VOGONS


First post, by Kahenraz

User metadata
Rank l33t
Rank
l33t

The BIOS in my P2E-M motherboard detects -5V despite the power supply I'm using lacking the pin for this rail.

I tested the same power supply in my ASUS P2B and its BIOS reports that there is no -5V.

I would check this with my POST diagnostics card but it does not test for this voltage.

Do some motherboards generate the -5V used by the ISA bus on its own?

IMG_20210910_072920_resize_80.jpg
Filename
IMG_20210910_072920_resize_80.jpg
File size
176.03 KiB
Views
878 views
File license
Public domain
IMG_20210910_073000_resize_97.jpg
Filename
IMG_20210910_073000_resize_97.jpg
File size
153.96 KiB
Views
878 views
File license
Public domain

Reply 1 of 15, by wiretap

User metadata
Rank Oldbie
Rank
Oldbie

Measure at the ISA slot with a multimeter to confirm it. Some boards may have a linear voltage regular installed to provide -5VDC, as rare as it might be.

My Github
Circuit Board Repair Manuals

Reply 2 of 15, by frudi

User metadata
Rank Member
Rank
Member

It's more likely just a phantom reading that coincidentally reads close to -5 V in your case. Try a different PSU or connect different components to the board and you'll likely get a completely different voltage. I've seen motherboards reporting even -60 V and similar senseless results.

Reply 5 of 15, by cyclone3d

User metadata
Rank l33t++
Rank
l33t++

Or try an ISA card that requires -5v. Or use a multimeter to see if the ISA slot is being supplied -5v.

Yamaha modified setupds and drivers
Yamaha XG repository
YMF7x4 Guide
Aopen AW744L II SB-LINK

Reply 7 of 15, by snufkin

User metadata
Rank Oldbie
Rank
Oldbie

This might be daft, but would it need to have an actual regulator? Looking around, I think the current requirement for -5V was only around 300mA. Going from -12V to -5V would mean burning around 2W maximum, and you can get things like zener diodes in packages that can handle that (e.g. 1N5342B https://www.farnell.com/datasheets/1796584.pdf , although that's a minimum 6.46V drop, so might leave -5V a little high out of the 10% spec, especially if -12V is also high). I'm wondering if it's posible the designers may have found a way to fudge a cheaper/smaller -5V supply without going down the 7905 route.

Really need to get a multimeter and check the ISA slot pin (B5). That or find where the voltage monitoring is done. The fact that the -5V reading in the BIOS varies makes it sound like something real is being measured. Do any other voltages always change at the same time?

Reply 8 of 15, by BitWrangler

User metadata
Rank l33t++
Rank
l33t++

BIOS programmer instructed thus IF NOT EXISTS -5V THEN Fudge.Reading ELSE Tech.Support DOSed by Panicked.Customers.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 9 of 15, by snufkin

User metadata
Rank Oldbie
Rank
Oldbie
BitWrangler wrote on 2021-09-10, 21:55:

BIOS programmer instructed thus IF NOT EXISTS -5V THEN Fudge.Reading ELSE Tech.Support DOSed by Panicked.Customers.

I suppose I was assuming that a firmware fudge would just set it to -5V, so it wouldn't ever read -4.9V. It's not like they'd take the time to add some pseudo random noise to it. Maybe they fudge it by taking the +5V reading and just inverting it, but you'd see that because they'd change in sync with each other. If they have to add any hardware to fudge an input to a voltage monitor (diode and a couple of resistors?) then they may as well just generate the -5V anyway. I know motherboard designers occasionally did odd things for power supplies, or at least on one motherboard (Abit KA7) they did something I found odd of not using the PSU 3.3V, but generated it locally from the +12V (maybe they wanted a more stable supply than they could get from the PSU).

Reply 10 of 15, by the3dfxdude

User metadata
Rank Member
Rank
Member
snufkin wrote on 2021-09-10, 21:49:

This might be daft, but would it need to have an actual regulator? Looking around, I think the current requirement for -5V was only around 300mA. Going from -12V to -5V would mean burning around 2W maximum, and you can get things like zener diodes in packages that can handle that

Yes... I guess if that's who you trust to do design 😀 let's burn power for something that is unused, when the ATX supply could have provided it if it was really needed.
The main point is, if the motherboard does it assuming ATX spec has changed, there is going to be a device you can point to on the board creating it. It would be interesting if it is true. Because like I just said above, having the ATX PS supply do it is a better answer than plopping any device on a board, that aint gonna get used anyway.

Reply 11 of 15, by cyclone3d

User metadata
Rank l33t++
Rank
l33t++

What's so hard about testing the -5v pin on the ISA slot?

I looked at pics of the board and didn't notice anything hi g that stuck out as far as providing -5v.

Yamaha modified setupds and drivers
Yamaha XG repository
YMF7x4 Guide
Aopen AW744L II SB-LINK

Reply 12 of 15, by Kahenraz

User metadata
Rank l33t
Rank
l33t

I stick a multimeter into the slot and verified +5V on pin B 03 and -12V on pin B 09. But there was no voltage on pin B 05 which should have -5V. It's so WEIRD that not only does the BIOS show this voltage but also that it actually fluctuates slightly. It's not even mirroring with +5V ; it looks like a normal voltage rail.

Reply 13 of 15, by snufkin

User metadata
Rank Oldbie
Rank
Oldbie
the3dfxdude wrote on 2021-09-10, 22:53:
snufkin wrote on 2021-09-10, 21:49:

This might be daft, but would it need to have an actual regulator? Looking around, I think the current requirement for -5V was only around 300mA. Going from -12V to -5V would mean burning around 2W maximum, and you can get things like zener diodes in packages that can handle that

Yes... I guess if that's who you trust to do design 😀 let's burn power for something that is unused, when the ATX supply could have provided it if it was really needed.
The main point is, if the motherboard does it assuming ATX spec has changed, there is going to be a device you can point to on the board creating it. It would be interesting if it is true. Because like I just said above, having the ATX PS supply do it is a better answer than plopping any device on a board, that aint gonna get used anyway.

Well, it'd only burn power if current was being drawn (power proportional to current), the 2W (actually 2.1W) would only be burnt if the full 300mA was being drawn (7v * 0.3A = 2.1W). I was just speculating that they might not have used a normal regulator chip, so there could be device on the board to generate -5V, but it wouldn't be obvious.
The motherboard's from when? 2000ish? So -5V was still a requirement of ATX PSUs (pre ATX 1.3), but I expect there were already discussions about dropping it. So for the sake of adding one small component, a motherboard designer might have decided to generate it locally so it'd still be available if someone replaced their PSU in a couple of years time. Just in case someone has some obscure ISA card that still needed -5V. Don't think I've ever used any card that needed -5V.

But now we've got an actual measurement from Kahenraz, and there's definitely no -5V on the ISA slots. But the value in the BIOS does show changes, so it's almost certainly not just a value fixed in firmware/software. Maybe it was easier to reuse a voltage monitor chip from a previous design and add a couple of resistors between e.g. -12V and GND to fake the -5V input to that one chip, than to add one line to the BIOS code to fix the measured value at -5.

Definitely weird.

Reply 15 of 15, by imi

User metadata
Rank l33t
Rank
l33t

if you have a SB 2.0 and there's no audio output (or just very very faint barely audible audio) for example...

an easier test would be to get a multimeter and measure if the voltage is present.