A quick overview of the PCI-Express bus slot interface

PCI Express, (often abbreviated to PCIe) is the current interface commonly found on PC motherboards for high performance cards such as the those in the Spectrum Instrumentation range.

PCIe is based around serial links called lanes. With PCIe version 1 (used on Spectrum's M2p cards) the data lane can theoretically carry 250 Mega Bytes per second in each direction, however the full rate is not possible in reality because of transfer overheads. Each single-lane (commonly denoted as x1-lane), typically provide speeds in the range 140 to 180 MB/s. The M2p cards have the advantage of a x4-lane connector, potentially giving a total data transfer rate of over 700 MB/s. PCIe also has a major advantage where multiple cards are installed on the same computer motherboard as there is no sharing of data bandwidth, each slot can work at its full capability and so too each connected card.

The release the second generation PCI Express (version 2) has doubled the theoretical data rate of each lane from 250 MB/s to 500 MB/s. This version is used by Spectrum's M4i series of cards, which also benefit from having a x8-lane data connector. This allows allowing a total data transfer speed with the host PC in excess of 3 GigaBytes per second for a M4i digitiser card streaming to the PC. It is about 20% slower in the other direction, which is what is required for transferring data to a M4i waveform generator card, however this still represents a huge bandwidth for data.

Note that any with multiple lanes card must have a physical slot long enough to suit. It is possible to fit a card with less lanes into a longer slot, so for example the x8-lane card will fit and work in a x16-lane slot, but with no increase data transfer performance. Also note that some motherboards have physically x8 slots or x16 slots which are electrically only connected as x1 or x4 lanes. The M4i and M2p cards do fit into these slots and work fine, but may have a limited transfer speed.

PCI-Express cards are also versatile in terms of version compatibility. So for example a version 2 card will work in a version 1 slot, but its data transfer will be limited. It can be fitted in a version 3 slot, but it will still be operating at version 2 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 12.09.19