AGP, AGP Pro, PCI and PCI-X voltage keying (WIP)

Just today this came up on 3 different lists, so it's definitely time for a blog entry!

Contents:
- Voltage keying of 32-bit and 64-bit PCI
- Voltage keying of 64-bit PCI-X 1.0 and 2.0
- Voltage keying (and headaches) of AGP 1.0 (2x/1x), 2.0 (4x/2x) and 3.0 (8x/4x)
- Power ratings of AGP and AGP Pro slots

- Voltage keying of 32-bit and 64-bit PCI

Nearly all "commodity" mainboards are still have 5V-only 32-bit PCI slots. This is because most PCI cards at the superstore are still either 5V or universal (3.3V with 5V tolerance). Although universal PCI cards are becoming commonplace, there is virtually no such thing as a universal PCI slot on a mainboard -- they are almost always 3.3V or 5V.

• 5V 32-bit PCI cards have a "cut" at the "front" (away from the bracket)
• 3.3V 32-bit PCI cards have a "cut" at the "back" (towards the bracket)
• Universal 32-bit PCI cards have both notches, typically being designed for 3.3V with a tolerance for 5V input.

• 5V 32-bit PCI slots have a "key" at the "front" (away from the rear of the mainboard/case)


• 3.3V 32-bit PCI slots have a "key" at the "back" (towards the rear of the mainboard/case)


• There is virtually no such thing as a universal voltage PCI slot

64-bit PCI is the exact same, with a 3rd notch/key. This separates the additional 64-bit address/data lines from the 32-bit address/data lines. This is not a voltage or other compatibility key, but so 64-bit PCI cards can fit in 32-bit PCI slots (they notch on the card just allows insertion at the "front end" of the 32-bit PCI slot). It's important not to confuse this notch on a 64-bit PCI card at the 5V key on a 32-bit PCI slot.



SIDE NOTE This discussion has nothing to do with clock signaling of PCI. PCI may be 33MHz or 66MHz, and there is absolutely no keying for clock signaling. In the overwhelming majority of cases, a 66MHz capable PCI card changes its signaling to 33MHz in a 33MHz PCI slot, and a 66MHz capable PCI slot changes its signaling for a 33MHz PCI card. Now this may have unforseen consequences as the entire bus changes its signaling (reducing performance of all other boards).

- Voltage keying of 64-bit PCI-X 1.0 and 2.0

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- Voltage keying (and headaches) of AGP 1.0 (2x/1x), 2.0 (4x/2x) and 3.0 (8x/4x)

Through AGP 2.0 (4x/2x), AGP was as simple as PCI. Unfortunately, starting with AGP 3.0 (8x/4x), it became far more complex as keys were re-used. So it almost becomes completely useless today and technical consumers need to be dillgent to check technical manuals (many of which are severely lacking for AGP x8 cards).

In a nutshell, AGP is as follows ...

(add images for keying through AGP 2.0, and the re-use for AGP 3.0)

	Commonly Known Features
AGP	Width	Modes"	Signal (MHz)	DTR (GBps)
1.0	32-bit	2x/1x	133/66	0.5/0.25
2.0	32-bit	4x/2x	266/133	1.0/0.5
3.0	32-bit	8x/4x	533/266	2.0/1.0

	Mainboard		Card
AGP	Voltage	Tolerance (Optional)	Voltage	Tolerance (Optional)
1.0	3.3V		3.3V
2.0	1.5V	3.3V	1.5V	3.3V
3.0	0.8V	1.5V	0.8V	1.5V

The real kicker is that the AGP spec defines the support, not the "mode" (speed). An AGP 4x card could be anything from a 3.3V tolerant card to one that requires 0.8V, because it could be an AGP 2.0 1.5V card with 3.3V tolerance, or even an AGP 3.0 card that requires 0.8V without any 1.5V support. In most cases, AGP 4x cards do make the best selections because they typically support all mainboards.

AGP 8x on the other hand is the current issue. Many, many AGP 8x cards are coming out with only 0.8V support. The GPUs are designed for 0.8V only, and it's up to the board manufacture to add 1.5V tolerance. Most do not due to added cost, hence the requirement of an AGP 3.0 mainboard these days for most, newer cards.

- Power ratings of AGP and AGP Pro slots

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