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VMEbus Mezzanine Modules

By John Rynearson, Technical Director, VITA

Question. "How can I add more functionality to my system without adding more VMEbus modules?"

In the beginning there was the board and it contained all the functionality that there was. But soon market needs demanded more functionality than would fit on a single board. Ingenious board designers quickly realized that in a three dimensional world they could move up as well as out. Thus the concept of the mezzanine module was born. A mezzanine module resides on the base level module and in a bus system fits in-between other modules much like the mezzanine floor in a hotel that is between the first and second floors. Mezzanine modules allow functionality to be added without requiring additional bus slots.

In the early days of VMEbus, mezzanine modules were also called daughter boards, daughter cards, or even piggyback modules. Most VMEbus manufacturers started making mezzanine modules for one of two reasons. When the transition to 32 bits occurred, limited space on a VMEbus board did not allow all functions needed for a particular application. For example, one early 68020 design by one VMEbus manufacturer used up all the board real estate for CPU design, bus interface logic, and memory interface leaving no room for serial ports. The mezzanine module concept was used by this manufacturer to add this very necessary serial I/O. Another reason for using mezzanine modules was to meet customer demand for different types of I/O on the same base level board. Vendors were forever having customers request a base level module but with different I/O. The economies of scale favor production of a single type of module, but customers require different functions for different applications. It occurred to a number of manufacturers to produce a base level CPU module and then to use mezzanine modules to customize it for many different requirements.

While the mezzanine concept has added more flexibility to the VMEbus, it has done so at the expense of compatibility. Because no standard for mezzanines was available in the beginning of VMEbus, each manufacturer developed their own proprietary mezzanine bus architecture. Many simply used a variant of the 68000 microprocessor bus since it was the local bus upon which their design was based. As a result none of the mezzanine boards among the hundreds of VMEbus manufacturers is compatible even though many address the same functionality needs. An attempt in the late 1980's by VITA to build consensus for a common mezzanine bus standard met with no success. Manufacturers felt that proprietary mezzanine buses were one way to differentiate their products.

Since that time however several things have happened. First, Greensprings introduced IndustryPack into the market place in the late 1980's as a defacto industry standard mezzanine bus for I/O functions. While not generally accepted by VMEbus vendors in the beginning, IndustryPack received an important endorsement in early 1992 when Motorola introduced the MVME 162 single board computer with four slots for IndustryPack modules. Since that time other VMEbus vendors have developed products that accept IndustryPack.

In Europe at almost the same time MEM Mikro Elektronik GmbH introduced the concept of M-Modules. In the 1990/91 time frame MEM and several other European manufacturers founded MUMM (Manufacturers and Users of M-Modules), an independent association to promote the development and use of M-Modules worldwide.

Second, a standards effort for a common mezzanine card (CMC) specification was started in the IEEE last year. Known as P1386, this effort is an attempt to define a common mechanical and connector format for mezzanine modules to be used by VMEbus, Futurebus+, Multibus II, and other embedded real-time modules. Within P1386, P1386.1 referred to as PMC, defines the PCI bus on the CMC form factor and P1386.2, referred to as SMC, defines the S-Bus on the CMC form factor.

PCI because of its wide acceptance by major semiconductor manufacturers will most likely become the default local bus of choice for many microprocessor board designs. In the VMEbus world, local buses provide for an orderly board design. In the past the local bus was usually the bus used by the microprocessor. Because so many microprocessor chips and peripheral chips will adopt the PCI bus, VMEbus board designers will be forced to use PCI as a local bus to remain competitive on a functional and performance basis. Recently, Newbridge Microsystems announced their intention to develop a VMEbus to PCI bridge chip, thus providing a one chip link between the VMEbus and the on-board local PCI bus. Look for new VMEbus products featuring the PCI bus as a local bus to be introduced during the next several quarters.

With the standardization of PMC and the ready availability of many peripheral chips with embedded PCI interfaces, a market for compatible PMC boards should quickly develop. In addition because the PMC bus is an open standard, users will be able to design their own cards if none exist in the open market.

In the past each VMEbus manufacturer developed their own proprietary mezzanine bus. In the future, look for PMC to become the standard mezzanine bus for VMEbus, Futurebus+, and Multibus II.

This page last updated: Sep 19, 1999

Reprinted from the VITA Journal with permission from VITA.

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