5150: The -5V is used for dram refresh and cassette circuitry depending on the revision. It was also passed through to the ISA connector. This made the -5V a spec item for years to come as the PC was originally designed with it.
https://minuszerodegrees.net/5150/misc/5150_power.htm

5160: The -5V is only passed through to the ISA connector.
https://minuszerodegrees.net/5160/misc/5160_power.htm

Any ISA supporting system: Some expansion cards require the use of the -5V. For the XT there were some stock expansion IBM options that may use it.

There is no risk at not having -5V in the XT, but it is possible things won't work fully. You need to pull out your cards and trace the -5V, then you'll know.

Ok, thanks I can proceed into testing.

I gather VGA cards are not known to use this rail?
Btw. I do not have an IBM XT, it's a XT clone from 1986 that's more closer to 5160 than 5150.
https://theretroweb.com/motherboards/s/olivetti-m19

If I'm not looking wrong and the -5VDC is the 5th rail from the top on the backside, looks like it's not wired on any of my cards. Great 😀

I would not rule out all video cards entirely... but yes, I have not seen a vga card using -5V.

It seems that there are VGA cards using -5V - Re: The Mystery -5V Rail

Seems my system doesn't use -5V at all.
I am about to make an adapter. But I don't know what exactly the blue "T" contact is. Any hints?

https://imgur.com/a/ly5hRqV

Sound cards like Sound Blasters (8-Bit) and PAS16 need -5v, too.

If you can't afford a proper PSU, you can still use something like a Voltage Blaster©®™.
Though I'm not a fan of this design myself, personally.

I did not see the mainboard label (on the imgur link) when I posted this question. As displayed, the system doesn't have/require -5V at all.

I'm good with picoPSU I just wanted to get extra info about what that "T" line could be. Everything else I can break out from the pico.
There seems to be no potential between T and G when system is powered. The others are what they say they are on the board label.

No idea what it's for, but it's not crucial maybe.

https://forum.vcfed.org/index.php?threads/oli … addition.69496/

Since PSU is in the monitor, normally, this could be monitor related.

On the other hand, I heard (read) there was an extra PSU for hard disk users available..
Because, the monitor wasn't strong enough in some cases.

Looking through a stack of old PSUs , I see that -5 is very low current. -12 is slightly more but still very low.
I'd guess that the '-' supplies are used for bias of some sort or other very low current applications
- I don't think I've ever seen logic/drives or other hardware actually powered from a '-' supply.

Given that PC PSUs are designed to support "worst case" (fully loaded system) - I'd guess that you
could just regulate -12 down(up?) to -5 .

You should of course check the actual current draw on -12 to be sure this doesn't overstress it.

Dave

XT machine with DIP DRAMs may need that -5V for them, but it depends on what kind of memories it uses.

DaveDDS wrote on 2024-01-24, 02:45:

Given that PC PSUs are designed to support "worst case" (fully loaded system) - I'd guess that you
could just regulate -12 down(up?) to -5 .

A lot of PSUs from that era were designed that way anyhow ; the -12volt rail is produced by the SMPS output transformer, and the -5volt rail is derived from a linear regulator (7905) connected to that.

giantclam wrote on 2024-01-24, 03:05:

DaveDDS wrote on 2024-01-24, 02:45:

Given that PC PSUs are designed to support "worst case" (fully loaded system) - I'd guess that you
could just regulate -12 down(up?) to -5 .

A lot of PSUs from that era were designed that way anyhow ; the -12volt rail is produced by the SMPS output transformer, and the -5volt rail is derived from a linear regulator (7905) connected to that.

Exactly.
BTW. There is useful page with schematics of various ATX and AT PSUs: https://danyk.cz/s_atx_en.html
You can check that the 7905 linear regulator approach is used in the PC SMPS AT, cca 200W, among others.

zyga64 wrote on 2024-01-24, 08:34:

giantclam wrote on 2024-01-24, 03:05:

DaveDDS wrote on 2024-01-24, 02:45:

Given that PC PSUs are designed to support "worst case" (fully loaded system) - I'd guess that you
could just regulate -12 down(up?) to -5 .

A lot of PSUs from that era were designed that way anyhow ; the -12volt rail is produced by the SMPS output transformer, and the -5volt rail is derived from a linear regulator (7905) connected to that.

You can check that the 7905 linear regulator approach is used in the PC SMPS AT, cca 200W, among others.

...sigh .. thanks for posting that ~ remarks to a fair simpler time, wherein all we really cared about (or monitored), was whether the +5volt rail was behaving itself or not ... takes me back to the smell of rosin solder =)