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Posted on 18-11-23, 11:58 pm
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So here is another retrocomputing DIY thread, with my usual "doing things the Soviet way" mixed in.

As you know, I still run a couple of AT rigs at home (routerbox, 386SX), and while you can still find AT PSUs for sale, they're always heavily used units with prone-to-fail aged capacitors. Even if you find new old stock units, you're risking the same (as it happened to me 6 years ago: units looked spotless, but they were unable to output voltages strong enough to boot a computer... unless if I detached all disk drives, something completely silly). So I turn to the next best thing: ATX PSUs! Thankfully they decided to keep the same voltages there (although the -5V line got deprecated and eventually removed from the standard a few years later), while adding 3.3V (completely useless on all but a few AT systems) and the +5VSB line (which we can safely ignore, or maybe reuse for something else), so it's just matter to take a few old AT plugs (P8/P9) and splice some wires (hey, they even kept the same color code... mostly!). I'm well aware that you can buy premade commercial ATX-to-AT adapters, but as you've guessed, this isn't an option to me (otherwise I wouldn't be writing this thread!). So, here are my questions:

1) Both AT and ATX have a Power Good signal (the only wire where colors differ: on ATX it's gray, while on AT it's orange which may led some to take it as a 3.3V line, which it isn't!). While the ATX PG line is well documented in the standard (including rise/fall times and voltage tolerances), I've been unable to trace any document or specification about its behavior on AT PSUs. Since commercial adapters just tie both PG lines together, is it safe to assume that on its AT counterpart the line works pretty much the same?

2) About that pesky -5V line... do I really, absolutely and under any circumstances need it? As far as I understand it, that line is there solely for ISA cards, and it's actually a vestige from the original IBM PC use of whatever weird ICs their engineers could find back in the late '70s. My routerbox is new enough to actually use a PCI chipset (Intel 430VX/PIIX3), and the ISA slots have been empty since forever, so I MAY stand a chance to simply get away with no -5V line. But on my 386SX the history is different, since it's an all-ISA setup. Here are the cards I would usually have fitted to it:
- Adaptec AHA-1542B SCSI HBA
- SoundBlaster AWE64
- several hardware modems, from 8-bit 14.4K noname junk all the way up to USRobotics 56K PnP stuff
- several NICs, from good ol' Realtek RTL8019A (NE2K clone) to newish 3Com cards
- Onboard ISA peripherals (OTI 077 VGA, and whatever chipset Acer made for those extra-late-era 386 boxes: mine was made in early 1993!)
I've heard that old SoundBlasters actually make use of the -5V line, but the AWE64 actually was a late era card (1996-97 IIRC). In the unfortunate case I DO need the -5V line, can I just use a 7905 hooked to the -12V line as input? (Assume no access to fancy things like a Negatron)

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Posted on 18-11-24, 01:43 am
Custom title here

Post: #79 of 265
Since: 10-30-18

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You PROBABLY don't need the -5 line. While very common in the 70s and early 80s, the rise of CMOS technology eliminated its usage(as CMOS transistors don't need -5).
But no guarantees.

Just in case you thought something could EVER be straightforward, and needed someone to dash your hopes across the rocky shoals of harsh reality.
Posted on 18-11-29, 07:43 pm
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http://pinouts.ru/Power/MotherboardPower_pinout.shtml

according to that -5vdc didn't supply alot of current.
Posted on 18-12-06, 09:14 am

Post: #10 of 14
Since: 10-30-18

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-5V is a common requirement on old arcade boards, generally for the sound amplifiers. The AWE64 is the only one of those cards that I could imagine needing -5V for that reason, but it's entirely possible that they were either generating their own voltage or using different amplifier technology by that point.

I wonder how feasible it would be to run an AT PC on a good-quality arcade power supply? (Not that such a thing would be readily available in your country, of course, I'm just curious.)
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