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Checking the standard ATX connector pinout it mainly gives out "standard" positive voltages, like +3.3VDC, +5VDC and +12VDC. There is one exception, pin 14, which actually gives out -12VDC:

pinout

Based on the ATX12V specification I can see that it only needs to supply 500mA of -12VDC (compared to the other voltages where even the +5V standby voltage is at least 2A), but I cannot find which component of an ATX computer requires this unusual voltage to operate.

(There is also pin 20 with -5V, but that pin is deprecated, and the deprecation note clearly say that this voltage was used to power some features in old ISA cards. -12VDC is still not deprecated and required based on the spec, so I guess it's still used somewhere in a modern computer)

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  • Probably for a true RS232 port that needs a negative voltage and that avoids a negative charge pump. But someone can probably confirm and give a better answer.
    – PeterJ
    Commented Nov 12, 2014 at 13:33
  • Perhaps somebody with better searching skills than I can find the question that was asked in the last 90 days that is nearly identical to this question. I know it asked about the -12v rail on power supplies and why it still exists.
    – Ramhound
    Commented Nov 12, 2014 at 13:38

2 Answers 2

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AFAIK there are 3 type of devices on a motherboard that require -12V.

  1. RS232, RS485 ports without build-in circuitry for generating the negative voltage.
    These are starting to become very rare... Haven't encountered any for the last 5 years or so.
  2. (mini-)PCI slots. They are slowly being phased out in favor of PCI-e, but there are still a lot of new computers made that still have them.
  3. Some high-end sound-cards require -12V as a reference voltage. These are usually powered from a PCI slot, which brings us back to point 2.
    (There have been a few motherboards that had such a card integrated in the motherboard itself.)

PSU manufacturers can't know in advance if the motherboard will actually need the -12V so they will keep adding the -12V line to the plug as long as the ATX standard requires it.
And it is not only a matter or whether it is needed or not: If you don't include it, your PSU will not be ATX compliant and you can't sell it as such.

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It's a deprecated rail much like the -5V one once was. It used to be used to power the RS232 serial ports, but most newer equipment can supply its own negative voltages internally.

So basically, it's useless for today's equipment.

Source

EDIT: As it it was pointed out, the -12V rail is also used in the PCI specification to power it. As PCI is being phased out in modern hardware, the -12V rail will likely follow in its footsteps.

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  • electronics.stackexchange.com/a/57382/5830 says -12VDC is needed also for PCI.
    – user
    Commented Nov 12, 2014 at 14:16
  • @MichaelKjörling True, but given the prevalence of PCI-E and the move to include more components previously added by PCI cards to the motherboard itself, it's fairly useless on modern equipment. Some boards don't even have PCI slots anymore.
    – Nathan C
    Commented Nov 12, 2014 at 14:21
  • Agreed that PCI slots are mostly obsolete on many systems, but your answer said it was used for RS232 (and by implication nothing else).
    – user
    Commented Nov 12, 2014 at 14:26
  • @MichaelKjörling Updated my answer.
    – Nathan C
    Commented Nov 12, 2014 at 14:29

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