RTC Magazine

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.