CompactPCI® and CompactPCI® Express System Architecture

2009-02-06, Manfred Schmitz

The original CompactPCI® architecture is based on a parallel bus which connects the system slot to the peripheral slots. The CompactPCI® bus makes it possible to mix 32-bit- und 64-bit-assemblies, though in the last instance the parallel bus limits the data rate. For this reason, the AGP interface (Accelerated Graphics Port) has been especially developed for graphics applications as a fast parallel point-to-point connection. AGP has not been implemented in CompactPCI® and is only used on the boards. For multiprocessing CPUs on peripheral slots are controlled via non-transparent bridges. With these bridges certain memory areas can be shared so that they can communicate. However, the software overhead required for this is considerable. There are no standard solutions.
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A CompactPCI® extension - PICMG 2.16 - introduced Ethernet on the backplane as another transmission medium beside the parallel PCI bus. This way each board could communicate with the other assemblies over two Ethernet links via a special switch board, so that for multiprocessing the software problem was solved. The Ethernet signals were led to the J3 connector making PICMG 2.16 only applicable for 6U boards.

CompactPCI® Express finally turns away completely from the parallel PCI bus and is based exclusively on PCI Express® as a fast serial point-to-point connection. Ethernet was not taken into account. Legacy CompactPCI® plug-in boards are connected via a bridge board. The routing of the PCI Express® links can be controlled via switch boards. Like for Legacy CompactPCI®, special, non-transparent PCI Express® Bridges with special software requirements are needed for multiprocessing.

CompactPCI® evolves further. The architecture adapts to modern chipsets. The proven mechanics of the IEC 1101 are adopted. Existing boards can be continued to be used. CompactPCI® in conjunction with the extensions PlusIO and Plus is a future-safe platform for industrial computers.


PICMG 2.16 complements CompactPCI® by Ethernet, enabling easy multiprocessing. PICMG 2.16 is only defined for 6U systems.


CompactPCI® Express is exclusively based on PCI Express®. Ethernet is not supported. Switches organize the routing of the signals. A PCI-Express-zu-PCI-Bridge is required for supporting Legacy CompactPCI® boards. For multiprocessing non-transparent bridges are needed.

Manfred Schmitz, Technical Director MEN Mikro Elektronik

German version on ElektronikPraxis: Systemarchitektur von CompactPCI und CompactPCI Express

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