ANSI Mezzanine Modules Provide Instrument Design/Replacement Flexibility
by Dr. Fred Harrison, Gary Guilbeaux, and David Clark, C&H Technologies
The mezzanine approach to placing multiple functions in a single card slot has been around for a long time both in proprietary and open standard forms, and valid arguments can be made for both of these approaches. One open standard gaining increasing popularity for instrument applications is the M-Module. This standard, originally developed in Europe for VME applications, has been embraced as ANSI/VITA 12-1996. The basic form of these modules is shown in
Figure 1.
Figure 1. Single-Wide M-Module
In addition to the single-wide form, M-Modules can be developed in double-, triple-, and quadruple-wide configurations. Because of the standard’s genesis in the VME world, it is sized so that four fit in a 6U module and two in a 3U module. Conveniently, because of the way other backplane standards have evolved, four units easily fit the front-panel space in VXI and 6U cPCI/PXI, two in the front-panel space of 3U cPCI/PXI, and up to eight in a 1U LXI rack-mount carrier.
At the present time, a number of instruments are available in the M-Module form factor. A representative sampling includes pulse generators, function generators, arbitrary waveform generators, digital word generators, counter/timers, GPS timing receivers, precision voltage sources, and switching.
The M-Module also has a relatively straightforward set of electrical and mechanical specifications. This enables an engineer to design a function that might be required without having to become an expert on VXI, PXI, or LXI since carriers can port the design to the backplane or bus of the test system in use.
Supporting the Standard
As with any mezzanine card, a means must be provided to adapt the card to a backplane or higher-level interface. Such a device generally is referred to as a carrier. These come in two types: nonintelligent and intelligent.
Nonintelligent carriers perform simpler functions such as mounting and providing power as well as more complex operations like handling translation between bus types and protocols, routing of triggers and interrupts, and making each mezzanine appear as a separate instrument to the host backplane. Intelligent carriers generally accommodate all of the functions of the nonintelligent plus pre- or post- processing of data, allow the combination of multiple instruments into composite instruments that then may be controlled at a higher level, and translate commands from older instruments to facilitate replacement of legacy instruments.
Figure 2 shows a typical VXI carrier, and
Figure 3 is an LXI carrier.
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Figure 2. VXI M-Module Carrier |
Figure 3. LXI M-Module Carrier |
Of equal or greater importance in the support of the mezzanine is the software. Most M-Module instrument types come with VXI/PXI plug-and-play or IVI drivers. However, a number of the more control-oriented M-Modules are supported only with C drivers. Actions are underway that will allow application of the plug-and-play drivers across multiple platforms.
Benefits of Modularity
Perhaps the greatest advantage of an M-Module mezzanine
instrument is the ability of both the vendor and the user to
become platform agnostic. From the vendors’ perspective, it is
only necessary to develop one instrument such as a pulse
generator and, with the use of carriers, sell the same product
into VXI, PXI, VME, LXI, and other applications. This greatly
reduces costs when compared to the development of pulse
generators for multiple buses. Figure 4 depicts platforms where M-Modules currently are supported.depicts platforms where M-Modules currently are supported.
Figure 4. Platforms Where M Module Support Is Available
M-Modules make it possible to use the same pulse generator in a factory test set that is VXI based and a field test set that is PXI based, reducing the chances of problems that invariably occur when two different pulse generators are used. In the long term, the user also will derive cost benefits because the vendor has not had to develop as many instruments to serve the different bus environments.
When a great new backplane or bus catches on as VXI and PXI have in the past, it only is necessary to develop a new carrier to allow migration of the M-Module to the new environment. A good example is LXI. By developing an intelligent LXI carrier, C&H’s 25 M-Module products were supported in the LXI environment.
In parallel with an M-Module development effort, a driver architecture has been developed that splits bus and OS functions from instrument functions. For instance, a driver for a new instrument can be developed one time yet run on all 10 platforms depicted in Figure 4.
Supporting Legacy Requirements
The modularity and reconfigurability gained with M-Modules can be appreciated when replacing legacy instruments that require amplifiers and buffers that are not part of a newer instrument. A recent case occurred when one C&H customer had to replace two different BCD-controlled pulse generators, circa 1965 to 1970, which had very specialized signal generation capabilities. The general nature of the signal could be met with a C&H MA204 Pulse Generator; however, it could not provide the necessary output levels and isolation.
Working with the customer, who did the driver software for the new composite instruments, two amplifier M-Modules were developed. This enabled the customer to replace two pulse generators with common carriers and M-Module pulse generators but with unique amplifiers, placing all the special signal amplification and isolation on a single M-Module.
The approach has been taken a step further on a number of legacy projects where intelligent VXI carriers have allowed users to port command translation software directly on a carrier to facilitate replacement of an old instrument. Composite instruments also may be created by mounting multiple instruments on an intelligent carrier and embedding their drivers and application code on the carrier’s processor and then writing a much higher-level driver that runs on the host with much less primary bus traffic. This same approach may be used with intelligent LXI carriers.
Future Direction
In general, we will continue to see increasing functionality and performance in M-Module form factors as the semiconductor technology allows these gains to be practical. Further, the availability of power PC cores within devices such as Xilinx gate arrays adds a level of flexibility and power that makes synthetic instruments in M-Module form factor a reality. These capabilities, both existing and planned, along with the capability to quickly migrate M-Module instruments to new platforms when necessary, bode well for the increasing utilization of the ANSI/VITA 12-1996 standard for future instrument development and applications.
References
• ANSI/VITA 12-1996, “American National Standard for The Mezzanine Concept M-Module Specification,†VMEbus International Trade Association,
http://www.vita.com
• Guilbeaux, G., et al., “Use of Intelligent VXI Carriers for Legacy Instrument Replacement in VXI Systems,†Autotestcon, 2003.
• Listing of M-Modules from all vendors,
http://www.chtech.com
• “M-Module Instrument Driver Architecture,†C&H Technologies,
http://www.chtech.com/pdf_misc_paper/M_Module_Drivers_Architecture_WP.pdf