BIOS insists there's -12V on the -5V rail

**Bartender** · 07-19-2006, 07:34 AM

Good morning -
Here's a weird one.
Have a 2000 vintage (or thereabouts) U.S. Micro PIII 450 MHz PC w/ an ASUS P3V4X motherboard. I really like this old PC. It's running W2K/ Ubuntu Dapper. I decided to trade the OEM PSU out for an Antec Tru-Power 380, removed from a Sonata. When I booted it up, the process stopped and gave an alert about the Hardware Monitor, which is part of BIOS. Under "Power", the Hardware Monitor reported the -5V rail was running -11.87V (or something like that). Unsure whether this was an immediate threat or not, I turned it off and swapped the OEM PSU. Everything normal again.
Cracked open "Upgrading and Repairing PC's" (13th ed.) where Steve sez the negative voltages really aren't used for much of anything anymore... "The -5V is simply routed to the ISA bus on pin B5 so that any ISA cards can use it. Today, though, not many do." The Hardware Monitor in BIOS gave me the option of "Ignore" for the -5V rail, which would be very odd indeed if it were an important parameter.
So, it looks like I could probly pull that pin or ask BIOS to "Ignore" the reading. But I'm not inclined to do that. The Antec PSU isn't noticeably quieter than the OEM PSU, which was modified with a Nexus fan.
The real question this morning is "how could I get -12V on the -5V pin?" The ATX main plugs look identical (well, the Antec PSU has one of those 20+4 plugs but you know what I mean) and the physical dimensions of the 2 PSU's are identical so it's not like the OEM was some weird SFX/NLX/etc. form factor. The Antec PSU was powering another PC until a coupla weeks ago without any weird problems.

**UncleMacro** · 07-19-2006, 12:09 PM

An Antec TruePower 380 doesn't provide -5 volts so the input to the voltage monitoring chip is floating. That can cause that voltage to be misread. -5 volt support has been optional for many years and very few modern power supplies implement it. It's extremely unlikely that even an old motherboard like a P3V4X requires -5 volts and the only other way you would need it is if you have are using an ISA card (mostly soundcards) which needs it. If nothing is using -5 (which is highly likely) then it doesn't matter that it's not being provided. I'd just disable the alarm in the BIOS and see if everything works okay.

**Bartender** · 07-20-2006, 07:27 AM

Hi, UncleMac -
Sound card is a PCI Creative CT4810. No ISA devices. Thanks very much for the analysis. I'll try it again and ask BIOS to "Ignore" the -5V reading.

07-19-2006, 07:34 AM	#1 (permalink)
Bartender Mentally divergent Join Date: Jun 2005 Location: Chehalis, WA, USA Posts: 1,285 OS: W2K, Ubuntu 8.04	BIOS insists there's -12V on the -5V rail Good morning - Here's a weird one. Have a 2000 vintage (or thereabouts) U.S. Micro PIII 450 MHz PC w/ an ASUS P3V4X motherboard. I really like this old PC. It's running W2K/ Ubuntu Dapper. I decided to trade the OEM PSU out for an Antec Tru-Power 380, removed from a Sonata. When I booted it up, the process stopped and gave an alert about the Hardware Monitor, which is part of BIOS. Under "Power", the Hardware Monitor reported the -5V rail was running -11.87V (or something like that). Unsure whether this was an immediate threat or not, I turned it off and swapped the OEM PSU. Everything normal again. Cracked open "Upgrading and Repairing PC's" (13th ed.) where Steve sez the negative voltages really aren't used for much of anything anymore... "The -5V is simply routed to the ISA bus on pin B5 so that any ISA cards can use it. Today, though, not many do." The Hardware Monitor in BIOS gave me the option of "Ignore" for the -5V rail, which would be very odd indeed if it were an important parameter. So, it looks like I could probly pull that pin or ask BIOS to "Ignore" the reading. But I'm not inclined to do that. The Antec PSU isn't noticeably quieter than the OEM PSU, which was modified with a Nexus fan. The real question this morning is "how could I get -12V on the -5V pin?" The ATX main plugs look identical (well, the Antec PSU has one of those 20+4 plugs but you know what I mean) and the physical dimensions of the 2 PSU's are identical so it's not like the OEM was some weird SFX/NLX/etc. form factor. The Antec PSU was powering another PC until a coupla weeks ago without any weird problems.

07-19-2006, 12:09 PM	#2 (permalink)
UncleMacro TSF Enthusiast Join Date: Jan 2005 Location: San Diego Posts: 2,133 OS: Vista SP1 My System	An Antec TruePower 380 doesn't provide -5 volts so the input to the voltage monitoring chip is floating. That can cause that voltage to be misread. -5 volt support has been optional for many years and very few modern power supplies implement it. It's extremely unlikely that even an old motherboard like a P3V4X requires -5 volts and the only other way you would need it is if you have are using an ISA card (mostly soundcards) which needs it. If nothing is using -5 (which is highly likely) then it doesn't matter that it's not being provided. I'd just disable the alarm in the BIOS and see if everything works okay.

07-20-2006, 07:27 AM	#3 (permalink)
Bartender Mentally divergent Join Date: Jun 2005 Location: Chehalis, WA, USA Posts: 1,285 OS: W2K, Ubuntu 8.04	Hi, UncleMac - Sound card is a PCI Creative CT4810. No ISA devices. Thanks very much for the analysis. I'll try it again and ask BIOS to "Ignore" the -5V reading.