A quick overview of the PCI-Express bus slot interface

PCI Express, officially abbreviated as PCIe (and not to be confused with PCI-X, which stands for PCI Extended), is the latest interface to be found on PC motherboards for high performance cards such as the those in the Spectrum instrumentation range. One of the advantages to PCIe is its almost universal inclusion on current PC motherboards

PCIe is based around serial links called lanes. With PCIe 1.1 (used on Spectrum M2i and M3i cards) the data lane can theoretically carry 250 Mega Bytes/second in each direction, however the full rate is not possible in reality because of transfer overheads. Single lane Spectrum PCIe cards typically provide speeds of about 130 to 160 MBytes/second, however this is still about 30% to 60% faster than the older standard PCI slot, hence suitable for many signal capture or waveform generation applications. All Spectrum single lane M2i and M3i cards can physically fit in PCIe slots with up to 16 lanes. PCIe has the major advantage over PCI/PCI-x in that there is no sharing of data bandwidth, each slot can work at its full capability and so too each connected card. Note that for speed critical applications where single boards are in use, it is can be more advantageous to use PCI-X, the fast version of the PCI slot, but consider too the newer M2p and M4i ranges of cards (see below) based on multi-lane PCIe.

The release the second generation PCI Express (Gen 2.0) doubled the theoretical data rate of each lane from 250 MB/s to 500 MB/s. A PCIe 2.0 PC slot is still compatible with PCIe 1.1 as a physical interface slot and from within software, so all Spectrum PCIe based cards are able to work in machines fitted with this slot version and whilst there is no performance advantage for the M2i and M3i ranges, M4i can take full advantage of the additional bandwidth, furthermore it has eight data lanes allowing data transfer speeds with the host PC totalling in excess of 3 Gbytes per second! Note that any M4i card must have a physical slot of at least 8 lanes. This card can also work with Gen 3.0 slots. The M2p cards by comparison is a 1.0 generation x4-lane connector. Even so, it too can work with later generation slots and can still provide a data transfer rate of over 600 MB/s.

Please note that some motherboards have physically x8 slots or x16 slots which are electrically only connected to x1 or x4 lanes. The M4i and M2p cards do fit into these slots and work fine but are of course limited in transfer speed.

Should you wish to read a more detailed examination of PCI-Express please see here; https://en.wikipedia.org/wiki/PCI_Express

DataQuest Solutions Ltd August 2013
Last modified 10/05/18