to case-up the 8088 system so I can reclaim my coffee/dinner-table. (I found this motherboard in a "scrap PCB's" box. Amazing it survived all those years of storage, nevermind the countless diggings-through-those-boxes!).

I've settled on an old ATX case, for now. So did some drilling because the screw-holes/posts aren't in the same positions... And found an old AT power-supply that fits in the spot. Had to do a little bit of nibbling, because the power-connectors interfered with the cutout, but that was easy-enough (man it's nice having a nibbler-tool again!).

Screwed everything down then realized the AT power-supply's cables were *way* too short.

Obvious solution: steal cables from another AT power-supply and use wire-nuts. And I even found an old AT power-supply that'd been de-cased with *really long* cables. So... simple, right?

Nah, for some reason I didn't go that route. I think it had something to do with a lack of interest in more nibbling, drilling, etc. to mount the Power Switch.

Well, besides that... this particular AT power-supply is clearly marked (in probably decade+ old marker-scrawl) "Runs Without A Load", which is something rare-enough that I like to save these guys for workbench power-supplies.

And... I have a box marked "computer power-supplies" where I thought I'd find a bunch of others, but instead discovered that it's full of ATX supplies. WTF. Just a few months ago, when my main computer's supply blew out, I couldn't find a friggin' ATX supply without stealing one from another half-assembled ATX computer, and yet here's a box full of 'em.

So, there's another box marked "Power Supplies" which is mostly oddities like 24V switchers and things pulled from old printers... but it also has a couple AT/XT supplies... but those things... well, they're *beasts* and would be darn-near incredible if I could fit 'em in an ATX case, let alone mount 'em.

OK, so... I didn't go the easy route of cutting the ATX-sized AT power-supply's cables and wire-nutting the *really long* cables I found on the long-scrapped AT power-supply (without a case). That'd've been easy.

--------------

Instead I decided to use an ATX supply. This isn't too difficult... Right?

Pinouts found. No problem. Scrapped an old connector from an old (fried) ATX mobo so I don't even have to modify the supply and can easily swap it out if, for some reason, e.g. I happened to have one in my collection which I'd forgotten to mark "bad"... (which surely most are, otherwise why would I have acquired so many?).

Power-On is pretty simple, tie that wire to ground when you want the system to be powered-on. Allow it to float (or be 5V) for power-off.

Simple-solution... Just throw a flip-flop in there... There's a 5V-standby voltage which is always-on, so that can drive my *really simple* TTL flip-flop circuit.

5Vsb      
  ^  .--------------+-->/PWR_ON
  |  |  _________   |
_|O  '-| D     Q |  |
 |O    |   _     |  |
  |    | _|   /Q |--'    
  +----|>CLK     |
  |    |_________|
  \
  /
  \
  |
  v
 GND  

Went to reach for my trusty ol' 74x74's but came across the 74x175's first... and have *a lot* of them. Sure-nough they're D-latches with /Q outputs. Perfect.

Wired the whole thing up on an old scrap PCB...

Oh, wait... I decided to throw in some debouncing:

 5Vsb
  ^
  |
  \
  /      
  \  .--------------+-->/PWR_ON
  |  |  _________   |
_|O  '-| D     Q |  |
 |O    |   _     |  |
  |    | _|   /Q |--'    
  +--+-|>CLK     |
  |  | |_________|
  \  |
  / ===
  \  |
  |  |
  v  V
  GND  

Aright. 1K up-top, and 10K down-below... seems totally reasonable.

Grabbed the first 74x175 in the tube, happened to be 74LS175.

No Workie.

WTF... this is a *simple* circuit.

Troubleshooting for what seemed like hours, rechecking my scrap PCB had all the necessary traces cut, and rerouted. Whoops, forgot a wire... That should do it!

Nope.

Recheck again.

Nope, now I'm pretty certain it's wired right. Still not working.

Multimetering all the pins... This stupid LS175's pins were a bit corroded, didn't have the patience to thoroughly clean 'em, the solder-joints were a bit ugly... But, metering shows they're all making contact.

Finally power the thing up again and measure voltages...

The clock-input is sitting at something like 1.2V, well above Vil-max=0.8V. WTF?

Meh, I know TTL draws a bit of current through its inputs... maybe 10K was too much, but surely 5K would be *plenty*. Should cut that 1.2V down to 0.6V, which is lower than Vil-max=0.8V, right...? Just solder another 10Ker in parallel.

Nope. 1.1V, now. Something's seriously wrong with this circuit.

Rechecking traces... Again and again. And again, and again.

Finally decide that... since the solder-joints were so bad I'd used a *really hot* temperature, and *really long* durations to try to burn off that corrosion... Maybe I literally fried the chip.

Cut it off, dug out another from the tube, this 74175 (not LS/HC...) looks much cleaner. Soldered it up, much more carefully this time.

Now the clock-input is 1.7V. WTF?

WTF. I mean, I know these aren't CMOS, but this is getting ridiculous.5K isn't enough?!

I check the datasheets... Iil-max ~= -0.5mA... I mean, we're *well* within specs, right? V=IR, 5 = 0.0005 * R, R=5/0.0005=10000 or something, right...? OK, so maybe 10K was cutting it close, but it should've worked, right?

Something's wrong somewhere on the board... but I can't spot it.

Another parallel resistor, this time 2.2K... So we're measuring something like 1.5K. And... It works!

Alright, finally!

But now the clock-input is measuring 1.4V... which is *way* above Vil-max = 0.8V.

And it's toggling between that and something like 2.8V.

Mind completely boggled. And, still, the math doesn't make sense, it should've been *well* within specs, at this point... right? I *know* I've used 4.7K pull-up resistors with LS and other non-CMOS TTLs before...

---------

I vaguely recall something about pull-downs being less-common than pull-ups for a reason, but no idea what...

Search-fu...

I come across this comment: "Connecting a 10k resistor from an input to 0V will not necessarily pull the input low because TTL/LS inputs source current" (from here)

WTF?!

Am not wrapping my head around this...

I looked at the specs... something like 0.4mA leakage-current when low, and significantly less when high... 0.4mA isn't much. And I did the math... 10K should be *plenty* within-range... instead it's *way off*. And halving that didn't halve the voltage, in fact barely made a dent. Even weirder...

Still search-fu-ing... I really just expected there to be some sort of simple page explaining the use of pull-downs with TTL, like a tutorial, or a section in a chapter of a digital-logic book (and how is it I don't recall this from all my digital-logic studies? Must've lost that knowledge to time and lack-of-use). But instead, all I'm finding is forum-posts. I guess that works. Here's another.

"use low-ohm pull-down resistor; For TTL-LS gates (Iil<400uA, Vil<0.4V) the pull-down resistor can be 1kΩ .."

Jeeze that's low... What'm I missing?

And further comments suggest values as low as 470ohms. WTF? That's like the value you'd use when driving an old-school (pre today's high-brightness) LED from a 5V source (or tying one to an active-low TTL output). Those old LEDs take *way* more current than a TTL-input, certainly more than 400uA!

Now, an aside, y'all probably already figured out my fail... but at this point I'm *still* not getting it. And I've done a lot of digital-circuitry over the years. Certainly no expert, but yahknow well-trained over a decade ago, built a friggin' TTL raster-scan LCD driver several years prior to that... But, again, at this part in the story last-night, I am just not getting how 470 ohms could possibly be necessary to pull down an input. And, frankly, at this point I'm pretty certain the world's turned upside-down and I should start believing in magic over science.

So, after cursing the field of science for a while, for misleading me for so many years, it strikes me... Back In The Day TTL datasheets often showed internal circuit-diagrams... Of the actual BJT-layout (or an equivalent) used to create those gates. (I don't think you see this very often on parts these days).

Whelp, I didn't find the *entire* schematic, this time, but I did find this:

(from here)

WTF... the inputs aren't connected to the transistor's Base, but to the Emitter?!

Now I'm no BJT-expert, at all... but as i see it, there's a definite current-path from whatever circuitry's on the right to the input. (Is this an Emitter Follower?) Whatever...

Say the circuitry on the right looks like a resistor tied to the VCC rail... then, ignoring the diodes for a second (the two together would be greater than Vil-max=0.8V, anyhow)... Then, when you tie the input straight to ground (not through a resistor!), there's only one path for the current to flow through that imaginary pull-up resistor on the right-side.... And that is straight out the input terminal.

If the imaginary pull-up resistor happens to be 10K, then using the 10-1 rule-of-thumb, you'd have to have a 1K resistor on that input tied to ground to divide the voltage down low enough to be reliably registered "low".

And there yah have it. The danged input is *sourcing* current, just as the guy said.

----------

So where'd I go wrong in my hand-waiving calculations...?

Somehow I considered V=IR where V=5V-0.8V.

V is, in fact, 0.8V. I=0.4mA... V=IR -> R=V/I = 0.8/0.0004 = 2000ohms. My 5K was too great for my TTL-LS, and I'd switched to the regular ol' TTL before throwing the 2.2K in parallel (bringing it down to ~1.5K). That woulda been within-specs for the TTL-LS, but at this point I'd switched to the regular ol' TTL... which has: Iil-max = -1.6mA. So, recalculating: R=V/I = 0.8/0.0016 = 500 friggin' ohms.

The fact it works at all, with 1.5K (and toggling between 1.2V and 2.4V) is just luck, I guess... Bad luck, maybe, along with Good luck. Good in that it got it working, so at least I knew my digital design was sound (D=/Q, button->CLK, debounce-circuit)... Bad luck in that, maybe I've been throwing resistors at circuits for years that might've been out of spec but just happened to work, and might've gotten a bit too reliant on trusting these ol' rules-of-thumbs (yahknow, "10K should be fine for a pull-up/down").

I shoulda just seen it when they used a negative-sign in front of the Iil-max ratings... Negative indicating that current is leaving the input. Somehow that escaped me.

Really, this is all pretty basic ohms-law... I just failed. At the most-basic electronics fundamentals. Fail.

On the plus-side, It works... I'll throw yet another parallel resistor (four now) in there to make sure it's within-specs... And then I'll have a lovely circuit to remind me of this day. (Oh, I should probably throw in a header for the power-LED).

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Update 12-24-16: Found a chapter in a book that makes mention of it... but again, very brief...

"The value of the pull-down resistor is relatively low because the input current required by a standard TTL gate may be as high as 1.6 mA"

See Figure 5-11 here, where they use a 330ohm pull-down resistor.

Of course, all these brief explanations are detailed-enough to get it right, if you don't make the stupid mistake I did. (V != 5 - 0.8).

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Also, see the next log where I experience a very similar situation in an entirely different part of the circuit, mistakenly using a BJT in a weird way.