Archived from groups: alt.comp.hardware.homebuilt (
More info?)
On Mon, 19 Jul 2004 20:07:59 GMT, kony <spam@spam.com> put finger to
keyboard and composed:
>To more directly answer your question, yes you can just short the
>PS-On signal to ground. For long-term, permanent modificatio it
>might be better to use a resistor between the two instead,
>perhaps 1K would suffice though it's just a guess,
I don't think this is a reliable approach.
Intel's ATX PSU spec states the following:
===============================================================
PS_ON# is an active-low, TTL-compatible signal
The power supply shall provide an internal pull-up to TTL high.
VIL, Input Low Voltage 0.0V - 0.8V
VIH, Input High Voltage (Iin = -200 µA) 2.0V
===============================================================
If the PSU has an internal 4K7 pull-up resistor, then a 1K resistor
would result in a PS_ON voltage of 5 x (1 / 5.7) = 0.88V which is in
TTL limbo.
> I"ve done it
>before as a test but unfortunately don't recall the maximal value
>possible and it may easily vary per particular PSU.
I think so, too, as the spec does not specify a value for the internal
pull-up resistor.
>What will it matter if you're shorting signal coming from mobo
>since that's exactly what mobo does when it keeps system running?
That's true if the signal comes from an open collector driver, which
it probably does, but not if it is generated by a regular TTL gate.
>When system resets it keeps signal pulled low too, so it's more
>an issue of expected power failure and recovery (as I initially
>mentioned, why you feel the need to do this).
If a BIOS solution is unavailable, I would lift the PS_ON pin from the
power connector and wire it to a ground point on the mainboard. For
example, you could attach a ring terminal to a mainboard mounting
screw.
- Franc Zabkar
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