COTS comes calling

Today's aerospace manufacturers face the daunting challenge of maintaining pace with the rapidly evolving world of electronic systems. There is an ongoing migration toward serial switched architectures that support high data rates to realise advantages such as eliminating differential skew in parallel data paths, and switched fabrics are replacing shared busses because of the greater transmission efficiency and lower latency. While commercial silicon continues driving bandwidth and processing power, embedded computing platforms in the high reliability world of aerospace require rugged solutions capable of withstanding vibration, thermal extremes and other rigors of the application.

Modern aerospace embedded computing systems depend enormously on the board-level interconnects. These support flight, mission, and life-critical applications. While commercial connectors push the leading edge of speed and density, they are designed for benign, climate-controlled equipment rooms and not the rigours of flight. Conversely, traditional mil-aero connectors are rugged, but don't offer the speed and density required. The latest aerospace connector designs blend the best attributes of high speed commercial connectors with lessons learned from military/aerospace applications, offering the optimal combination of elevated data rates, dense packaging, and mechanical robustness.

Designing a high speed connector is a challenge in electrical engineering, requiring controlled impedance to prevent reflections, crosstalk, and other forms of signal degradation. High speed commercial connectors can meet demanding electrical requirements, but struggle to meet the mechanical ruggedness of aerospace requirements. The goal then, is to take the best of COTS technology and ruggedise it.

Connectors for aerospace embedded systems must be modular and flexible, capable of handling single-ended, differential, power, RF and optical signals. Modularity provides flexibility and scalability - building blocks that can be combined to meet a variety of application needs. Demonstrated ruggedness adds the final dimension.

One example of commercial technology ruggedised for aerospace applications is TE Connectivity's High Speed Ruggedised (HSR) connector. Derived from commercial Advanced Telecommunications Computer Architecture (ATCA) components, the HSR supports data rates of 10Gb/s. Shown in figure 1, the connector uses a common module size with inserts supporting differential and single-ended signals, power, RF and optics. A stout shell provides additional mechanical integrity, making the HSR suitable for line replaceable module applications.

COTS to VITA

The VMEbus International Trade Association (VITA) is a primary source for standardised embedded architecture development in aerospace & defence. VITA has guided the progress from the original VMEbus to the latest standards, such as VITA 41 VXS and VITA 46 VPX systems. VXS was the original multi-gigabit switched fabric VITA standard, but has been largely superseded by the even more capable VPX. VITA 47 defines environmental and mechanical ruggedness for VPX systems, including temperature cycling, vibration, shock, altitude, and more.

The VITA 46 VPX connector is the MULTIGIG RT2 connector from TE Connectivity. Using a pinless wafer design in place of pin contacts is well established in commercial applications. Wafers, available for differential, single-ended, and power needs, can be easily modified to support specific customer needs for characteristic impedance, propagation delay, and other electrical parameters.

The MULTIGIG RT2 connector supports speeds up to 10Gb/s, well above VPX's 6.25Gb/s. The design is highly modular to support both 3U and 6U configurations and the mixing and matching of differential and single-ended signals, power, RF, optics, and guideposts and keying.

Several other VITA standards extend VPX with modules for mezzanine boards, power, RF and optics.

While switched mezzanine cards (XMC) are popular for extending and customising VPX printed circuit boards, the original version of XMC (VITA 42) wasn't designed for aerospace applications. TE's Mezalok connector, designed to VITA 61, is a significantly improved version of XMC, offering superior electrical and mechanical performance. In capitalising on existing commercial technology, the new VITA 61 XMC 2.0 connectors (figure 2) use ball grid array (BGA) style solder ball board attach with specially designed compliant contacts.

The MULTI-BEAM XLE power connector, proposed as the interconnect for the VPX VITA 62 power supply standard, is also derived from a commercial design that offers 50A and 20A contacts. Even compared to earlier versions in the same connector family, the XLE delivers up to 40% more power in the same space. The design is hot pluggable, tolerates mating misalignment and has lower mating forces.

Two additional emerging VITA standards were developed to further expand the VPX ecosystem by defining modules for optical and RF applications. Both demonstrate the advantages of using proven, existing technology and adapting it to new applications.

VITA 66 for optics, gives users the choice of MT array connectors, ARINC 801 termini, or expanded beam (EB) contacts using a common module. Each style of termini has aerospace pedigree and offers different benefits in terms of density, ruggedness, reparability and other characteristics.

Pushing the envelope

While VPX is the prevailing standard for high speed embedded computing, newer designs are also coming. VITA 75, a VPX variant now under development, is a ruggedised small form-factor standard aimed at creating smaller, lighter, less power-hungry systems. One connector proposed for VITA 75 is the TE Fortis Zd connector (figure 3), which represents the leading edge in ruggedised COTS technology. Fortis Zd connector marries the 12.5Gb/s technology of TE's Z-PACK TinMan with the Mini-Box receptacle contact, representing the state-of-the-art in high data throughput and high reliability.

As sophisticated as these high performance aerospace connectors have become, some of the most elegant design is in the board attach. The backplane connectors have adopted the mainstream assembly techniques enabled by solderless press-fit (or compliant pin) contacts. Through-hole plating and diameters follow industry standards and do not require special processing. Standard press fit tooling can be used. Such compatibility also means standard routing rules can be used on the board. Similarly, the surface-mount VITA 61 XMC 2.0 connectors use industry-standard board design rules and reflow processing. With their advanced design, these rugged interconnects bring COTS manufacturing efficiencies to the world of aerospace.

Adapting and evolving commercial interconnection technology has proven to be the preferred way to meet the evolving needs of embedded computing in aerospace applications. Commercial technology often leads in achieving new levels of speed and density, needing only to be sufficiently ruggedised to meet the stricter requirements of aerospace. While such ruggedisation may not be a trivial matter, it's still much lower risk and faster to market than starting from scratch. Rather than proving a technology works at all, you need only demonstrate that the technology - already widely used - works in a more rigorous environment.

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