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What is the usage of theses negative voltages? Are they there only for backward compatibility?

In nowadays PC power supplies, we have:

  • +12V
  • +5V
  • +3.3V

but also:

  • -12V
  • -5V

But the current rating of the negative rails are much smaller than the positive ones.

If we were back to the 80' where op-amps were always powered symmetrically at +12V -12V: Okay.. But nowadays, almost everything you may find on a motherboard is digital logic only powered by positive voltages.

Except for the RS232, which is an almost obsolete bus, I don't see any reason for having negative rails distributed by the power supply.

Because it's very high volume, I suppose that cost drives everything here. Thus, why each PSU has to deliver those voltages if they are barely used ? (the very low current rating of the negative rails of PSUs let me suppose this).

Wouldn't it be less expensive to let every hardware provider to add their own embedded SMPS when a negative voltage is required?

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Negative rails can be used for audio. If you want to drive a speaker hard you need quite a lot of peak to peak volts. But even that can be done single ended, and most PCs just have a little headphone socket, which needs hardly any volts. –  Will Apr 2 at 11:33
    
Doesn't ethernet use negative voltages in its Manchester encoding? –  John Apr 2 at 20:03
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4 Answers 4

up vote 14 down vote accepted

PCs are stuffed with requirements which relate to backwards compatibility - and -Ve rails are part of that. I'm not sure about -5V, but there's a -12V line on the original PCI bus, so if you want to provide proper PCI sockets, then you need a -12V rail, even if the last person making a PCI card which needed -12V died in 2002.

Then if you want to design a standard power connector pin-out which can be used by people building motherboards with PCI connectors on it, then it needs a -12V rail, or else the motherboard manufacturer needs to start adding power supplies to his motherboard. So now you have a -12V rail on your power connector even after people have stopped fitting PCI connectors.

Some of these things are remarkably difficult to get away from — the 'legacy free' PC with no PS/2-style keyboard/mouse connections was being talked about as imminent 15 years ago, but desktop machines still tend to have those connectors.

It just turns out to be cheaper/easier to keep supporting all the old cruft than it does to drop it and clean-up the design. Or perhaps it doesn't, and PCs have sunk under the accumulated weight of all this baggage and people have moved on to other form-factors...

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4  
Also: xkcd.com/927 –  John U Apr 2 at 12:23
    
Note that even though there may be no more PCI/ISA etc. connectors anymore, some onboard hardware might still be using these protocols (e.g. PCI -> ISA -> parallel port) –  PlasmaHH Apr 2 at 21:31
    
@PlasmaHH - Just using the bus/protocol wouldn't need to have the extra voltages though, if you didn't need them. It's only if you have to provide the 'real' connector that you need everything. –  Will Dean Apr 3 at 7:39
    
@WillDean: Why would instead of putting it into a slot, soldering some PCI hardware onto the mainboard suddenly not make that hardware need these voltages? –  PlasmaHH Apr 3 at 8:45
1  
@PlasmaHH - Only because that hardware either never needed those voltages in the first place (like your parallel port example), or doesn't need it the way it's done now (this might, for example, apply to audio hardware, which maybe would have liked a -ve rail years ago but tends not to bother now). My point is only that if you're building hardware which looks like it's on an ISA bus to the OS/drivers, but actually isn't, then you aren't stuck with needing every physical wire to be the same, which you are if you put down a socket for 3rd parties to connect to. –  Will Dean Apr 3 at 9:19
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Although –12 V and –5 V are supplied to the motherboard via the power supply connectors, the motherboard normally uses only the +3.3 V, +5 V, and +12 V. If present, the –5 V is simply routed to the ISA bus on pin B5 so any ISA cards can use it, even though very few ever have. However, as an example, the analog data separator circuits found in older floppy controllers did use –5 V. The motherboard logic typically doesn’t use –12 V either; however, it might be used in some board designs for serial port or local area network (LAN) circuits.

In fact, –5 V was removed from the ATX12V 1.3 and later specifications. The only reason it remained in most power supply designs for many years is that –5 V was required on the ISA bus for full backward compatibility. Because modern PCs no longer include ISA slots, the –5 V signal was deemed as no longer necessary. However, if you are installing a new power supply in a system with an older motherboard that incorporates ISA bus slots, you want a supply that does include the –5 V signal.

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-5V are needed for dynamic RAM. With early PCs based on 8 bit processors, 64kB was a typical "maximum" memory size, implemented using 32 16kBit RAMs (4116). Only with the advent of 64kBit RAMs was the out-of-rail voltage (which was +12V and -5V for the 4116) generated on-chip with charge pumps.

Similar requirements for early EPROMS. So there is little surprise in the bus system for the first IBM PC having those voltages.

+12V also was popular for the motors of disk drives, both because of the larger power than on the +5V rail as well because of fewer consequences to the computing from power surges. -12V, in contrast, was almost only used for RS232 circuitry.

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In short, because the ATX spec says so. ATX is an improvement on the old standard of AT, and does go through revisions. -5V rails became optional in ATX12V 1.2. The original ATX specifications were released by Intel in 1995 and have been revised multiple times. Currently at 2.3. But it is slow to update, like any standard. -12V will be phased out, eventually.

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