To be -5v, or not to be -5v? That is the question

[socketa]

All of 'my' boards are socket A
Some have just a 20-pin connector, and others have 20-pin connector + the 4-pin 12v CPU connector.
Would any of them need the -5v rail?
Would it be better for the Mobo if it was not present? - i.e. if SWIM was to remove it?
If the 5v rail is connected, and it is not required, then it would be unnecessarily applying voltage to the mobo, perhaps prematurely wearing out the components that are connected to this rail.
I can recall reading somewhere that this type of connector (with -5V) should not be used on newer motherboards, but i don't know if these mobos are classed as new - as the terms "new", "modern" and "old" can be quite ambiguous.

[mariushm]

Re: To be -5v, or not to be -5v? That is the question

-5v was used by ISA slots (for example opamps on ISA sound cards may have used +/- 5v).

I'm not aware of other components on a motherboard (except maybe onboard modems?) that would use -5v.


-12v was used by serial ports .. and some motherboards still have serial ports or still have headers for the bracket with connector.

-5v was made somewhat optional in atx psu standard v1.2 in 2002 and in v2.01 (2004) is not required at all in the standard.

See page 3 here (v2.01):

V2.2 is here: (yeah the url is fucked up by them)

[japlytic]

Re: To be -5v, or not to be -5v? That is the question

On some old floppy/HDD controllers, -5V was used by the data separators.
Could -12V be deleted from the ATX standard, considering the fact that it was used on PCI slots but not on PCI Express slots?

[mariushm]

Re: To be -5v, or not to be -5v? That is the question

Yeah, it could be deleted/made optional, the motherboard could simply add an inverting dc-dc converter to make -12v from 12v. Nothing complicated, the current power supplies only offer 5-10w at best on -12v which is very simple to do.

I would in fact give up on -12v AND 3.3v and go only with 5v, 12v and 20v (+/- 5%).

Why 20v? Because it's standardized for usb 3.0 and it would also make it fairly easy to reuse laptop adapters/chargers to power up motherboards that have everything onboard (think itx, microatx with kaveri or similar processors, boards that don't go over 100w etc etc)

No sense wasting 4 wires or whatever the amount is to move a few amps on 3.3v - this can be easily done by a dc-dc converter onboard, it's already there in some cases because system receives 5v standby and there has to be 3.3v or 1.8v produced for bios, chipset whatever while in standby.

They could go further and make some i2c link (or some protocol that's open) between computer and power supply and just let the power supply default on 12v through all atx connector pins meant for 12v or 20v and let motherboard tell it if it supports 20v on just the pins meant for 20v or to send 20v through all 12v+20v pins or a subset of 12v pins (you may still want 3-4 12v wires for the pci express slots and usb 3.0 in case device can't handle 20v and falls back to 5v or 12v).

[999999999]

Re: To be -5v, or not to be -5v? That is the question

Some OEM motherboards and aftermarket (Creative Labs for one) sound cards had audio amp chips meant to direct drive speakers, also made use of the -12V rail to give 12V/0/-12V supply.

Back to the original question, if -5V is connected to the board but not required, meaning not used, there is nothing it could damage. There is no ATX compliant motherboard which will have a problem with -5V being present on the corresponding PSU connector pin, not newer or older it makes no difference.