Mezzanine cards have held an entrenched position in military embedded systems since the early days of SBCs and standard backplanes. They enable system developers to mix and match the functions they need and by doing so create a semi-custom solution using off-the-shelf products. Meanwhile, processor-based mezzanines pioneered the idea of separating computing functions from I/O and application-specific functions has become a core theme in military applications-a notion that's extremely attractive for applications with long design cycles like the military. Today, the long popular PCI-based PMC and Processor PMC (PrPMC) each have their respective successors in the form of XMC and PrXMC.

The VITA 42 XMC set of standards provides backward compatibility with legacy PMC modules while allowing PCI bus products to integrate switched fabric architectures. The standards build on the existing PMC standards by adding switched fabric interconnects to the existing PCI bus interface. XMC has a conduction-cooled option that piggybacks off the VITA 20 Conduction-Cooled PMC standard.

To support gigabit serial interfaces, notice that both P15 and P16 connectors define 10 full-duplex differential pair lines. The VITA 42.0 base specification does not dictate signal types, data rates, protocols, voltage levels or grouping for these signals. Instead, it leaves that up to the several sub-specifications that are part of the VITA 42 family. This allows XMCs to evolve as new interconnect technologies and protocols emerge.

Over the past couple of years, FPGAs have become a fixture in mezzanine card designs. As the product roundup on the next couple of pages shows, FPGAs are a dominant part of most of the latest crop of XMC products. FPGAs offer a collection of resources ideally suited for peripheral I/O functions. FPGAs may be configured to implement numerous electrical interface standards as well as a variety of protocol engines.

Reconfigurable FPGAs can be used to enable an I/O board to replace several legacy products, while adapting to future standards and protocols as well. This helps to mitigate product obsolescence, both at the board level and at the deployed system level. Thanks to the magic of today's level of semiconductor integration, multi-function board products have emerged enabling military system designers to blend a variety of I/O functions onto a single XMC card. The challenge has been to choose I/O technologies that are suited for use together.

In applications that depended heavily on signal acquisition, raw resolution and bandwidth are only effective if the analog front end and the acquisition subsystem maintain good signal integrity as the signal is moved into the digital domain for processing. Here, XMC mezzanines help that issue as the analog components can be physically on a separate card from the digital processing components on the carrier card.

An example of a system suited to take advantage of XMC-based FPGA processing solutions is the E-2D Advanced Hawkeye (Figure 1). The data recording and playback systems for the E-2D can scale up to dozens of modular, heterogeneous input/output channels and FPGA-based protocol engines to support application-specific processing in real time during record and playback. As storage technology and FPGA technology advance, that approach allows the system architecture to boost throughput and storage capacity through reuse of the modular building blocks within an open standard framework.