RTC Magazine

The VPX backplane architecture represents a major leap forward for system integration flexibility through its support of flexible hybrid configurations. These configurations include flexible topologies, multiple signaling protocols and hybrid core architectures, such as mixed VPX and legacy VME64x configurations, in addition to multiple power choices. Earlier backplane specifications, such as VME, strictly defined slot usage. These previous backplane architectures defined how connector pins would be used by a given board, and how each card slot would be connected to the next card slot. These backplanes limited the system capability because key architectural features were defined rigidly from the start. For example, decisions about system connectivity (how boards are connected to each other) determined a specific interconnect topology. VPX was designed to enable end-users to employ any one or a combination of the popular interconnect topologies in a single backplane to best fit any given application.

The VPX Core standard provides for the development of hybrid backplanes because it is designed to simultaneously support a mix of bus segments. For example, these integrated bus segments can be configured in full mesh, pipeline or single or dual star topologies. It is also permissible to have some slots configured as legacy parallel VME. Of the seven connectors in each slot, numbered 0-6, connectors J1 to J6 may be implemented for either differential signals or single-ended signals. This flexibility allows a user to use exactly as many pins and connection configurations, etc., as are needed for the specific application. VPX defines a standard card layout and standard mechanics, electrical utilities and a range of fabric options but lets system engineers connect the dots between them so as to conform to the exact needs of their application.

The term, “hybrid backplane” typically suggests either bringing together heterogeneous backplane architectures such as fabric-based VPX and parallel VME64x (legacy hybrid), or the mixing and matching of different types of network topologies, such as mesh and stars (hybrid topologies). There are, in fact, four different types of hybrid backplanes. In addition to the hybrid types just mentioned, VPX adds the support of hybrid protocols, which involves mixing different fabrics, for example Serial RapidIO and PCI Express, on different channels or bus segments. It also supports a hybrid power approach that allows the integrator to choose the primary voltage for his application from the choices: 3.3 VDC, 5 VDC, 12 VDC or 48 VDC power. Thus, the VITA 46 (VPX) backplane architecture uniquely embraces all four of these hybrid concepts into a single flexible backplane standard.

Legacy Hybrid

The idea of legacy hybrid has been a familiar one in VME development over the years. For example, in a VXS backplane it is possible to combine side by side, legacy VME64x boards with the 2 mmP0 connector alongside fabric-based boards with the differential Multi-Gig J0 connector. In the legacy VME64x slots, boards such as an SBC could be running StarFabric over the 2 mm HM P0 connector, while other systems could populate the legacy VME64x slots with cards based on Ethernet or Myrinet serial protocols. Either of these legacy choices could be combined with multiple slots of newer VPX fabric cards based on Serial RapidIO. For VPX users, the legacy hybrid approach is a transitional, bridging approach that makes it easier for people to use the available VPX boards immediately by combining them with other existing legacy boards. Some early adopters have expressed an interest for hybrid backplanes with large numbers of legacy slots and smaller numbers of VPX slots, For example, one proposed backplane offers nine slots of legacy VME and three slots of VPX. As more VPX boards become available, it’s expected that the ratio will switch toward more VPX slots and fewer VME.

Implementing legacy hybrid support requires a VPX standard that maps signal locations for parallel address and other system signals required by older VME boards onto one or more VPX slots. These special VPX slots are for cards that are capable of communicating across both a serial VPX protocol as well as the parallel legacy VME protocol. Again it can be noted that not all VPX slots in such a legacy hybrid backplane would be required to support dual architecture cards. Part of the challenge in developing a legacy hybrid VPX/VME backplane involves proper wiring to ensure that electrical signals are assigned to the VPX slot in a way that meets the constraints of signal routing while maintaining signal integrity by keeping sensitive signals away from each other. The backplane in Figure 2 has a single VME slot and it is designed to support a card such as the Curtiss-Wright VPX-185 in slot 1 (Figure 1). Any or all of the VPX slots could be configured in this way, however, there are other uses for the signal positions that this would monopolize.