By: Clarence Peckham, GE Fanuc Embedded Systems
A continuity of specifications across a quarter of a century has enabled VME to grow and adapt new technologies and increase performance while keeping compatibility with established designs.
VME at 25. I don’t think anyone would have expected that to happen! Good planning? Good luck? Actually I believe the answer is pretty simple. The VMEbus specification was written to meet the needs of the market and has been modified since the early 80s to continue to meet the requirements of the users.
The VME specification was developed by the VITA Standards Organization to support early 16-bit microprocessors—most notably the Motorola 68000. These processors required a lot of MSI/LSI technology support logic. The earliest VME systems utilized separate boards for CPU, Processor and I/O with all communications between the boards occurring over the VMEbus.
In the VME specification, support was provided for both 3U (160 x 100 mm) as well as 6U (233 x 160 mm) board formats. The 3U board form-factor never gained much popularity for two main reasons. First, the single DIN connector only allowed for a 24-bit address and a 16-bit data bus—adequate, but only for the 8/16-bit processors of the time. The second, and more important reason, is that the single DIN connector on the 3U board did not have any available pins for I/O signals on the backplane. This left only the front panel available for I/O connectors. The 6U format, on the other hand, was much more user friendly, with its support for 32-bit address and data implementation as well as providing 64 I/O pins on the second DIN connecter.
As technology developed and programmable logic and 32-bit processors became readily available, the VMEbus and 6U board form-factor became the platform of choice for building 16- and 32-bit systems. Eventually the technology reached a point where the processor memory, disk and Ethernet I/O were on one board for performance reasons, and the VMEbus was typically used to communicate with I/O cards or between processor cards since the bus architecture supported multiple processor cards via an arbitration system.
One of the highlights in the history of the VMEbus was the availability of the VME interface chip. The first globally available interface chip was the VIC chip designed by a consortium of board vendors and Cypress Semiconductor. Other chip solutions followed, but it was the VIC chip that released the board designer from designing a VME interface based on programmable logic designs. The VIC chip, even with its “warts,” provided a common interface, one that provided the biggest advance in interoperability on the VMEbus since the specification was released. Since the VIC chip there have been other designs all leading to the latest VME interface chips from Tundra Semiconductor—The Universe II and Tsi148 Chips.
Another feature of the VMEbus is that it is the only standard that encompasses environmental requirements as well as mechanical and electrical specifications. The VITA 47 specification provides support for all of the environments required for VME-based systems—both conduction- and convection-cooled.
So where are we with the VMEbus specification? Well if you want to build a system based on PowerPC or Intel Pentium processors with lots of I/O, graphics or communications devices, the released specifications provide a variety of solutions. In Figure 1, a traditional use of the VMEbus, one or more processor cards and a card cage full of I/O and other function cards is shown. In the Figure a commercial type configuration is shown for air-cooled applications and a military conduction-cooled system is shown as well. For the first fifteen years these configurations would have been the majority of the systems.
In the past ten years the expansion of the VMEbus to include support for faster transfers (VME64/VME 2eSST) and PCI peripherals with the inclusion of PMC modules on all cards has opened up new configurations of VME systems. The addition of the P0 connector provided more I/O pin options, as well as allowing other connection methods to be implemented. The end result is that each card can now be considered a system. A single board computer can have two PMC cards (Figure 2) that can provide extended capabilities such as high-performance graphics, FPGA-based computing, I/O or another processor using a processor PMC. The end result is that the VMEbus chassis provides interconnect between the “system” cards and supports the environmental requirements of the application.
So what is available today to make VME still an attractive solution for building systems? The basic VME specification still provides a viable solution for both commercial and military applications. The list of features that make VMEbus a viable solution are:
• Basic 32-bit address/32-bit data bus supporting data transfers from 8 bit programmable I/O to 320 Mbit/s block transfers
• Ability to support multiprocessor solutions sharing common I/O assets
• Proven environmental specifications for convection as well as conduction-cooling. VMEbus systems have been qualified for many ruggedized military applications and deployed in many military platforms
• Continued support of technology advances such as switched fabric backplanes via VITA 41.x family of specifications
• Large supplier base providing board and system level solutions
• Backward compatibility. Older VMEbus boards can be used with the latest approved VMEbus specifications. Allows reuse of older I/O boards in new systems
The key factor in the VMEbus future is technology insertion. Moving from simple bus model and adding block transfers, VME64 extensions and support for 320 Mbit block transfers, were all major enhancements. However, the addition of support for switched fabric backplanes via the VITA 41 family of specifications was a major shift in technology. Still supporting legacy VMEbus, but at the same time providing support for high-performance serial links such as InfiniBand or serial Rapid I/O, enables VITA 41 to support much higher performance systems. Figure 3 is an example of an InfiniBand-enabled system supporting multiple processors and legacy VME boards with the ability to connect multiple systems via InfiniBand.
What is being developed for the future? VME-based products will continue to support serial fabrics as a method of transferring high bandwidth and large packets of data. Future developments rest on the latest specification, and the one that is the most dramatic for the future of VMEbus—VITA 46. The VITA 46 specification is the first enhancement that will not support legacy boards or systems because a new high-performance connector structure is defined as well as support for fabric switches. The 3U and 6U form-factors are maintained in VITA 46 but there are a couple of options on card spacing.
Since VITA 46 will not support legacy VME cards, any requirement for I/O that was available will require a relay out of the card. On the surface this sounds like a less than ideal solution, but with the new RoHS requirements as well as the need to provide support for another 10-25 years for VITA 46 applications, it makes sense to refresh some of the older technology VME cards.
Another VITA specification that will affect future VMEbus efforts is VITA 47. Efforts by the embedded industry to set uniform standards for ruggedization levels (temperature, shock, vibration, etc.) were, at best, hit-and-miss until VITA published ANSI/VITA 47-2005. This standard sets a clear, precise and comprehensive set of guidelines to which every manufacturer in the industry can adhere. Perhaps more importantly, it allows customers to compare products from various manufacturers by applying the same set of benchmarks to everyone.
Now that a uniform, open standard has been approved, ANSI/VITA 47-2005 should make it possible for both the makers and consumers of rugged electronics to compare products from multiple vendors with far more accuracy and simplicity. However, due to the historical lack of consistency, it will continue to be critical for some time (possibly years) for customers to clarify with their suppliers the precise levels of ruggedization for individual products that were released prior to the ratification of VITA 47. For products produced going forward, though, the process of compliance verification will become much more straightforward.
In some ways, VITA 47 was a matter of organizing and sorting out an existing body of standards and test or manufacturing methodologies. Those explicitly defined by VITA 47 are often based on earlier MIL specifications. When the commercial off-the-shelf (COTS) movement took effect, many military specifications were no longer updated, and so VITA 47 was partially an effort to reaffirm or redefine many testing methods, manufacturing methods and certification levels that had lapsed or fallen into a certain degree of disarray. In some cases VITA 47 was also a kind of negotiation among the members/vendors as to the levels of ruggedness that should be formalized—usually levels that were already in use.
What is the outlook for the next 25 years for VME-based applications? Will traditional VME bus systems continue to be used on new design wins? Will VITA 46-based systems provide a new line of design wins? These are all tough questions to answer. Clearly VME-based systems will continue to ship for the foreseeable future as there are many current design wins that use the system. Overall I do not see a lot of competition for VME, but users will have to consider competing specifications when they review VITA 46 since it is all new. That may open the door for CompactPCI or MicroTCA system designs.
Overall, I predict that VME-based applications will be shipping for at least another ten years. After all, VMEbus is a proven specification, there are hundreds of diverse products that can be used to build a system and users know how to construct systems using VMEbus specifications.
GE Fanuc Embedded Systems
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