VME or CompactPCI?

Contributing editor John Collins looks at the choices that designers have when integrating off the shelf board level products into new products.

Whether designing a missile detection system, an advanced piece of telecom equipment or integrating a scientific test apparatus, it makes sense to use off-the-shelf standard bus boards. To aid this task, there is an established market ready to assist, offering backplane-based computer boards employing standard buses.

Now 17-years-old and still thriving, VME bus still dominates the high-performance standard bus realm. While alternative bus architectures such as CompactPCI are displacing VME in a few market segments, VME continues to enjoy success.

Meanwhile, many new telecom and telephony system designs are embracing CompactPCI. For test and instrumentation applications, VME and now CompactPCI, have both spun off versions specifically for such use.

Since its inception in 1995, CompactPCI has secured its place as an important bus architecture. Its appeal lies its combination of desktop PCI technology and VME form-factor concepts. CompactPCI is a high-performance industrial computer platform based on the standard PCI electrical specification. It uses the same rugged Eurocard packaging as VME combined with a 2mm hard-metric pin and socket connector.

THE OLD STRUGGLES

At first, the struggle between CompactPCI and VME looked like a good, old-fashioned bus war, reminiscent of the days when VME and Multibus II used to slug it out. Today, the industry seems comfortable with the idea of CompactPCI and VME not as competitors, but rather as variations on a theme. Indeed, many longtime VME board vendors now make both CompactPCI and VME products. Few board vendors put themselves solely in either the VME or CompactPCI camp.

Demand for reliability and ruggedness in telecomms narrows the choices to CompactPCI and VME. Both bus/form-factor standards use the industrial grade Eurocard standard as a mechanical form factor. Since CompactPCI arrived on the scene, there has been endless debate about where the buses are positioned. Telecom has been a key target market for CompactPCI. Consequently, the same debate rages at a higher octave in the telecomms market.

The PCI Industrial Computer Manufacturers Group (PICMG), the association that drives the CompactPCI standards, has crafted a Computer Telephony specification. This extends the capabilities of the CompactPCI architecture to support specific application needs of telecom and telephony applications. Complying with the Enterprise Computer Telephony Forum (ECTF) H.110 specification, it defines a standard interface for bringing telephone signals into CompactPCI systems.

The ability to hot-swap boards is a key requirement in telecommunications, which require continuous operation at some level. With the completion last year of a hot swap specification, CompactPCI users now have a scalable approach for hot swapping that can be used in a range of platforms from embedded industrial controllers to high availability servers.

For its part, VME has some important advantages for telecom designers. There is an installed base of VME chassis, and a whole infrastructure available to support VME with a wide variety of different boards. Support for telephony data buses is also included. Four years ago, the SCSA standard was implemented on a VME bus. SCSA, created by Dialogic is a bus specifically targeted for moving voice data.

Like CompactPCI’s hot swap spec, the VME community has its own standard for live insertion on the VME bus, called HA (High Availability) -VME. The general idea for HA-VME involves an active backplane that can detect a card’s presence. But because most applications require a passive backplane, the HA-VME approach has enjoyed limited success.

For over 10 years, a spin off from VME called VXI (VME eXtentions for Instrumentation) has gained acceptance as a modular instrumentation platform for very-high-performance automated test systems. VXI, in particular has been embraced in military and high-volume manufacturing test systems. Because VXI systems are typically costly, in the $10,000 range, usually only larger companies have engineering resources in-house to put the systems together and maintain them.

Borrowing the VXI concept, National Instruments decided to craft an instrumentation spin-off for CompactPCI. Two years ago it created PXI (PCI eXtensions for Instrumentation) and the bus has been gaining industry acceptance ever since. Last year, a group of over 40 industrial test and measurement companies formed the PXI Systems Alliance. PXI is essentially a superset of CompactPCI, so it leverages desktop components and the PCI bus. As such PXI boards cost less relative to VXI. PXI uses the smaller 3U board form factor.

PXI incorporates distinct specifications and recommendations on environmental testing. For example, a PXI product must undergo some temperature testing. The supplier must provide the temperature rating and documentation that indicates what tests were run. There are recommendations also for shock, vibration, and humidity. The instrumentation-specific electrical extensions of PXI are similar to those of VXI. These include a trigger bus, a 10MHz reference clock, a local bus, and a star trigger feature.

In developing PXI, National Instruments learned from some of the mistakes with VXI. In VXI there is no interoperability at the hardware level between VME and VXI modules. In contrast, the electrical extensions in PXI offers complete interoperability with standard CompactPCI systems. As a result, you can have CompactPCI cards and PXI cards in one system.

As far as the defense industry is concerned, VME is the de facto standard because it is independent of any processor, bus silicon, or operating system constraints.

GETTING A MOVE ON

Over its history, the VME community has evolved. VME speed-ups started early this decade with the emergence of VME64, a protocol that allowed VME to run at up to 80Mbytes/s. Over the past few years, there has been interest in moving to new, high-speed VME protocols.

First there was 2eVME, a protocol that increases VME bandwidth to 160Mbytes/s. Now, work is underway on 2eSST, which doubles that bandwidth to 320Mbytes/s, with potential for 533Mbytes/s, and perhaps even 1Gbyte/s. Simultaneously, a clever scheme for sending VME transactions over PCI, called vPCI, is in the works.

These latest speed ups to VME have kept military customers from straying over to CompactPCI. Even though technologies like 2eVME and VME320 are not widely deployed, military designers like the comfort of knowing there is an upgrade path in the future.

In the military board market, the emphasis on Commercial Off The Shelf (COTS) products has become a driving trend in military systems. US military leaders are increasing their reliance on COTS hardware and software to contain costs in spending, and the drive to COTS has supercharged the market for VME boards.

Many military programs, after years of development, are just now moving into volume production stage. And while interest in CompactPCI has been low amongst military designers, some interest is emerging. A few vendors are now rolling out their first military-ruggedized versions of CompactPCI boards.

Racal instruments Tel: Slough (01344) 388000

{{Some Useful Bus Board Links

GroupIPC http://www.groupipc.com

I2O Special Interest Group http://www.i2osig.org

PICMG Home Page http://www.picmg.com

PC/104 Consortium Information http://www.controlled.com/pc104/consp1.html

PCI SIG Home Page http://www.pcisig.com

STD32 SIG Home Page http://www.std32.com

The 1394 Trade Association http://www.1394TA.org

Universal Serial Bus Home Page http://www.usb.org

The VMEbus International Trade Association (VITA) http://www.vita.com

Industrial Automation Open Networking Alliance http://www.iaopennetworking.com}}