mubs said:
What's the deal on AGP speeds? Will a 4x/8x card throttle back in a 1x/2x motherboard? Some cards (especially Matrox) list all the speeds, like 1x/2x/4x/8x. Anybody know?
I really wish I had a complete (not to mention guaranteed correct) answer to that question.
I did a lot of reading this morning to see if I could come to any solid conclusions. In fact I did reach two solid conclusions. Here's the first one: the answer to this question was likely abundant all over the web back in the year 2000. Questions about compatibility with 1x/2x seem to be about 4 years late.
I tried though. Here is my second solid conclusion:
AGP specifications involve too many numbers.
Hope that's helpful.
If not, here's my totally nebulous, possibly incorrect, and most certainly vague third conclusion:
AGP
speeds are theoretically fully backwards compatible. However, the reality appears to be a bit different. AGP speeds are sometimes backwards compatible between 2x and 1x, 4x and 2x, and between 8x and 4x. Sometimes 8x is also backwards compatible to 4x
and 2x, but not always. But an 8x card will never be backwards compatible with an original AGP (1x) slot.
Why the discrepancy between theory and reality when it comes to backwards compatibility? Voltages. I think. Allow me to “explain”:
What speeds are there?
AGP (now apparently referred to as “AGP 1x” due to all the changes that came along)
AGP 2x
AGP 4x
AGP 8x
What voltages are there?
3.3V
1.5V
0.8V
Simple enough so far. Here's where things get confusing, primarily due to the fact that Yogi Berra was the head of the AGP design group.
AGP in the original specification worked with 3.3V. From what I read this morning, back then you could find AGP motherboards and graphics cards that handled 1x or 2x speeds at that 3.3V level. However, based on my memory and some common sense about how things develop, I can only assume that it was just AGP (1x) at first and that AGP 2x didn't turn up until some time later. I sure don't remember facing any dilemma as to which AGP “speed” to choose back when I was first in the market for AGP ... heck, it was just AGP or PCI ... none of this speed nonsense.
Anyway, that was apparently AGP in the original specification. Because there have been specifications released since then for newer AGP variants, people now seem to refer to the original AGP specification, running on 3.3V, at either 1x or 2x speed, as being belonging to the AGP
1.0 specification.
See what I mean about my second conclusion above? It gets worse...
Then the AGP specification 2.0 was released.
And all hell broke loose.
Hah. Just kidding. All hell didn't really break lose. But it should have, because things were relative straightforward until this point.
The AGP 2.0 specification introduced AGP at the 4x speed, which is ok if you're into that sort of “faster” thing. But it also did something evil. It declared a new voltage level: 1.5V.
Furthermore ... and god knows what group of idiots allowed this to happen and when ... it also seems that around this point it became possible to find AGP 2x cards that operate only at this lower voltage too. I guess some manufacturers of the slower speed cards figured they may as well adopt
some part of the AGP 2.0 specification, even if they didn't adopt the higher 4x speed.
So, for graphics cards and slots, we now have these possibilities:
AGP at 3.3V
AGP 2x at 3.3V
AGP 2x at 1.5V
AGP 4x at 1.5V
The introduction of 4x happened around, what, the year 2000?
Anyway, at this point, the AGP monkeys were still somewhat interested in backwards compatibility. At least a little. Remember, the
speeds are supposed to be backwards compatible ... it's the voltage that's the potential killer. (Hah! Get it? Potential killer? Nevermind.)
AGP slots are “keyed.” AGP slots with the vertical key nearest the backplate are 3.3V slots. AGP slots with the vertical key furthest from the backplate are 1.5V slots.
Just for the hell of it, there are also motherboards out there that apparently have “universal” AGP slots that can supply either 3.3V
or 1.5V. The AGP slots on these motherboards have no keys at all! Great.
Now, because the AGP 2.0 specification didn't just blindly introduce 4x without also requiring speeds to be backwards compatible, if you see an AGP 4x card, it
might be able to work at the slower 2x speed. Does this have something to do with the voltage, you ask? Who the
hell knows. I really have no idea. I'm looking at pictures of these 4x cards that are keyed for
both 3.3V and 1.5V. Now what the hell does that really mean? As best as I can tell, they'll be
electrically tolerant should you stick one of these “double-keyed” 4x cards into an AGP 1x slot, but who knows if it will actually work. All you
might be guaranteed is that the graphics card and/or your motherboard won't burst into flames because the card is
apparently telling you that it can at least shrug off 3.3V without causing damage, if not actually use it for something.
So, in my case, I'm just going to buy a freaking $40 AGP 4x card and ram it into my original AGP slot and see what happens. I'll post pictures here if doing so results in anything ... interesting.
I may as well add a comment about that 0.8V option. That's the new voltage level introduced by the AGP specification 3.0. Backwards compatibility was really broken at this point. If I read correctly, an 8x card, despite being an 0.8V device, is keyed for 1.5V. That just means it's tolerant of being stuck in a 1.5V slot. It does
not mean it's guaranteed to function in any way, shape or form. Technically it's an AGP 3.0 device, running at 8x, and it needs 0.8V. So if you don't stick it into an obliging slot, you're in uncharted territory. That said, again from what I read, most graphics card manufacturers seem to be pulling the required tricks to make sure their cards do work if you happen to do this however (presumably by making each of their cards capable of using
either 0.8V or 1.5). I can only assume that it should therefore also be possible for an 8x card to be tolerant of a 2x slot (again, whether it will work or not is up to the manufacturer). That, however, assumes you've got a 2x slot that is keyed for 1.5V! Apparently there's no way in heck you're going to get an 8x card working in either a 2x slot at 3.3V or a 1x slot at 3.3V.
Confusingly, much like when AGP specification 2.0 came out and you apparently wound up seeing AGP 2x out there at both the earlier 3.3V and the newer 1.5V, with the release of AGP specification 3.0, apparently AGP 4x now exists at the original 1.5V level, and this new 0.8V level.
So that leaves us with this:
AGP at 3.3V
AGP 2x at 3.3V
AGP 2x at 1.5V
AGP 4x at 1.5V
AGP 4x at 0.8V
AGP 8x at 0.8V (but sometimes the capacity to use 1.5V is apparently snuck onto the card by the manufacturer)
And then there's AGP Pro.
Cards that have that weird key swooping out the bottom like a tiny hockey-stick have included that thing so that they'll work in an AGP Pro slot. Beyong that, I'm not going to give AGP Pro a second thought. I'd never even heard of it this morning, which means one of the following:
1) I'm completely out of the loop these days with respect to computer technology
2) It died a horrible death and nobody actually uses it anymore
Ok, so both of those are possible.
My final thought:
I should have stuck with ISA.
I'll let you know how this $40 shot-in-the-dark goes. Until then, you can sit back and wait for all the posts from other people informing me how incredibly wrong I am about all this stuff.
