Powering old motherboards with 5V only? \ VOGONS

Powering old motherboards with 5V only?

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First post, by xorlof

Posted on 2023-11-17, 20:27

xorlof Offline

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Looking at the major chips on a 286 motherboard I have beside me, I haven't found anything that looks like it needs voltages other than 5V. With a careful selection of surrounding components, do you think it possible to boot and use such a system with 5V only? Can anyone think of any way there would be damage to the system if I tried?

Aside from the motherboard itself, I need to power
- the ram and processor, which are 5V
- a vga card--the one I have on hand looks to be 5V only; it doesn't even have isa pins for -5V, -12V, or +12V
- a sound card--ditto, although a handful of cards use -5V at the very least
- the keyboard, which should be fine
- a compactflash drive, which should be ok with 5V
- an isa i/o card to interface with the drive--This is the biggest question mark for me. The multi i/o card I have on hand (typical assortment of serial, parallel, floppy, ide, game) does look like it wants +12V and -12V, but I'm not sure which devices need which voltages. I'm guessing serial serial might not work, but how about the IDE port?

Thoughts? Thanks in advance.

Reply 1 of 4, by BitWrangler

Posted on 2023-11-17, 20:41

BitWrangler Offline

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Probably the serial is most likely, disable on the i/o card and in CMOS if possible to avoid weird errors and hangups. If possible you might wanna stick a 10 kilohm to ground on the 12V to avoid anything that reads it or references it going into "undefined" state, i.e. putting spurious bits on a bus somewhere. Floaty pins are best avoided.

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 2 of 4, by Cyberdyne

Posted on 2023-11-19, 18:27

Cyberdyne Offline

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I have ran many motherboards with only +5V. Some do need to route 5v to POWER-GOOD line. Some motherboards do not need even that. Had good results with 486 to even P3 motherboards. So made minicomputers with a 5v power adapter and a cheap newer ISA sound blaster clone, usually they also do not need other voltages. Some sound amplifiers do.

I am aroused about any X86 motherboard that has full functional ISA slot. I think i have problem. Not really into that original (Turbo) XT,286,386 and CGA/EGA stuff. So just a DOS nut.
PS. If I upload RAR, it is a 16-bit DOS RAR Version 2.50.

Reply 3 of 4, by xorlof

Posted on 2023-11-20, 15:32

xorlof Offline

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Thanks for the replies. I'll give it a shot and will be sure to hook up power good.

BitWrangler wrote on 2023-11-17, 20:41:

If possible you might wanna stick a 10 kilohm to ground on the 12V to avoid anything that reads it or references it going into "undefined" state, i.e. putting spurious bits on a bus somewhere. Floaty pins are best avoided.

Anything special about that 10K value? I've got a bunch of 2.2Ks on hand. Good 'nuf?

Reply 4 of 4, by BitWrangler

Posted on 2023-11-20, 19:16

BitWrangler Offline

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Not super special, I just go higher than a kilohm because some of the extra super low power CMOS can backfeed off a milliamp. I'd use 2 in series of the 2.2K I guess.

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

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