Comment by cesarb | Hacker News Reader

Comment by cesarb | Hacker News Reader

> The secondary circuitry produces the four output voltages: 5 volts, 12 volts, -12 volts, and 3.3 volts. [...] The power supply also provides a negative voltage output (-12 V). This voltage is mostly obsolete, but was used to power serial ports and PCI slots.

And even older PC power supplies also provided a -5V voltage, the corresponding pin on the ATX connector is now (according to Wikipedia) a reserved pin.

This progression also shows in the expansion slot standards: the ISA slot had pins for -5V and -12V; the PCI slot removed the -5V pin; and the PCIe slot finally removed the -12V pin. That is, a motherboard without any ISA or PCI slot, and without a RS232 socket or header, has no use for the -12V voltage.

And there's already a newer power supply standard, called ATX12VO, which simplifies the power supply by providing only 12V (and a separate standby 12V). There's already at least one motherboard built for that standard: https://www.anandtech.com/show/15763/first-atx12vo-consumer-...

> -12v, -5v

What prevents one from just wiring mobo.5v to port.gnd and mobo.gnd to port.-5v? That would effectively create a -5v potential between port.gnd and port.-5v

-12 V was used for RS-232, which uses +12 V and -12 V levels (with a very wide tolerance band). Having a separate supply for this was a design decision which was pretty much instantly obsolete with integrated charge pumps: All RS-232 outputs are running off of charge pumps running at (-2) and (+2) ratios, so +-10 V levels with a 5 V supply.

I don't know what -5 V was used for. Maybe some analog circuits somewhere, very early MOS logic also often had awkward supply voltages (charge-pumps made this obsolete too, just like high-voltage supplies for EEPROMs).

Early DRAM chips like the MK4116 required -5 V, +5 V, and +12 V. (-5 was the substrate back bias for the chip.) So the ISA standard required -12 V, -5 V, +5 V, and +12 V.

-5v was often used in sound cards for audio amps, I believe. I've got an arcade machine with an AT supply that uses -5v for the audio amp anyway.

I have a number of 80s and early 90s computers. They had some circuits that used ECL, which required a negative voltage. One I had to recently adjust had a power supply that outputted (when properly adjusted) +5V and -5.2V.

That would create a floating ground, and systems needs to be designed for it. Because likely motherboard ground is connected to chassis, and it connected to earth, while power supply most likely have it's own ground connected to earth.

You just shorted the 5V rail. most PSUs will just turn off their output/shuts itself down.

Then you'd also need a +10V supply for the port's +5V input, and to convert logical levels for communication between them. That would be trickier than just amplifying them because low/high of 0V/5V would become 5V/10V. Constant voltage offsets are generally a real pain.

If there's no other connections to the device, then sure you could do that.

However usually there is. For example, the device might need +5V or +3.3V as well.

Well you can't supply +5V and PSU ground as separate leads, because almost always multiple ground connections are internally connected in a device.

Thus that would mean the +5V rail gets shorted to (PSU) ground by the device.

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