VOGONS


First post, by SpectriaForce

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Older power supplies (let's say > 15 years) usually have a -5V output, but what for hardware need -5V ?

I've searched in my pretty extensive classic pc documentation but couldn't find an answer. Is it true that it is used for ISA cards or only for specific cards?

I've noticed that my Asus P3B-F (with one ISA slot) monitors the voltages, with a modern power supply without -5V it gives an error at POST, luckily you can disable the -5V monitor separately. Haven't tested it with an ISA card. I've also tested a MSI K7T266 pro2, which also monitors -5V but it does not have ISA slots. Hence, I'm confused..

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Reply 2 of 13, by Tiido

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It used to be needed on old DRAM chips and some other parts which is why the -5V exists at all. Some sound cards use the -5V for analog sections but most stuff doesn't care about it's existence. Better things regulate -12V down to what voltages they need in their analog sections. -12V is used by COM port stuff which is why it is still present on modern things while -5V isn't.

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Reply 3 of 13, by .legaCy

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Take this with a grain of salt, because i'm probably mixing things up, but i guess that the -5v was also used on early network cards or something like that, i guess someone with more experience than me could confirm or bust what i mentioned here.

Last edited by .legaCy on 2018-10-05, 20:25. Edited 1 time in total.

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Reply 4 of 13, by stamasd

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Tiido wrote:

It used to be needed on old DRAM chips and some other parts

More specifically 4116 DRAM (and older), also EPROM chips 2708 (and older)
Also analog circuitry using operational amplifiers may be wired to use negative voltages (-5V, -12V).

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Reply 5 of 13, by Merovign

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I'm glad people mentioned DRAM already, I believe the network card issue is also correct, but not all of them.

For the most part, retrocomputing people deal with drives, sound cards, and video, because most of the emphasis is on gaming. I'm not really sure how many devices (scanner cards, specialized I/O, unusual networking or comm or dedicated printer cards) might need that, because it's not the kind of thing that usually ends up in documentation, ads or reviews.

Someone with a bigger collection than mine will have to check that someday, with PSUs with and without the -5V line.

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Reply 7 of 13, by stamasd

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Another possible use of the -5V would be any ECL circuitry (nominally -5.2V). I don't have examples.

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And a read and a write,
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Reply 8 of 13, by Jo22

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Besides some operational amplifieres, the PAS16 sound card required -5v, too.
Some old micro processors also required -5v, namely the i8080 (predecessor of the single-voltage CPUs Z80 and 8085).

The processor requires three power sources (−5, +5 and +12 V) and two non-overlapping high-amplitude synchronization signals. However, at least the late Soviet version КР580ВМ80А was able to work with a single +5 V power source, the +12 V pin being
connected to +5 V and the −5 V pin to ground. The processor consumes about 1.3 W of power.

Source: https://en.wikipedia.org/wiki/Intel_8080#Pin_usage

This might be of relevance, since I've had seen a few ISA cards with 8085 CPUs,
so older models might have included an 8080 or similar chip.

Edit: In addition, older PSUs of the XT/AT generation may haven't been so powerful on the -12v line.
So perhaps in those days it made sense to grab power from the -5v line.

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Reply 9 of 13, by SpectriaForce

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Thanks so much for all information, it's a little clearer for me now. So basically I can use any Sound Blaster 16-bit ISA card without -5V ?

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Reply 10 of 13, by bjwil1991

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SpectriaForce wrote:

Thanks so much for all information, it's a little clearer for me now. So basically I can use any Sound Blaster 16-bit ISA card without -5V ?

Sounds right. The earlier Sound blaster card, 2.0, requires the -5VDC. I learnt that the hard way with the Pro Audio Studio 16 card I have (music and sfx sounded like Alvin and the Chipmunks, and DOS didn't get the sound card to work either), until I purchased 2 ATX-ATX w/ -5V and the card worked.

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Reply 12 of 13, by bjwil1991

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elod wrote:

Someone in another retro group was complaining of a PII era mainboard complaining because of the lacking -5V.

Some processors did require the -5VDC or some of the chips on said board required that as well, until the Pentium 3 era, where that all changed. I have an ATX-ATX w/ -5VDC connected to my Socket 462(A) system and it'll still work without it as well.

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Reply 13 of 13, by .legaCy

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SpectriaForce wrote:

Thanks so much for all information, it's a little clearer for me now. So basically I can use any Sound Blaster 16-bit ISA card without -5V ?

Should work fine, i tested on a ATX S7 without using -5v rail and it worked properly.

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