"James Ellis" said:
Hi all,
I just upgraded my system with an Asus A8V and Athlon 3000 cpu. Problem is
that when I plug in the power cord there is power to the system. I didn't
even hit the power switch. And I can't turn it off with the power switch, I
have to unplug it. Adding to the trouble, the monitor gets no signal, there
are no beeps, the hdd led is steady red but I do not hear any hdd activity.
There is no POST, no nothing. When I reinstall my old motherboard and cpu
the system works fine.
Can anyone shed some light as to where to start looking?
Thanks,
Jim
Momentary
Contact---------- Chip 1 -------------------Chip 2-------------+
Front Panel Converts to steady More "Wake On" |
Switch logic level. "Wake On" logic to modify |
logic can modify level level |
|
PSU "PS_ON#" <--- THERMTRIP or equiv ------- AGP_Warn ---------+
input pin Processor cutoff Cutoff Circuit
Logic "0" or
GND switches ON.
Above, is a vague sketch of elements that affect the power path.
The two chips that support "Wake On" features, such that the computer
turns on by itself, are the Southbridge and the Super I/O chip. The
Super I/O has "Wake On keyboard" and "Wake on Ring Indicator", and
the Southbridge has a whole bunch of "Wake On" features, including
the PME signal received from a Wake On LAN ethernet chip.
I don't know whether the Super I/O or the Southbridge comes first
in the chain above. The order is not that important to the
discussion.
The momentary contact switch is conditioned by the first chip it
connects to. If the switch is held down for 4 seconds or longer,
the behavior changes. If the front panel switch was stuck, chances
are something different would happen, but what I cannot say.
The bottom two parts of the power path, are cutoff circuits. If
an illegal 3.3V-only video card is plugged into some of the older
AGP based motherboards, the power supply is turned off. Similarly,
if the processor overheats, that can be used to turn off the power
as well. (Your motherboard doesn't seem to have the AGP Warn circuit,
at least not that I can see in a picture of the motherboard. AGP
Warn is also not present on PCI Express boards.)
OK. So that is the power control path. At the end of the path is the
"PS_ON#" pin. Grounding that pin turns on the PSU.
Now, try to imagine some faults. If the user does somethin creative
with the ATX 20 pin or ATX 24 pin power connector, such that it is
improperly plugged, and as a result, the PS_ON# pin on the cable
of the PSU gets connected to a ground pin on the motherboard, the
PSU will turn on immediately. With the 20 pin connector, that would
be pretty hard to do, unless the connector was offset and "hanging
over" the end of the motherboard connector.
I've had one computer here, where I bumped an IDE cable and it was
only half inserted. That caused the symptoms you describe, where
the power was on immediately, and I can only assume the half plugged
IDE cable was electrically stressing the Southbridge. I was
surprised that there was no permanent damage - fulling seating
the cable solved the problem for me.
Another possibility, is an extra brass standoff underneath the
motherboard, one that doesn't line up with a plated hole on the
bottom of the motherboard, is shorting to something important
on the bottom of the motherboard. You can check for that,
by removing the motherboard from the computer case, and
assembling a minimal set of components on your table top.
That so-called "cardboard test" will eliminate any potential
shorting problem. I unscrew the PSU and just sit it on the
table next to the motherboard.
So, anything stressing (shorting) the Southbridge or the Super I/O
could do it. Obviously, a hardware defect anywhere in that path
could also do it. If your front panel power switch stayed depressed,
either the computer would shut off after 4 seconds, or it would
never turn on in the first place, depending on how the switch
conditioning logic works.
I don't think the presence or absence of the ATX12V 2x2 power
connector, will do anything to affect the outcome of this problem.
If the ATX12V 2x2 is disconnected, the processor cannot POST
and there will not be a beep. But that power cable should not
affect whether the computer comes to life immediately after
the PSU is switched on via the switch on the back of the
computer.
HTH,
Paul
