Pretty sure it will be fine 😀

I don't know... -10% is a serious percentage IMHO 😜

ATX voltage tolerances:

+3.3V, +5V, +12V: 5%
-5V, -12V: 10%

Couldn't find anything on XT/AT, which is probably more relevant when it comes to LAPC-I...

thanks 5u3 , basically I should re-phrase my question to: is -4.5v fine for a LAPC-I ? 🤣

also I took a look in the LAPC-I manual, it states that the card needs:
+5v / 550mA
-5v / 50mA

Usually I was of the opinion that "'WATT''s less, can't kill but may not work either" and "'WATT''s more, may work but also can kill"... 🤣

What I meant to say is that I'm more daring when it comes to testing electronics with less power, but I may be wrong. h-a-l-9000 may clarify this up.

But of course, testing the LAPC-I is a big exception - it's too rare to even try anything on it.

5u3 wrote:

ATX voltage tolerances: […]
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ATX voltage tolerances:

+3.3V, +5V, +12V: 5%
-5V, -12V: 10%

Couldn't find anything on XT/AT, which is probably more relevant when it comes to LAPC-I...

According to IBM, the +5V and +12V can vary by 5%, and the -5V and -12V can vary by 10% in PC/XT/AT power supplies. I think its more important that you can provide 500mA than an exact voltage.

I see, thanks for the replies guys 😀
I have triple-checked and tested a 1999 250W ATX psu for now that delivers -5.02v and seems it's quality built with japanese caps and everything... all the newer ones are -4.5 with the exception of a 2002 model that delivers -4.82 (have put it aside for now)
I might be getting paranoid here but I am not prepared for a lapc-i search 🤣

ultimately I might take the advice I was given in vintage-computers forum, to reconstruct my own -5v line from a +12v line , I might grab a quality psu (a corsair or something similar) and do this mod to have some peace of mind...

Maybe it's because currently nothing is using the -5 line?

Do you have a SB 2.0 or PAS16? Both use -5V.

yeah the -5v line has been optional in the ATX standard for almost 7-8 years now....
I have a LAPC-I Mau1wurf1977 , that's why I am searching this matter so much... if I didn't have it then I'd just use a random good atx psu and get over it 😁

Sorry I didn't explain myself well.

What I was trying to say is you know when you put load on a power supply the voltage drops? So maybe that's what will happen with the -5V. It might be higher because it has no load at the moment...

So if you have SB 2.0 or PAS16 give it a go with one of these cards inside.

I am measuring the voltage with no ISA cards installed at all Mau1wurf1977 , infact I don't even have a vga installed on the test mobo, it just powers on and beeps I guess (no speaker connected 🤣) , also doing the same test with a PSU tester, I get the same results. So I am pretty sure that if more cards would drop the voltage then the -4.5v psus will be terrible.

But you see the voltage is negative. So dropping it will move it towards -5V.

With no cards installed the PSU has almost no load and voltages will be high across the board. What are you getting for the 12V and 5V lines?

Are you sure that under load it will drop towards -5 ?
+12/+5v lines are normal on all psus

Not sure but worth testing if you have any cheap cards that use -5V. You never know 😀

Hmmm..I've been using the LAPC-I in a few systems for a few years now, mostly with those using modern ATX PSUs, ranging from 300 - 500W with -5v rail , with the ATX to AT converter cable+switch for use with AT motherboards, and sometimes directly with an ATX motherboard. So far so good and not noticed any problems. The only instance that I need to change is when using with faster CPUs, I had to change the 8-bit I/O recovery time value in BIOS to make it sound right, on certain motherboards.

I never actually checked if the PSU is giving out exactly or around -5v.