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High-Speed and Hybrid Backplanes
VXS Processor Mesh Architecture: Powerful, Flexible, Compatible
Although the VXS Processor Mesh architecture does not define any new backplane pin assignments beyond those already defined within the VITA 41.0 base specification, bandwidth capabilities exceed 112.5 Gbits/s of aggregate throughput within the processing mesh in a single chassis.
MICHAEL MUNROE, ELMA BUSTRONIC
VMEbus has morphed once again. One of the most promising developments is VME Switched Serial (VXS), also known as VITA 41. Like many of today s specifications, there are a host of subsidiary documents to support the platform, which is VITA 41.0. Other specifications define various signaling protocols ”such as InfiniBand, RapidIO, Ethernet and PCI Express ”all of which can utilize a single backplane physical architecture that is defined by the base document.
The latest VXS innovation is the VXS Processor Mesh specification, VITA 41.7, developed by Elma Bustronic. This new backplane architecture, first demonstrated at BusBoard 2006 in Long Beach, will provide a higher level of connectivity for VXS than has ever been previously considered. Perhaps its major significance is the fact that it does not define any new slot mechanics, connectors or channel definition.
VXS defines within VITA 41.0 two new classes of cards: payload cards and switch cards. The payload card implements a single new differential connector in the P0/J0 position that exists on conventional VME64x cards today. The switch card defines a slot that is filled top to bottom with the new high-speed, differential MultiGig connectors.
Standard Slots, Flexible Configurations
Processor Mesh defines a set of five fully meshed switch card slots, with four bi-directional channels ”a total of 32 differential pairs ”between each slot and every other slot. This configuration became a reality when a VXS switch card vendor began discussing its application with Elma Bustronic. The two switch slots on a conventional VXS dual-star backplane are enough for most applications. However, in an especially challenging requirement, as many as five of these processor boards would probably be needed. In addition, a greater level of connectivity between each of these powerful cards was desirable.
The challenge was how to integrate five switch cards into a larger VXS system in a useful way that would be flexible enough to support the widest range of other, similar applications. At the same time, the processing resources on those cards had to function at their highest capability. The result became VXS Processor Mesh. The new backplane topology integrates conventional VXS switch cards in a mesh configuration. Therefore, the switch cards are no longer connected directly to any payload cards, but only to other switch cards. The connection paths between each and every one of the four mesh slots consist of four direct, unswitched channels, which together offer a bi-directional 40 Gbit/s path. They could also be used as 32 differential pairs in a single 80 Gbit/s unidirectional pipe.
This meshed section is for special processes and is in addition to the two conventional dual-star fabric slots that are still required to support the A ? and B ? fabric connections to the payload slot in a system (Figure 1).
With this new architecture, VXS backplanes are no longer limited to a single pair of fabric switches. Most importantly, the payload cards and switch cards in VXS Processor Mesh can be the exact same cards that are used in the classic dual-star VXS systems.
VXS supports the continued use of existing VME64x cards. This is particularly important when special cards have been developed by end customers to serve as a custom interface to a special type of sensor or output device.
For example, if those cards used a 2 mm HM connector in the P0 position, it can now be replaced with a single differential MultiGig P0 connector that supports 20 times the bandwidth of the existing 2 mm HM center connector. Both Processor Mesh and Payload Mesh VXS backplanes have been produced with one or more slots equipped with the legacy 2 mm HM center connector, in order to accommodate existing VME64x cards that do not currently justify being redesigned (Figure 2).
FPGAs Prefer Direct Connections
Another consideration is the fact that many high-performance VXS cards are FPGA-based. FPGAs from Xilinx, Altera and others are most effective when used in point-to-point topologies. FPGA-based designs require less code and deliver more performance when used with direct, streaming serial protocols such as Aurora and SerialLite. Although the VXS standard was designed to support a switched fabric topology, VXS backplanes will not always be used for this purpose.