Author: Manfred Schmitz, CTO MEN Mikro Elektronik
CompactPCI Serial, CPCI-S.0, is the only standard for modular computers that unites PCI Express, Ethernet, SATA and USB on the backplane. PCI Express is a relatively common backplane interface, while USB seems to be rather unusual.
USB is in fact a widely used standard for the connection of peripheral devices like keyboard and mouse, USB Flash drives and external hard disks especially to mobile computers. Nearly all modern laptops also provide so-called Express card slots. These allow to retrofit the computer externally, for instance by GSM/UMTS modems. Express cards are based on USB and alternatively on PCI Express. Internally, laptop computers also use both interfaces to connect so-called PCI Express Mini Cards. Practically all WiFi modules inside laptops are based on this standard, which in turn is electrically built on PCI Express AND USB.
In order to be able to use all PCI Express Cards and PCI Express Mini Cards even in a modular 19" computer, you must also support these electrical interfaces – both USB and PCI Express in one slot. This way you can equip CompactPCI systems with wireless technology.
To keep things most simple for the users, they should be able to put any board into any slot. This requires that every slot defines identical interfaces, as is the case with CPCI-S.0. Every slot supports USB and PCI Express (as well as Ethernet and SATA). The CPU centrally controls USB and PCI Express, and distributes the signals to each of the eight peripheral slots via the backplane. This architecture is called a star topology and in the case of CompactPCI Serial does not need any further hardware like switches, bridges or fabrics, making it simple and inexpensive.
Modules with a higher bandwidth usually use a PCI Express interface (e.g. WLAN), while USB is sufficient for modules with a lower bandwidth such as GSM/UMTS. With just one link (i.e. one differential receive and one transmit line), PCI Express already achieves 250 MB/s. USB 2.0 falls far behind with its 480 Mb/s. However, USB 3.0 reaches bandwidths similar to PCI Express. CompactPCI Serial defines both, USB 2.0 and USB 3.0.
Where PCI Express and USB are available concurrently, inexpensive technology can be used around modern, modular, industrial computers. The support of USB 3.0 is impressive evidence of how future-proof CompactPCI Serial really is.