Date: Sat, 11 Sep 1999 18:31:19 +0800 From: Peter Wemm <peter@netplex.com.au> To: Chuck Robey <chuckr@picnic.mat.net> Cc: Mike Smith <mike@smith.net.au>, Luigi Rizzo <luigi@labinfo.iet.unipi.it>, hackers@FreeBSD.ORG Subject: Re: damn ATX power supplies... Message-ID: <19990911103119.4253B1CB1@overcee.netplex.com.au> In-Reply-To: Your message of "Sat, 11 Sep 1999 00:22:53 -0400." <Pine.BSF.4.10.9909110020500.28670-100000@picnic.mat.net>
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Chuck Robey wrote:
> On Thu, 9 Sep 1999, Mike Smith wrote:
>
> >
> > > any idea on how to force ATX power supplies to restart after a power
> > > outage without having someone press the 'power' button on the front
> > > panel ? All the motherboards i can find now have their bios with two
> > > options:
> > >
> > > Disabled
> > > no automatic restart on power failure
> >
> > You _should_ be able to change this.
> >
> > > none of them is satisfactory especially for picoBSD things such as
> > > routers or firewalls where an UPS is overkill...
> >
> > You can always hotwire the supply; go dig up a pinout for the ATX power
> > connector and you'll see that if you ground the power-on line the PSU
> > will come up...
>
> It's not just a ground, the line that brings up the power is a momentary
> switch, so a longish (about 1/2 second) pulse would do it. That would
> make it easier for you to set it up so you *could* turn it off if you
> actually did want to. Of course, if you did turn it off, and used this
> pulse idea, the next power fluctuation would turn your PC back on ...
> kinda inverted behavior!
The power switch on the front panel is indeed a momentary switch, but
that is not what's connected to pin 14 (PS_ON in the connector specs).
The momenary switch is connected to an electronic device that is powered
by the continuous small transformer inside the ATX power supply and it
does an electronic "on/off" toggle and does pull PS_ON to ground for the
entire duration that the system is running.
On older motherboards, this device was pretty dumb and was little more than
an on/off toggle. On newer motherboards, it's addressable on the SMB bus
(along with the SIMMS, the LM78/LM75/etc, the embedded LM75 in the newer
CPU, etc). Anyway, the newer devices are programmable to do things like
the 4-second power off delay, auto-on with AC, maintain previous state when
AC restored, alarm clock time auto start, as well as the usual "turn off
now" command from the APM bios.
Anyway, I repeat, PS_ON (pin 14, usually numbered on the connector and
usually the only green wire) is grounded to make the ATX power start, and
the moment you disconnect it, the power supply shuts down.
BTW; it is possible some motherboard bios's could get upset at this
("hey, I said to turn off damn it!!"), but I'm yet to see one.
Cheers,
-Peter
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