Feature Article - Prototyping and Testing for Switched Fabrics

Products continue to be developed for the new switched fabric technologies, such as AdvancedTCA (PICMG 3.0), StarFabric (PICMG 2.17, 3.3), and soon VXS (VITA 41). Backplanes, switch cards, and chassis designs are becoming more prevalent for these technologies. But sometimes, products for prototyping and testing new technologies are overlooked. Without these products, how will developers test and debug their products based on these technologies?

Products like extender cards and load boards are not the only items that need to be used for prototyping. Development units in backplanes and chassis are also critical. Usually smaller and simpler (and certainly a lower price) than "standard" units, development units can be a cost-effective way to do test and debug. We'll look at what we may see on the horizon for prototyping of some of the new standards-based technologies. Plus, we'll take a look at new ways companies are dealing with the high-speeds of fabrics for their testing.

Accessories for Fabrics

How does one do prototyping and testing for new switched fabric designs? Adapter cards are one way for board designers to use existing line cards while prototyping new fabrics.

Traditionally, adapter cards were used to implement cards of one form factor into a chassis of another form factor. For example, some allow a 6U x 220mm VME card to plug into a 9U x 340mm system. Adapter cards are being developed today to act as bridges between the legacy traffic and new fabrics.

One example is the StarFabric Adapter card. This device allows one to forego having to develop a new StarFabric switch or line card for prototyping. An adapter card can allow one the ability to use their existing CompactPCI line cards for developing a StarFabric system. (see diagram #1)

StarFabric Adapter Cards convert the PCI bus to StarFabric links, acting as PCI-to-StarFabric bridges. Standard CompactPCI cards can be plugged into the StarFabric Adapter Card, which in turn, is plugged into a PICMG 2.17 compliant backplane. The adapter card takes the cPCIbus traffic, serializes it, and sends it across the backplane in two 2.5 Gbps StarFabric links. Both 32 bit/33 Mhz and 64-bit/66 Mhz traffic can be converted via StarGen's SG2010 chip which resides on the adapter card. These cards are a useful tool in prototyping a PICMG 2.17-compliant StarFabric system. Adapter cards for many of the other new fabric-based technologies can also be designed.

Extender cards are another tool for prototyping. Testing within a chassis can be problematic. Reaching all the way in a chassis for testing signals is difficult and even dangerous for your system. It is easy to touch the wrong signals and get a reading on a different pin and not realize it. Or worst case, you can even fry your backplane or card. Extender cards bring the board being tested all of the way outside of the chassis for easy access. (See Diagram #2 on how this is done in a VME64x system.) Further, both side of the card is more accessible. Often, extender cards have posts used to attach test probes. These are clearly marked with the appropriate signal for each pin. Many extenders allow the signals to be individually switch isolated. Bustronic recently also developed a rear cPCI extender card for extending signals out the back of the backplane.

ATCA test extender cards would be helpful for the industry to develop. One can test the current topography of their standard card. The firmware and software can be tested, allowing the user to determine changes that are needed. However, using extenders with these high speeds may be problematic. Distance is an issue with high-speed signals and extenders are designed to create this kind of space. Further, in this market environment, the connector companies are less willing to create new designs that are risky. The standard front-loading extender card would require right-angle receptacles (for interfacing with test cards) in the new connector formats. The connector vendors are not likely to be enthusiastic about tooling these rarely-used variations. We'll have to see what innovative concepts appear in the industry to resolve these issues.

For system developers, a load board can be a useful testing tool. A load board provides significant time and expense savings by assuring a system's operating specifications. The load board functions to test a system's cooling capabilities by first applying the load to the power supply for verification and finally creating the necessary heat to confirm chassis cooling. By locating hot spots in the chassis, a system designer can verify where to optimally redirect the airflow to prevent overheating. The load board increases productivity by quickly and accurately characterizing systems at low cost. For PICMG 2.17, one must be careful when using a load board. A standard cPCI version can be used, but you must stay away from the Fabric Slots, which allocate pins in positions that conflict with the PICMG 2.0 pinout. Careful usage in other slots is perfectly acceptable. For PICMG 2.16, a standard load board can be used in all slots. A standard VME/VME64x load board can be used for VXS systems, as the P1/J1 and P2/J2 signals are the same.

Dummy cards (air block filler cards) are available for many of the fabrics. These are used to fill slots, which blocks and redirects airflow in the chassis. PICMG 2.16 and 2.17 can use standard 6U x 160mm cPCI versions. AdvancedTCA dummy cards are also now available in the 8U x 280mm form factor.

Testing Signal Integrity in Switched Fabrics

Another consideration for switched fabric designs is the high speeds inherent in these designs. Using switched serial interconnects and the multiple Gbps speeds bring unique design challenges. On the backplane level, they can greatly affect variables such as stub length, trace width and length (dielectrical losses and skin effect), impedance, vias (stub effect), and cross-talk. The use of new high-speed connectors, differential signaling, and serial interconnects are but a few of the culprits. Backplane designers need to understand the affect of these influences to maximize performance. With new connectors like the Metral line, Multi-Gig, ZD, and others, challenges in testing the effects on the signal must be overcome. The new switched interconnects, connectors, etc create new hurdles for high-performance backplane designers and manufacturers.

The Elma backplane group (comprised of Elma TreNew in Europe and Bustronic in the USA) developed a research backplane for high-speed architectures.(see Diagram #3) It allows us to see the complete signal path (cards, connectors, backplane) as one entity. This high-speed R&D backplane contains a reference area, link area, and crosstalk area. The reference area allows one to analyze the traces without the influence of the high-speed connectors. The differential lines (edge coupled/broadside coupled and single ended lines) are implemented with different trace widths on different layers. The link area shows the effect of the top high-speed connectors (Metral, ZD, VHDM) and the 2mm HM connector within different trace widths and lengths. Finally, the crosstalk area allows us to view the crosstalk levels of the different connectors.

By observing the segregated and combined affects of the various parameters mentioned above, we can see how small changes in one area can affect another. At high speeds, a change of only one of these elements implies a new adjustment of all of the others.

A high-speed R&D backplane can be used to prototype many of the new fabric technologies and help determine the optimal design. One can determine the best aggregate design outcome for modifications in stub effect, skin effect, dielectric losses, crosstalk, differential pair routing, and matching impedances.

Conclusion

With the focus of the developers on the new fabric-based technologies and core products, sometimes peripheral tools and development systems are overlooked. There are already some of these products hitting the market for AdvancedTCA, PICMG 2.16 and 2.17, VXS, and more. There are some potential design problems for accessories, but the industry has tremendous talent in overcoming these obstacles. Keep an eye out for new developments on the horizon.