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Is the -5v rail on Supermicro P6DBE required?

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First post, by millerjs

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Hi All,
I recently got my hands on a Supermicro P6DBE system and I have been having a lot of fun playing with it. My only complaint so far is that the PSU included in the chassis, a "Zippy Emacs" HP2-6460P has a tremendously noisy fan. I can replace the fan if I need to, no big deal, just a normal 80mm, but it's not speed controlled so it will always be somewhat noisy. I have another PSU I'd like to swap in its place, a Seasonic SS-600HT, which has a big quiet bottom-mounted 120mm fan and a 3pin fan speed control cable that can be attached to a motherboard header to monitor it, which is appealing. I also just trust a Seasonic PSU a bit more than a "Zippy Emacs", for whatever that's worth.

The P6DBE won't boot with the SS-600HT connected, however, and I suspect it's because that PSU does not have a minus 5v rail. I know that the PSU works correctly, I have tested it on a newer motherboard and there is no issue. What happens on the P6DBE is that when I attach the ATX power connector and then press the power button, the machine starts to boot - it flashes the PS/2 keyboard LEDs, the CPU fans spin up, and the power LED on the board lights - then within a second it makes a shrill beep, and turns off - I'm presuming this is a power alarm?

I don't have any ISA cards in the system, and don't plan to use any, so while I don't care for the -5v, the board does. I have done some reading around and have found that some 440BX boards work perfectly fine without it, others throw an error at POST but continue to work, and some others have an alarm that can be disabled. I have read through the P6DBE manual and haven't found anything I can identify as a way to disable the alarm, so as far as I can tell right now I'm stuck. I am not against creating a -5v rail with a 7905 or similar, but if there's something I can do on the board to just ignore this condition, I would rather do that. Any help would be greatly appreciated!

I don't know if it matters, but here's the rest of the system configuration:

  • 2x Pentium III 850MHz
  • 4x 256MB PC100 ECC
  • AGP GeForce 4MX 64MB
  • PCI Intel Pro100 Ethernet
  • IDE CD-ROM
  • IDE HDD
  • 1.44MB FDD

Thanks in advance!
John

Reply 1 of 4, by majestyk

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These mainboards (like my own P6DBU) make a shrill beep sound that attunes to a higher pitch towards the end during shutdown. This is by design and I assume it´s one of Supermicro´s typical gimmicks.
I recently powered my P6DBU up with some cheapo ATX-PSU and it worked perfectly. (Not sure if this PSU had a -5V rail, though.)

There´s also a chance the Seasonic PSU doesnt´t reach "power good stage" fast enough for this finicky mainboard and it cancels startup early. Have you tried with just one CPU?

Reply 2 of 4, by millerjs

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Hmm, that is an interesting idea. The other board I tested the PSU on is also a supermicro (an X6DA8-G2) and it doesn't complain, but it is much newer.

I tried with CPU2 pulled by I see the same behavior.

Also, out of curiosity, I looked up the other PSU, the HP2-6460P and found a data sheet for it: https://images10.newegg.com/UploadFilesForNew … 44357078701.pdf

It listed the -5v pin as "NC", so I thought maybe there was a separate rabbit hole I was headed down, but sure enough when I manually start that PSU and measure that pin, -5v is present.

I did compare the two supplies and found that the Seasonic's PWR_OK only hits about 4.99v, where the zippy hits 5.17v. The seasonic could be sagging due to aging caps, or maybe it has always been that low, hard to say. What I think I should do at this point is try the P6BDE with another known good PSU that also doesn't have the -5v rail and see if it's able to start the board - that will help determine if the issue is the -5v or the seasonic.

Reply 3 of 4, by kheper

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The Supermicro P6DBE is an ATX 2.01 compliant board, and the ATX 2.01 power specifications indicate a -5 volt rail. In the link, scroll down to page 20 for the connector pinout. Look for "-5".

https://studylib.net/doc/18087611/atx-specification

Reply 4 of 4, by millerjs

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Yeah, I had read over the ATX 2.01 spec when I saw that in the P6DBE manual and saw the -5v there, but I don't know how much that really means, realistically. Plenty of other boards from this era seem to work just fine without a -5v rail, per other reports I've read.

Right now the main thing I need to do next is test another PSU that doesn't have -5v and see what it does, which will help me narrow down if the issue is with the Seasonic PSU or the missing -5v.

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