Computer Hardware AT/ATX PS question

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Subject Author Date
AT/ATX PS question Pelysma 07-10-05
---> Re: AT/ATX PS question Floyd L. Davids...07-10-05
Posted by Pelysma on July 10, 2005, 11:49 pm
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The computer on my workbench right now has an AT form factor, but the
motherboard has connectors for both AT and ATX power supplies. I want to
replace the 230w Linksys AT PSU that's in there now with a newer L&S 250w
ATX PSU that I have sitting here.

What I know about the system is what I can see looking at it. Here's what I
can collect:

VA-503+ motherboard
VIA chipset; the next line on the Northbridge chip is VT82C598MVP
K6-2 400MHz on 100MHz FSB
128 MB RAM in two 64-MB 168-pin DIMMs; four slots for 72-pin EDO are empty
6.4 GB IBM ATA HDD
Windows 98SE upgraded from W95

I'm adding two more hard drives, 8 GB each, but I'm keeping the 6.4 as the
system drive because there is software on it I want to use for a while and
have no way to install on another OS (notably MS Access 97). I'll be using
the Reset button as a start button as mentioned in another thread.

The computer has a role to play in my collection but won't really be turned
on very often.

Is there anything I need to know, or anything I need to reconfigure, before
unplugging the AT PSU and plugging in the ATX one?

--
P.




Posted by Floyd L. Davidson on July 10, 2005, 4:05 pm
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>
>Is there anything I need to know, or anything I need to reconfigure, before
>unplugging the AT PSU and plugging in the ATX one?

How is the front panel ON/OFF power switch wired?

With an AT supply there will be a (probably black) AC cord coming
out of the supply going to the ON/OFF switch. That cord carries
120VAC, and it is switched at the front panel.

With an ATX supply, no such AC cord exists, and instead the
green wire on the connector (routed through pins on the
motherboard) is switched to ground by the front panel ON/OFF
switch, and that causes the AC to be remotely switched inside
the power supply.

Very different arrangements, which have more to do with the case
and the power supply than with the motherboard. Your
motherboard was apparently designed to work with either. Which
probably means you could switch it to an ATX case. But swapping
the AT supply in an AT case to an ATX supply isn't that easy.

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com


Posted by ric on July 10, 2005, 10:28 pm
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"Floyd L. Davidson" wrote:

> With an ATX supply, no such AC cord exists, and instead the
> green wire on the connector (routed through pins on the
> motherboard) is switched to ground by the front panel ON/OFF
> switch, and that causes the AC to be remotely switched inside
> the power supply.

Not quite accurate. The PS_ON (green) wire goes to a logic circuit
on the MB. The on/off switch goes to this circuit as well. Engaging
the momentary on/off switch toggles the output of this logic circuit,
changing its state, and turning the PSU either on or off. (If the
logic output toggled low, the PSU turns on. If the logic output
toggled high, the PSU turns off.)

Beside, the OP said he was going to use the RESET switch for his
on/off switch. That will work just fine.


Posted by Floyd L. Davidson on July 10, 2005, 10:45 pm
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>"Floyd L. Davidson" wrote:
>
>> With an ATX supply, no such AC cord exists, and instead the
>> green wire on the connector (routed through pins on the
>> motherboard) is switched to ground by the front panel ON/OFF
>> switch, and that causes the AC to be remotely switched inside
>> the power supply.
>
>Not quite accurate.

You've just repeated the same thing I said with more (and
unnecessary) detail. How is it not accurate?

>The PS_ON (green) wire goes to a logic circuit
>on the MB. The on/off switch goes to this circuit as well. Engaging
>the momentary on/off switch toggles the output of this logic circuit,
>changing its state, and turning the PSU either on or off. (If the
>logic output toggled low, the PSU turns on. If the logic output
>toggled high, the PSU turns off.)

That's correct detail, but it doesn't add anything at all to
understanding the point about the switch.

>Beside, the OP said he was going to use the RESET switch for his
>on/off switch. That will work just fine.

It will work. Of course then he doesn't have a RESET switch,
only an ON/OFF switch that is mislabeled, and an ON/OFF switch
that is labeled but doesn't work... which is a nice recipe for
confusion.

Seems to me that finding another AT power supply is a *much*
better idea. Or just switching the whole thing to an ATX case.

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com


Posted by Eric on July 11, 2005, 12:25 pm
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"Floyd L. Davidson" wrote:

> >> With an ATX supply, no such AC cord exists, and instead the
> >> green wire on the connector (routed through pins on the
> >> motherboard) is switched to ground by the front panel ON/OFF
> >> switch, and that causes the AC to be remotely switched inside
> >> the power supply.
> >
> >Not quite accurate.
>
> You've just repeated the same thing I said with more (and
> unnecessary) detail. How is it not accurate?

You stated that the green wire is switched to ground by the front
panel switch. It is not. It is switched to ground by the OUTPUT
of a logic circuit. The front panel switch toggles the INPUT of
the logic circuit.

> >Beside, the OP said he was going to use the RESET switch for his
> >on/off switch. That will work just fine.
>
> It will work. Of course then he doesn't have a RESET switch,

And a reset switch is used oh so often in an ATX computer.

> only an ON/OFF switch that is mislabeled, and an ON/OFF switch
> that is labeled but doesn't work... which is a nice recipe for
> confusion.

For those who are easily confused. I was under the impression
that the OP would be operating this system. I think he would
get through the confusion.

BTW, a little simple surgery will turn a AT type ON/OFF switch
into a momentary ATX type ON/OFF switch.


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