Q: AsusTech : agp 8x video issues

In article , wrote:

Asus P4P800 Deluxe -


Within 1 year I have "RMA'd" 2 brand new retail "8X" video cards, one
nvidia and one ati...both developed boot display annomylies and video
coruption in windows...

A third agp "8X" nvidia card will not run properly on this board,
locks, lags etc...but it runs flawlessly on my "P4PE" agp "4X" board.

I am running an agp "4X" card now and so far things are fine

Q: Is there any record of problems with "AGP 8X" slots on any of these
Asus products ? Possibly some bad circuit, capasitor or the slot
itself which may be specific to the "AGP 8X" feature ?

My new "RMA" replacement card is here, but I will not install it
untill I get some info or you get your board back...


ps:

-retail Antec PSU
-retail APC UPS
-tested with all periferals disconnected
-tested full load with agp "4X", OK so far...(3rd week)

Thanks for any response

There is a power circuit on the motherboard that makes
1.5V for the AGP 4X/8X slot. The circuit remains at 1.5V
nominal, whether the slot runs at 4X or 8X. Both the I/O
pads on the GPU (video card chip) and the Northbridge
AGP pads, are powered by this supply.

You've probably read references to AGP using 0.8V . The way
that works, is when operating in 8X mode, the AGP lines are
terminated with load resistors. The resistors cause the
signal voltage to drop to 0.8V, while the I/O pads driving
them are still actually powered by 0.8V. In a sense, this
is still powering by 1.5V, but you'll still see references
to 0.8V, even though the circuit is not fed by 0.8V.

The power consumed by the I/O circuits increases, when you
are running 8X. If the motherboard had a defect in the power
circuit, and could not handle the load of running at the
8X setting, that could cause flaky operation.

You could go into "Advanced" in the BIOS, and somewhere down
there, you'll find "AGP VDDQ Voltage". It varies from 1.5V
to 1.8V, in 0.1V steps. If you put your AGP 8X card back
in there, you could try adjusting the voltage, and see
if a higher voltage makes things more stable. (Don't be
deceived by the BIOS settings - sometimes circuits like
this really only have a high and a low setting. They may
not in fact have four voltages values in actual hardware.
One Asus motherboard is like that on DIMM voltage - there
are really only two DIMM voltages.) Try 1.8V and see if
the 8X card you've got now becomes more stable. It shouldn't
really need a voltage boost like this, but the improvement
in stability could mean that when set to 1.8V, it might
just be barely making 1.5V or something.

There are two supply voltages that could be shared by the
video card and the motherboard - Vagp and +3.3V. And, either
the +3.3V could be coming direct from the power supply, or
the +3.3V could be made on the motherboard itself. Generally,
the other voltages, like +5V or 12V, are converted by
switching circuits on the video card, into voltages that
can be used by the chips, like, for example, the memory
chips for the GPU. As such, an out of spec +5V or +12V is
not likely to fry the video card, as the conversion circuit
on the video card will have some tolerance to voltage error.
But a Vagp error, like if the Vagp was producing more voltage
on the motherboard than it was supposed to, could lead to
the premature death of either the motherboard Northbridge or
the video card GPU.

Does your Northbridge run hot ? That might be a sign of
an over-voltage problem.

I'd recommend measuring some of the voltages on the board,
but am embarrassed to say that I haven't been able to positively
identify many of the power circuits on my P4C800-E (I really
need an oscilloscope for this work, but don't own one). In any
case, to do such measurements, you'd have to pull the motherboard
out of the case, and sit it on the bench.

Considering the problems you've had so far, you should consider
either taking the motherboard to a repair place, and have them
see if they can find Vagp and measure it. Or RMA the motherboard
and take your chances with another one. I hope Asus will believe
you, if you report the cause for returning the board is
"fries video cards" :-)

HTH,
Paul