More on the i845/i850 AGP Incompatibility Issue
By Van Smith
Date: November 3, 2001
We have obtained information from FIC, Intel, and Hercules regarding the i845/i850 AGP incompatibility issue we wrote about earlier this week. In particular, Francois Garet of Hercules has been most helpful in bringing us important information about the scope of incompatibility regarding Intel i845/i850 motherboards and certain AGP 4X plug compatible video cards.
It appears that the information upon which we based our original article is incorrect. The German site HardTecs4U (and the same information is reportedly given at the noted German publication heise online) presented a testing methodology for the grave incompatibility between certain Intel chipsets and an unknown number AGP graphics cards.
The methodology outlined involved measuring continuity between the AGP card's Pin A2 and ground. If A2 is not grounded, these publications stated that the so tested video card would be one of those dangerously incompatible with the Intel i845/i850.
This is not so according to Mr. Garet:
Regarding Pin A2 signal, this pin signal is read only on AGP universal bus (with no AGP 4X notch), and on AGP universal bus, our board switch to AGP signals in 3.3 volts, you can read this spec on AGP 2.0 specs (chapters 4.3.5 and 4.3.6), so this Pin A2 signal is not relevant for a test on a non universal AGP bus, as on DFI VC11, and as on other i845 motherboards.
The only conclusion you can make from Pin A2 signal is that, on a AGP universal bus (without the 4X notch), you use a 3.3 volts, but that is a signal voltage only for an AGP universal bus, it is not the signal for a i845 or i850 AGP bus.
Hesitantly taking Mr. Garet's advice, we plugged in the still working Hercules 3D Prophet 4500 into an Asus P4T-M i850-based motherboard that we had confirmed operational with a Pin A2 "safe" card. We are relieved to say that the system powered up without incident.
So which cards do hammer the Intel i845/i850 chipsets? According to Mr. Garet, not many:
The only graphics boards on which we have seen a problem with i845 and i850 motherboards are some old SIS based graphic cards, on which the Vref generation signal for 1.5 volt was not properly implemented. It is not due to the SIS graphics chipset, but to a mistake in the board design made by some boards manufacturers, so it doesn't mean one must ban SIS based graphics boards, but only some improper boards manufacturing.
The problem, therefore, seems to be much more limited than the "Pin A2" test would indicate. We will follow-up with more details shortly.
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