I have a Fujitsu Primergy TX100 S2 server and I want to add more network cards. The only PCI slot is already occupied with a NIC. I have available one PCI Express x4 slot and two PCI Express x8 slots.

On my local shops I can only find x1 cards and those for x8 are very very expensive and rare.

My question is: if I buy a PCI Express x1 card it will work on PCI Express x8 slot? I want 2 pieces so both x8 slots will be occupied.

The network cards will be used in VMware ESXi 5.0.

2 Answers 2


Any PCI Express card of a smaller size will fit into a larger slot. So an x1 card will fit in an x8 slot. Please see the Wikipedia article for more info on the spec.

  • +1 Also cards must support being inserted into a smaller slot than they're designed for (A x16 card will "work" in a x1 slot). Some slots support this, others do not, it depends on the physical slot.
    – Chris S
    Commented Dec 14, 2011 at 13:57
  • +1. I found this on Wikipedia later: "A PCIe card physically fits (and works correctly) in any slot that is at least as large as it is (e.g., an ×1 sized card will work in any sized slot);"
    – Stefan
    Commented Dec 14, 2011 at 14:33

PCI-e is significantly different from the old PCI standard (and is in fact more based on AGP than PCI); one of the more visible differences is that PCIe has no parallel bus concept like PCI does.

PCI-e buses consist of lanes; one lane (x1) equals one serial 5gbit/s link between the host and the device.
These links are fully asymmetric and hence do not suffer from synchronization issues - this explains their far greater speed compared to PCI.

What is less visible but equally important is that the device and the host do not have to physically match the number of lanes they offer - a device with two lanes (x2) can connect to a host bus with 2, 4, 8, or 16 lanes on a single slot.

To further complicate things, the connector on the mainboard (the host bus or slot) does not have to actually connect every lane that will fit; an x16 slot can (and usually does) connect only 8 of the lanes, making it an x8 bus in reality.

These tend to be referred to as "x16(x8)" or similar.

So, to make the short answer to your question a lot longer: yes, you can connect any xN PCI-e device to an xM PCI-e host bus, as long as M >= N.

  • x16 slots also tend to offer varying numbers of lanes in desktop systems, depending on the chipset and motherboard slot configuration (though that's getting off-topic for ServerFault).
    – Andrew
    Commented Dec 15, 2011 at 0:24
  • I didn't imply that they didn't - or did you mean between the x16 slots on one board ? Yes, those tend to be x8/x4 or x8/x8/x4 etc.
    – adaptr
    Commented Dec 15, 2011 at 13:03

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