Designing embedded architectures
04 January 2010
VPX and VXS architectures raise unique design and testing challenges, as outlined by Martin Blake.
The military is looking hard at VPX and VXS to solve a variety of high-performance embedded computing problems, but designing and testing such systems takes some creativity. The growth of these architectures brings new, creative backplane/enclosure design solutions in several areas, including development/test, backplane/chassis configurations and integrated solutions. The ecosystem is expanding for the newer VITA 41 (VXS) and VITA 46 (VPX) architectures. Their growth and design similarities (including related components such as the MultiGig connector) continue to bring comparisons.
VPX is the focus of much attention these days in Mil/Aero applications, and for good reason. Acceptance of the architecture continues to grow, thanks to its sharp increase in potential bandwidth, level 2 maintenance features, design flexibility, I/O options and more. However, VME/VME64x is still the dominant player in deployed systems, with many shipboard, submarine, and aircraft applications more than happy with legacy VME performance. Meanwhile, VXS’s share of the market is growing in niche applications as it offers a significant performance boost over VME64x and its backwards-compatibility is particularly attractive.
VXS vs. VPX
VPX is growing rapidly and being selected for a larger proportion of Mil/Aero designs than VXS. However, VXS is often the right choice where its balance of performance, compatibility and price meet the application’s demands, although VXS has often been perceived as a mere stepping stone to VPX. Despite that perception, it is a viable technology in its own right that will be successful for many years to come. The VITA 41 (VXS) architecture continues to expand and grow. While there’ve been many feature comparisons of VPX vs. VXS, its helpful to examine some of the less-discussed points from a system platform perspective.
Compared to VXS, VPX uses more of the MultiGig connectors than comparative-sized backplanes. Using a mesh topology in most cases, VPX typically has higher backplane layer counts, and thicker layers, than VXS, as well as more complex routing, which can lead to higher backplane prices, while on the chassis side, VPX can have significant power and cooling demands. VPX has a lot of flexibility in its specification, so the architecture is typically highly customised, potentially increasing costs.
Cost Differences
Mil/Aero programs are obviously not as price-sensitive as other sectors, but, for applications with less demanding bandwidth needs, VXS can be an attractive option. Since it is more conducive to backwards compatibility and has a larger ecosystem, the architecture is seeing design wins in various applications.
At the last count, there were seventeen VXS board manufacturers with a total of 54 blade products, compared to five VPX manufacturers with a total of 39 VPX blade products. Even given that, VXS only has a 6U board size while VPX has both 3U and 6U sizes. The VXS ecosystem is still significantly larger, although this doesn’t mean that system designers will necessarily favour VXS over VPX. VXS has carved out a nice niche and will remain an enticing technology, but VPX is rapidly gaining acceptance thanks to its combination of performance, ruggedness and flexibility. Both architectures have some very interesting and creative design concepts including backplane topologies, chassis solutions, and system accessories.
Overcoming Challenges
New system architectures naturally bring design challenges as they push the limits of performance, and VXS and VPX are no exception. One issue is enclosure cooling —particularly for VPX, which can theoretically reach 468 W per slot, although in practice most of today’s cards are around the 100 W area. Many applications for VPX designs will be ones where forced-air cooling may not be an option and conduction cooling for that amount of heat transfer can be challenging. To address that issue, VITA 48 is underway, offering liquid cooling flowing through the individual modules, which requires elaborate piping with quick disconnects and presents interesting design challenges. However there is a less costly, modular, and simpler method to dissipate today’s heat levels; one solution is to run the liquid through the chassis walls. The VPX card wedge locks will conduct the heat to the enclosure wall, which can remove the heat
The liquid-heat-exchange approach has been proved to dissipate up to 150 W per slot, redundancy and modularity can be built in and each side of the chassis can have an independent cooling mechanism, so that if one side is damaged, the other wall can provide cooling until repairs can be made.
Development and Test
Beyond the power, signal, and cooling challenges of VXS and VPX, there’s also the matter of testing, which can call for some creative thinking. There are several products developed specifically for development and testing for VITA systems. These include VME/64x load boards, test extenders, development chassis, and test backplanes. The development system is a good place to start. Development systems are available in portable-style chassis and facilitate test and debugging of the cards. There are also creative solutions for the test chassis, for example in VXS it is possible to do development on payload cards without using switch cards. By incorporating a point-to-point signalling topology, payload cards can be tested. If a couple of slots are configured as VME64x, then full testing can be done across new VXS cards and legacy VME64x. Figure 2 shows a backplane diagram along those lines.
Open Frame Systems
Development systems can come in open frame styles, where they do not have the side walls of the enclosure, providing easier access to the cards. However, care must be taken when selecting one for VPX as the VITA 46 architecture can have high power loads and require considerable cooling. A development chassis for VPX needs to have special considerations for more airflow and power options.
In additional to development systems, the VXS or VPX system platform that will go into the field need to be tested. Both of these architectures need a way to easily test the signal performance, cooling capability, power conditions, and to debug new cards. To make cooling and power testing easier, a load board can be devised for these systems. The board can aid designers in locating hot spots in the chassis and confirming the power and electrical connections meet the VITA specifications. Figure 3 shows a VPX Load Board with go-no-go indicators.
Using Flex Circuits
Another challenge for test and debug is the lack of right angle connectors for VXS and VPX extender boards. A way to overcome the absence of the required MultiGig connector format for extenders is by using flex circuits. Incorporating a rigid-flex-rigid PCB design provides a fix, allowing the VXS or VPX extender card to accept the board-under-test as a straight-mate alignment versus a right angle. Frames and injector/ejector handles can be used to securely hold the boards and facilitate plugging.
Finally, testing the signal across the backplane and/or full interconnect path is very important. Particularly with Mil/Aero systems and the more stringent testing requirements, designers cannot afford to risk that their system does not work properly. Some internal company labs do not have adequate equipment to accurately characterise the signal performance of today’s high speed fabrics. Or, the expensive lab equipment is so tied up there aren't resources available to do a “health check” of the system. SerDes test modules can be designed in VPX, VXS or other form factors to test the signal quality without tying up a lab. These boards can simply plug into an open slot and test the BER (Bit Error Rate), skew, jitter and so on of the board or backplane path. They can also be used for pattern generation, such as eye diagrams, ensuring the signal quality meets the specification.
Integration Challenges
Doing individual testing of the VXS or VPX designs is one thing, but the dynamics change when they’re all packaged together with in a system platform with other boards and software. Integration is another important step in the process. Depending on the application, the sub-system integration for embedded systems may be done in-house, at an integrator, or by the enclosure manufacturer if they provide that service. Integrating the SBC, switches, and other cards in a VXS or VPX system can be tricky; the cooling, power specification and signal issues can all cause headaches, as can high temperatures, shock and vibration, hot-swap, system management and other requirements. It is important to perform complete verification, validation and qualification testing along with all of the supporting documentation.
Figure 4 is an example of a fully-integrated VXS subsystem designed with the backplane, system platform, and integration done together. Troubleshooting and testing is much more seamless, allowing a quicker time to market, and less program risk.
It can be advantageous to start development with a turnkey bundle. This includes a completed and integrated subsystem that has been fully tested, which will typically provide time and expense savings. The VXS and VPX ecosystems continue to expand; there are creative solutions for handling the faster speeds, cooling and power demands and other barriers from the newer architectures. Further, new test tools are making it easier for designers to ensure the performance of their system. The future is looking brighter for VITA-based systems.
Martin Blake works for Elma-Mektron
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