PCI Express is not the same as PCI-X

| by Ken | in Technology Add comments

Perhaps this is obvious to all readers already but since it wasn’t to me, I thought it would be helpful to get more info about this out there.  PCI Express is not the same as PCI-X.  Even though “X” is often used as an abbreviation for words with “X” as the second letter and even though PCI Express specs frequently include an “X” after the word Express. In hindsight, it’s obvious how I was wrong but since it may not be for others, here’s what I found.

I was shopping for a SATA card (more on that in a later post) and I had my pick of 4 ports in the computer: a PCIe-x4, PCIe-x2, PCIe-x16, and PCI.  I knew that the “e” meant that they were “Express” ports and that the “x” was the size of the port.  And when you look at an Express port compared to a regular PCI port, you can see the difference and know which is which.  Here is a picture from Wikimedia in which you can see the regular PCI on the bottom and 4 Express slots above.

For various reasons (which I will describe in that later post), I had locked in to a specific chipset: the Silicon Image 3124 or “SiI3124” (or “SiL3124”).  Cards made with that chip were generally listed as working with either PCI or PCI-X.  That was confusing to me because I was well aware that any individual card that fit in a PCI slot would not fit in an Express slot and vice versa.  I assumed that it was sloppy spec writing and since I had my pick of all of the variants (as I assumed), I ordered the card and figured I would see what arrived.

What arrived was indeed a PCI-X card.  Not a regular PCI card nor a PCI Express card.  The “X” means that it is an extended version of a regular PCI slot.  So the pins start in the same location as they do for regular PCI but they go longer.  Oh crap.  Looking back at the picture on the product description page, I could see that the connector tab is longer than on normal cards but I just assumed it was a lousy picture.  But nope, good picture and the specs did say “PCI-X”, not “Express”.  Uh-oh.

But wait!  It’s okay!  because this particular card does truly work with either PCI or PCI-X.  There are pin headers on it that tell the card which slot it is being plugged in to.  Very nice!  Although the card didn’t come with any jumpers so I had to scrounge for some.  And how does the card fit in a regular PCI slot if it is longer?  Because the card’s connector tab has a break in it right at the end of the regular PCI connector length.  Now, in my computer, the card has most of the connector tab in the connector but some of it is hanging in the air beyond it.  That’s the 64-bit part that is not used when you tell the card to operate in regular PCI mode using the jumpers.

Now that I know to Wikipedia this info, I see the very helpful comment at the top of both the PCI Express page and the PCI-X page saying “not to be confused with”.  Too late! Wikimedia does have a helpful picture that shows the differences. Something I should have read before ordering my card!  The regular PCI slots and cards are on the left and the extended PCI-X slots and cards are on the right.

So generally speaking, I believe most PCI-X cards will fit in regular PCI slots.  They may not function properly, however, unless you can tell the card to operate in a different mode and you may need jumpers to do so.  The reverse should be true too, where a regular PCI card would fit in a PCI-X slot but I don’t think there is a lot of non-server hardware that has PCI-X slots in it so that’s probably not a concern for most people reading here.

Hope this information is helpful to someone either before buying or maybe after as it was for me!

2 Responses to “PCI Express is not the same as PCI-X”

  1. […] because I had a PCI slot in my computer and the card would work for either PCI or PCI-X.  (See the separate post for my full story on my realization and subsequent […]

  2. […] out for a Silicon Image 3124-based card for $80 from CoolDrives.  (Note that even though it is a PCI-X card, it works in the PCI slot when using jumpers.)  Even though my total outlay is for both cards, I […]

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