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AGP compatibility for sticklers: The Short Version

If an AGP card fits in an AGP expansion slot then they are compatible. But read the rest of this anyway. There are some rare exceptions where motherboard and video card manufacturers don't obey the rules.

Say I have a motherboard that only supports 1x/2x AGP, would 4x/8x AGP card work with that mobo? If so what would be it's disadvantage? Agp 1.0 = 1x and 2x agp 2.0 = 4x agp 3.0 = 8x the revisions are based on speed and voltage. By saying it requires a 2.0 slot, they are saying it is not 1x compatible, but will go down to 4x even though it is an 8x card.

AGP compatibility for sticklers: The Long Version

AGP Basics

In 1996, Intel introduced AGP 1.0. AGP was a modified version of PCI designed to speed up transfers to video cards. It was followed by AGP 2.0 in 1998 and AGP 3.0 in 2002. Each new version added new speeds and signaling voltages.

The signaling voltage is the voltage used to send data between the AGP card and the AGP motherboard. '1x' means '1 times'. 1x is the base AGP speed. 2x is twice as fast as the base speed, 4x is four times as fast, and 8x is eight times as fast. You can download the final AGP 3.0 specification from here. You may also want to look at the AGP 1.0 specification, the AGP 2.0 specification, and the AGP Pro 1.1a specification.

AGP Multipliers

AGP supports four different speed multipliers: 1x, 2x, 4x, and 8x. Looking at the various possible signaling voltages and speed multipliers, you might think that there are a large number of different kinds of video cards and motherboards supporting various combinations of voltages and multipliers. But it's actually much simpler than that. The AGP 1.0 specification requires that all implementations support the 1x speed multiplier at 3.3 volts. The 2x multiplier is optional. There's no such thing as a 3.3 volt video card or motherboard which only supports 2x. By default, when the AGP 1.0 machine powers up it selects the fastest speed multiplier supported by both the video card and the motherboard. If they both support 2x then they will run at 2x. Otherwise they run at 1x which is always implemented by all AGP 1.0 video cards and motherboards. There is often an option in the BIOS which allows you to limit the speed to 1x if 2x is not reliable. The AGP 2.0 specification has a similar requirement. 2x and 1x support at 1.5 volts are required and 4x support is optional. The AGP 3.0 specification requires support for 8x. The 3.0 specification isn't as clear as the 1.0 and 2.0 specifications on the subject of requiring the lower multiplier but all AGP 3.0 implementations that I've seen support both 8x and 4x. As a result, you can completely ignore speed multipliers when you're checking for compatibility between an AGP video card and an AGP motherboard. If the video card and motherboard both support the same signaling voltage then there is always at least one common speed multiplier supported by both at that voltage. You only need to make sure that the video card and motherboard have at least one signaling voltage in common.

AGP Connectors And Slots

Each AGP card has one or two slots in its card edge. If a video card has the 3.3 volt slot, then it can use 3.3 volt signaling. AGP 2.0 added the 1.5 volt slot on cards which could use 1.5 volt signaling. If the card has both slots then it can use both signaling voltages. AGP 3.0 added support for 0.8 volt signaling but it did not add a new kind of slot. If a video card supports either 1.5 volt or 0.8 volt signaling then it has the 1.5 volt slot.

The AGP connectors on the motherboard are keyed to prevent insertion of AGP cards which would be damaged if plugged in. An AGP 3.3V motherboard connector can only accept AGP cards which have the 3.3V slot. If you try to insert a card without a 3.3V slot into an AGP 3.3V motherboard connector, the card will bump into the connector key and cannot be inserted. Likewise an AGP 1.5V motherboard connector can only accept AGP cards with the 1.5V slot. An AGP universal motherboard connector has no keys and therefore can accept any kind of AGP card. An AGP card with both voltage slots can be plugged into any kind of AGP motherboard connector. If you can plug an AGP card into an AGP motherboard connector, then neither the card nor the motherboard will be damaged (assuming they obey the AGP specifications).

AGP Pro motherboard connectors were created to support video cards which use more power than a plain AGP connector can supply. There are extra connectors on both ends of an AGP Pro motherboard connector which allow an AGP Pro video card to draw more power. Plain AGP cards are fully compatible with AGP Pro motherboards, but it doesn't work the other way around. AGP motherboards are not compatible with AGP Pro video cards. High-end motherboards are often actually AGP Pro motherboards because they can accept both AGP cards and AGP Pro cards. But AGP Pro video cards are rarely seen outside the workstation market because they aren't compatible with AGP motherboards. Consumer-oriented video cards which need to draw lots of power usually require you to plug in a disk drive power supply cable. That way they can be compatible with all motherboards and still use lots of power.

Official AGP Compatibility

The table above gives Intel's official names for the various kinds of AGP cards allowed by the AGP specifications. Unfortunately, the technical specifications for a video card rarely use these terms properly (if at all) to describe the video card. They usually just list the fastest AGP multipliers it supports: '8X, 4X', or '4X'. From that information and the voltage slots on a picture of the video card, you can often figure out exactly what it is. I've seen plenty of video cards listed as 'AGP 3.0 cards' when in fact they are actually universal 1.5V AGP 3.0 cards. I've also seen cards listed as 'AGP 8X,4X 1.5 volt only' when there is really no such thing. What they are trying to tell you is that it's a universal 1.5V AGP 3.0 card and doesn't support 3.3 volts. It is required to support 0.8 volts if it supports 8X.

The table above gives Intel's official names for the various kinds of AGP motherboards allowed by the AGP specifications. The technical specifications for motherboards tend to be just as sloppy as they are for video cards. Sometimes they use the official motherboard type names properly and sometimes they do not.

The table above shows the results of all possible combinations of AGP cards and AGP motherboards.

Practical AGP Compatibility

The previous table shows that there are combinations of motherboards and video cards which can be plugged together but do not work. According to the AGP specifications there should be no damage, but the combination would not be compatible. You see people posting in forums worrying about this possibility all the time. There are also stories that you can get some 0.8 volt only cards which are willing to tolerate 1.5 volts but that you really shouldn't plug them into an AGP 4X motherboard even if they work. Given the previous table, the confusion is understandable. But there are two pieces of information which they're missing: nobody makes AGP 3.0 cards, and nobody makes AGP 3.0 motherboards. At least not any manufacturers I can find. Every single video card I could find which claimed to be an AGP 3.0 card was actually a universal 1.5V AGP 3.0 card. And every motherboard which claimed to be an AGP 3.0 motherboard turned out to be a universal 1.5V AGP 3.0 motherboard. It makes sense, if you think about it, because if anyone actually shipped a consumer-oriented product which supported only 0.8 volts, they would end up with lots of confused customers and a support nightmare. In the consumer market, you'd have to be crazy to ship a 0.8 volt only product. And as far as I can tell, there aren't any in the workstation market either. There may be some obscure product somewhere, but I sure can't find any. It's easy to find ones which are mislabeled as AGP 3.0 cards or motherboards but I haven't been able to find the actual item. If you remove the 0.8 volt only entries from the compatibility table, then you end up with the following table.

That's why 'If an AGP card fits in an AGP slot then they are compatible' is actually correct if you only consider stuff you can really buy.

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One practical matter which must be considered is the fact that some of the original AGP 1.0 motherboards do not provide enough power to operate some newer AGP video cards reliably. For example, some of the original motherboards using the first chipsets which supported AGP (like the Intel 440LX and 440BX) can become unstable if you install video cards which draw lots of power through the AGP slot. The motherboards can't always supply the necessary current for the newer video cards. So if you're adding a video card to an AGP 1.0 motherboard then it's a good idea to install a video card which doesn't consume very much power.

You can also occasionally get memory resource conflicts by installing a new AGP video card into an old AGP 1.0 motherboard. The video card will work properly until you install the display driver. Once you try to install the driver, a memory conflict shows up. The range of conflicting addresses varies from case to case. This problem is very unusual and when it happens it is rarely possible to fix it. I'm not sure exactly what causes the problem but apparently the motherboard and the video card are incompatible in some way which prevents Windows from properly assigning memory addresses to the video card. In the cases I've seen, there doesn't seem to be any way to predict from the video card chipset and motherboard chipset whether there will be a problem. Sometimes a particular video card chipset and motherboard chipset get along well and other times they don't. I'd guess that it's some kind of incompatibility caused by an outdated motherboard BIOS and possibly the video card BIOS. The one thing you can try is to flash your motherboard with the most recent BIOS. But since it's an old motherboard, the manufacturer will most likely not have anything but old BIOSes available. If you're running Windows 95, 98, or ME, it may be possible to manually assign addresses and get it to work but I've seen people try this and the process is about as enjoyable as a root canal and usually fails to fully solve the problem anyway. If you're running Windows 2000 or XP then it's probably impossible to fix because the newer versions of Windows almost always prevent you from manually assigning addresses, IRQs, etc. That's almost always true even if you select the standard PC HAL while installing Windows with the hope that it will allow you to assign resources manually. If you run into one of these memory resource conflicts then you should probably give up and try a different video card. It's rarely fixable.

AGP Cards

The table below lists the AGP graphics card type for just about all of the consumer-oriented AGP graphics cards. Note the complete lack of 'AGP 3.0 Card' entries. The graphics chipset determines what kind of AGP card it is; not the brand of the video card. If you would like to know what kind of video card is in your computer, open the 'Control Panel' and double-click 'Display' to bring up the 'Display Properties' window. Then select the 'Settings' tab, click 'Advanced', and select the 'Adapter' tab. Sandra Lite can also give you very detailed information about your display adapter.

Sometimes manufacturers don't obey the rules. I found a video card which had incorrect voltage slots. It had only a 3.3 volt slot when in fact the video card could also have accepted 1.5 volts. That card would work correctly in an AGP 3.3V motherboard but it also would have worked in a 1.5 volt motherboard had it been given the 1.5 volt slot. There also was a SiS video card manufactured years ago which had the wrong voltage slots. If you plugged it into the wrong motherboard, it would be destroyed. The 'wrong slots' case used to be extremely rare and any manufacturer which made that mistake quickly acquired a very bad reputation. Unfortunately building AGP video cards with the wrong voltage slots is becoming more common. It's not difficult to find no-name, low-budget, Chinese video cards on EBAY which have the 3.3 volt slot despite the fact that the graphics chipset does not support 3.3 volts. It pays to stick with competent manufacturers. As long as they obey the AGP spec, you cannot damage anything by plugging a video card into a motherboard.

AGP Motherboards

There are a few motherboards which do not use the correct AGP connector. This AOpen AK79G Max is a universal 1.5V AGP 3.0 motherboard and yet it has an AGP universal connector which accepts 3.3 volt cards. Fortunately, it also has circuitry which prevents damage when a 3.3 volt AGP card is inserted and lights up an LED to warn you that the video card is a 3.3 volt card. I think they built the motherboard this way as a defense against video cards with incorrect voltage slots. You can't damage the motherboard or video card even if the video card has the wrong voltage slots with this kind of design.

You may also come across the occasional motherboard which has an AGP universal connector covered by a sticker which says 'do not insert 3.3 volt cards'. According to the specification, they shouldn't do that. Perhaps they didn't want to go to the trouble of keeping 1.5 volt keyed connectors in stock. It's a shame users have to be careful about this because if manufacturers obeyed the specification, it wouldn't be possible to make a mistake.

I also found a picture of a motherboard from one obscure manufacturer which had a universal AGP connector rather than the 1.5 volt keyed connector that it should have had. I doubt it had any circuitry to prevent damage if you inserted a 3.3 volt card and there was no hint that you shouldn't insert one. It's a good idea to stick with established manufacturers. But that was only one motherboard out of hundreds I've looked at. If you're buying a motherboard from someone you've never heard of, it might be a good idea to check that they've used the right AGP connector.

The table below lists the AGP motherboard type for just about all the consumer-oriented motherboard chipsets. There isn't a single 'AGP 3.0 Motherboard' chipset in the list. If you're not sure what kind of chipset is in your motherboard, then you can use CPU-Z or Sandra Lite to find out. Also remember that even if a chipset supports AGP, it doesn't mean that a motherboard using that chipset will always have an AGP slot. Some low-end motherboards using AGP chipsets leave out the AGP connector to cut costs.

Final Words

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Keep in mind that there are plenty of things which can go wrong when you plug an AGP card into an AGP motherboard. The information given above just tells you about their signaling voltage and speed multiplier compatibility. If they're not compatible as described above then they definitely will not work together. But even if the signaling voltages are compatible, there are other things which can occasionally go wrong when you plug the card into the motherboard. The most common is a motherboard BIOS incompatibility which prevents the motherboard from fully recognizing the video card. The only way to solve that problem is by installing an updated motherboard BIOS. There are other more obscure problems like AGP signal strength and AGP timing issues which can make communications between the motherboard and video card unreliable. Those are very rare but they happen. There's no way to know ahead of time whether you'll have that kind of problem unless you can find other people who have tried the exact same video and motherboard hardware with the exact same BIOS versions.

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