AT Motherboard with ATX Power Supply

[SoCalSienna]

I'm trying to run an old AT motherboard from an ATX power supply. I have the conversion adapter cable (20-pin to 12-pin). But I can't figure out the other power lines. The old AT power supply had four additional lines going to the case main power switch. The conversion cable only has two additional wires (a green and a black). What do I do with them? My old motherboard has no pins for a power switch. So how would I power up the system?

Any answers are appreciated! I'd like to keep the system running for my own tinkering.

 

[Walter349]

I am not sure that you can do this. the power supply for the AT, required a hard on/off switch that controlled the psu directly and did not provide for the PSU logic control states used in the ATX PSU and ATX system boards.

The ATX requires that the PS_ON signal be dropped low by means of a contact switch, via the system board. that is what the green and black cables do, essentially the switch provides a make/brake contact that shorts the green and black cables, thus sending the PS_ON signal to the PSU and energising the PSU. Also the System Board will need a 3.3VDC(AUX) rail to provide the pull ups for the RAM and CPU to allow the machine to start up. Depending on the chipset, this maybe provided via an onboard voltage controller using the 5VDC(AUX) rail, or it may need it directly from the PSU. Also the later models require a seperate set of power rails to the CPU without which the CPU will not power up at all.

 

[G0AOZ]

Where did you get the "conversion cable" from? Maybe the supplier has some info on the green and black wires. See if you can trace where these two wires go. I'd suspect they'll go back into the PSU...?

If I was asked to guess how they're to be connected, I'd say wire them to a momentary switch, which would become your ON/OFF switch. However, this piece of advice comes with a "skull and crossbones" warning!


ROGER.

 

[Walter349]

The green and black cables connect to the power switch via the system board and provide the PS_ON signal to the PSU, see my explanation above

 

[SoCalSienna]

Thank you Walter and GOAOZ. Your info helps!

- I got the cable at Fry's. It was in a simple bag with no directions. I also saw it at an on-line store, but they had no directions about the power switch lines.

- I previously tried momentarily connecting the green and black wires. No go. (MB with video card and 12 pins connected black to black).

 

[edfair]

Would suspect that it would take a solid contact to keep it up. The momentary switch used to power up the ATX board activates power up on the board, which holds up till power down.

Ed Fair
Give the wrong symptoms, get the wrong solutions.

 

[ceh4702]

You have to have an AT Power Supply Switch to make this work. Normally two wires attach to the power supply and 2 wires attach to the motherboard.

They sell them at

http://www.directron.com

or maybe go back to fry's and get one. If they have an old AT power supply the wiring diagram would be on it.

If you do not like my post feel free to point out your opinion or my errors.

 

[SoCalSienna]

Thank you edfair regarding the solid contact requirement and ceh4702 regarding the AT switch. I do have an AT case with AT-style switch, and it appears to give a solid contact. The instructions on directron.com about the switch are helpful.

I tried the setup, but no success. Here's what I did:

- ATX power supply with an ATX to AT adapter connected to AT motherboard (12 pins).
- Connected the two lines from the adapter cable to an AT switch. (black to where black was, green to where white was). There were two additional posts in the switch for brown and blue lines, but the adapter cable only has black and green lines.

Oh well! Thank you for the ideas!

 

[edfair]

On the ATX power supply shorting pin 14 (green usually) and one of the blacks usually brings the PS up. That is why we suspected the green and black wires were a power up pair.
You could try it with the wires embedded in the molex connector, or you could try tracing the two loose wires and verify that they are really what we think they are.
The molex numbering is left to right, top to bottom, with the latch down. 1-10 on the top row, 11-20 on the bottom, with 14 as green. Assuming, of course, that it is a conventionally wired box.

Ed Fair
Give the wrong symptoms, get the wrong solutions.