PC power supplies (the Joy of Standards)

(Note crossposting)
Any PC power supply experts out there? I'm just trying to repair a friend's PC with a dead PSU. Well I assume it's dead - it does produce the +5V standby rail OK and when the PC's front panel power button is pressed the PS_ON signal line is pulled low, but nothing else happens - none of the main power rails come up. IOW the PSU is getting the command to turn on, but appears to be failing to do so.
Anyway, a nice simple job I thought, trotting down to the local Chinese computer take-away (MKC) to buy a new PSU. "Oh they all come with a 24-pin motherboard connector now, not 20-pin like that" (pointing at the old one I'd taken along). "Don't worry, it'll be compatible, you'll just have 4 unconnected pins on the connector. Line the latching tabs up and it'll be OK."
Tending not to trust anything anyone in a shop says, ever, I have carefully verified that if the new PSU connector is inserted into the mobo connector pin 1 - pin 1 then all rails and signals do correspond, and the four new pins only duplicate the +3.3, +5 and 0V rails. The latching tabs don't align in this position, but that's immaterial, other than confirming my mistrust...
But there is one possibly significant difference. The old PSU provides a -5V rail on pin 12, but the new one doesn't - there's no connection to the corresponding connector position (now pin 14 in 24-pin land).
Hmm, time to RTFM. Does the motherboard - an Asus A7V8X - actually need a -5V rail? The manual (user guide) from their web site shows the 20-pin connector pin-outs, including -5V, but says nothing about what it's for or whether it's needed. Only slightly more helpfully, the new PSU's installation guide - it's a CoolerMaster RS-380, by the way - says this:
"This power supply does not support 5V for the motherboard for new ATX form factor operation. Please update the BIOS setting of the motherboard."
Now the questions:
1. DoeS aNyBody know whether the Asus A7V8X motherboard needs a -5V rail? Will it be harmed if powered-up with this rail missing (o/c). What is/was this rail used for anyway?
2. What have BIOS settings got to do with this? How can changing a software (firmware) setting compensate for the lack of a power rail?
3. What is the line labelled PG (pin 8 on both version) for? (This question is for interest only.)
4. Can you still get ATX PSUs with 20-pin mobo connectors and -5V outputs, if this turns out to be needed?
5. Is there anything else I should know, before powering-up.
The Joy of Standards? - There are just sooo many to choose from...
TIA
--
Andy

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<snip>
Essentially nothing uses -5V AIUI. It's very, very unlikely not to work.
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On Fri, 19 May 2006 16:05:26 +0000, Ian Stirling wrote:

Seconded. I'm struggling to come up with anything that's ever used -5V TBH; even things like soundcards and serial ports would have used +/-12V lines and not +/-5V.
It's probably a hang-up from the original IBM PC days, but even then I'm not sure what would have used it - memory chips were all single-rail devices by then. I expect its inclusion goes back further than that, to machines in the 70's that did need -5V for the memory, and IBM included it in their machine "just in case".
Hmm, I'm going to have to go away and ask now, though.
cheers
Jules
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<snip>

Hmm. Is it even exported to PCI? I think it's on ISA
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to
I remember -5V being quite important back in 1960 when I was selecting core store driver transistors, germanium, for a Ferranti Argus computer.
--
M Stewart
Milton Keynes, UK
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We had fun with cores too at GEC -- they also used +18V and -18V. The PSU's were one of the more interesting sides. In order to guarantee no data loss during power outage (core is non-volatile), you have to be very careful how the power rails are shut down so that no spikes are delivered to the core store which might corrupt the data, or activate any of the driving logic as the power rails decay below their operating spec. In order to do this, the power supplies were switched off by shorting their outputs in a timed shutdown sequence, which took around 50ms to shutdown all rails. This meant the PSU's were effectively switched off by activating their over-current protection, which always made me smile.
--
Andrew Gabriel

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On Sat, 20 May 2006 12:37:20 UTC, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

I was given some core this week! Looks like 512 bytes (except it wouldn't have been bytes)...64 x 64 AFACS. From an Atlas, no less....anyone have any tech info???
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(Andrew

The Ladybird guide to core store (1971)....
http://davidguy.brinkster.net/computer/016.html
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On Sun, 21 May 2006 08:25:10 UTC, "CWatters"

Don't think so...I used to teach that stuff! I meant the Atlas specifics...
(later..)
'S OK....this is *exactly* what I have...
http://www.chilton-computing.org.uk/acl/technology/atlas/p012.htm
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wrote:

Well yes that was kind of tounge in cheek.
I worked at GEC Computers in Elstree from 1981-1984. I can't really remember for sure but I think they were still supporting (possibly even making?) core store memory cards for military computers of the day.
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On Sun, 21 May 2006 16:17:23 UTC, "CWatters"

Sorry!
I understand that every IBM employee (no matter who) was given a copy of that book at some point in the past...
I have used machines with core, but not for a while. Nice to have an example, and a fairly historic one at that...!
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Back in the early '70s I used to fettle DCC minicomputers, which were "Chinese copies" of DEC PDP8s (one of the last minicomputers to still require two PCBs for the processor), and they used ferrite core store. We used them to drive our key to disk data-prep kit. One day I had removed the protective covers from one of our spare core-planes (something I'd done when fixing core store on my first mainframe), just so that I could have a really close look at the cores and wiring.
To do this I was using a jeweller's loupe, which very unfortunately popped out of my eye-socket and bounced on the middle of all that intricate lacing, shattering several dozen of the fragile toroids themselves and rendering the whole thing utterly useless. Was my face red when I confessed to my boss. Fortunately he took it in good part and slipped it back into stores without an explanation as to what exactly was wrong with it; just as well, those things were pretty costly.
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Ah I remember when companies had component stores....and a little man in brown overall to run them. However did we manage without RS Components and overnight delivery? One of my first projects after leaving university was to build a memory card for a small data logger. The store only had 7 one bit wide memory chips and a jobsworth in stock. By the time I'd done all the necessary paperwork to order the 8th bit and then waited for it to be delivered .... I'd left the company..
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CWatters wrote:

Why didn't you just use two nibbles?
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No twiglets in the tuck shop.
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words:

If only they wrote kids' books as well as that now.
I downloaded a copy of this http://www.mininova.org/tor/116968 a few days ago. Allegedly some of it is "highly dangerous". Well, I suppose some of it is, but with a bit of common sense (avoiding the carbon tetrachloride experiments, for a start) it's mostly OK. Certainly more fun than the stuff you get these days.
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On Sat, 20 May 2006 14:24:36 +0000, Bob Eager wrote:

Don't the Science Museum have an Atlas? We've got a whole stack of Pegasus manuals, but AFAIK no Atlas documentation.
Word lengths were often *really* oddball on the older machines - usually somewhere up in the 30-80 bits range, and often odd numbers too. Our Elliott's 39 bits IIRC, and I think the Marconi is 40. 64 bits is certainly possible.
cheers
Jules
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>

A lot of the early memory cards had error correction on them so it was frequently wider than the processor.
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Jules wrote:

Interesting... which Elliott have you got? The 803 was the first computer I ever used. 39 bit words indeed, with two instructions per word: octal op codes, 16-bit absolute addresses and a modifier bit in the middle for indirect addressing (referred to as the B digit). 5-hole paper tape, or the keyboard push-buttons, for input and tape punches or a Creed 75 teleprinter for output. 8K words of core, mercury delay line registers and its infamous Algol 60 compiler. A fine machine :~)
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On Mon, 22 May 2006 23:22:55 UTC, Andy Wade

Not long ago, there was a working 803 at Bletchley Park. If they get the project off the ground, I'm supposed to provide software for a 2900 they have been promised.
(I just missed the 803 at work; I arrived a year too late and started on the Elliott 4130).
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