Author: Manfred Schmitz, Chief Technical Officer, MEN Mikro Elektronik
Rear I/O in the 6U area is very important for many applications. Telecom applications, for example, only allow front I/O for service purposes. The reason for the use of modular systems based on plug-in cards is actually the extremely short time that is needed to exchange boards in the case of service. Thanks to the hot-plug capability of CompactPCI Serial (CPCI-S.0), you don't even have to switch off the power to exchange a board. Many connections at the front, which would have to be removed before exchange, could completely undo this advantage. The solution is rear I/O.
In measuring technology there is yet another motivation for rear I/O. A board with an analog front end often needs an adaptation for measuring values that is different for every application. You can very well accommodate line drivers on a tailor-made rear I/O adapter. This leaves the front for convenient status displays.
Rear I/O technology also incorporates two major difficulties. In common CompactPCI systems, all signals must be led from the front to the backplane. The backplane's connectors route the signals on to the rear I/O adapter. With (parallel) CompactPCI the rear I/O and also the PCI bus signals are fixed to 2-mm connectors. These are not well suited to support the high bandwidths of digital communication or to handle highly sensitive signals in the field of measurement engineering.
This is why CompactPCI Serial completely does without a definition of the rear I/O connector and even leaves out the backplane in this area. The individual connector of the front board directly meets the corresponding connector of the rear I/O board.
This approach has only benefits: Having nothing to do with rear I/O, the backplane becomes smaller and less expensive. As not all the slots may use rear I/O, because they don't need it, no connectors are reserved for rear I/O at all. While there is still a discussion in the VPX area on how to support optical interfaces, CPCI-S.0 offers a trouble-free way to make this possible. The front board can directly meet any desired optical connector of the rear I/O card. The same goes for sensitive signals in the area of measurement engineering. Special connectors with a high signal-to-noise ratio of the front board can directly contact the rear I/O board.
This makes the concept for 6U rear I/O of CompactPCI Serial equally suited for applications like optical data transmission or the transmission of highly sensitive signals. In the same way you might as well employ common 2-mm connectors, for instance for hybrid boards. The rear I/O connector then leads the signals of the parallel PCI bus to the rear I/O adapter. In this special case, the rear I/O adapter is simply a common CompactPCI backplane.
CPCI-S.0 on the one hand offers a very high degree of interoperability by only a few options, while on the other hand putting hardly any limits to individuality.