Engineers Embrace CompactPCI

The rugged, high-performance CompactPCI and its instrumentation extension PXI are attracting many system developers.

Gil Bassak, Contributing Technical Editor -- Test & Measurement World, 10/15/1999

PXI’s main enhancements to CompactPCI fall into three categories: electrical extensions, mechanical extensions, and software extensions (see Fig. 2). The electrical extensions add timing and triggering features so multiple instrument boards can perform synchronous data acquisition. Because PXI is a superset of CompactPCI, boards made to one specification will work in both settings. A CompactPCI backplane, however, won’t carry PXI’s extra timing and triggering signals, nor would a standard CompactPCI module execute any timing or triggering signals carried by the PXI bus. With CompactPCI modules, you can’t have one instrument card trigger another without system intervention—you can with PXI cards. Figure 3 shows a PXI chassis with a mounted receiver that provides mass connection of signals.

The mechanical extensions of PXI require manufacturers to document environmental and regulatory specifications of their equipment. “These are not covered in CompactPCI,” Carsten Puls, PXI marketing manager at National Instruments, says “but when you build a complete system, it’s important to know how your test system will function in a given environment.” Rather than dictate conditions for environmental testing, PXI simply requires vendors to document the tests they run. And while PXI requires temperature testing, the specification merely recommends that manufacturers conduct tests for humidity, shock, and vibration as well as make those test results available. On the regulatory side, the PXI specification requires that products—both plug-in modules and chassis—meet the CE standard for radiated and conducted emissions, as well as electrical safety standards.1

PXI’s software extensions define standard software frameworks, mainly around Windows NT and, in the current version, Windows 95 (see “Revving Upward,” below). According to Puls, PXI instrument manufacturers must supply a software driver that runs in Windows NT, 95, or both. This avoids cases, similar to when CompactPCI first was released, where a manufacturer might supply hardware, describe the register set, and then leave it to the engineer to write a driver. That style, prevalent in the VMEbus world, does not sit well with test engineers who usually want to program a test system with the benefit of instrument drivers.

Engineers Sound Off
James Sari, a principal technical staff member at Johns Hopkins Applied Physics Laboratory (Laurel, MD), is working on a hydrophone datalogging project. He was pleased to find the 16-bit, 64-channel CompactPCI digitizer board he needed, something he could not locate in a standard PCI format. His card’s programmable input gain suited the 2.5-V signals he had to process. Although Sari would have liked tunable antialising filters for individual channels, he still considers the board’s $3,000 to $5,000 (with options) price tag more than fair.

Issa Nesnas, technical staff member at Jet Propulsion Laboratory (Pasadena, CA), develops and prototypes proof-of-concept software for the Martian Rover spacecraft. Nesnas found the frame grabber he needed in a CompactPCI card. Additionally, the CompactPCI’s eight-card capacity, system assembly, and low-cost, off-the-shelf, PC-compatible software all earned his praise. What irks him, however, is paying what he says is twice as much for a CompactPCI card than he would for a standard PCI board. Overall, a $4000 PCI system might cost up to $8000 if built with CompactPCI, according to Nesnas. But what about comparing CompactPCI to the VMEbus, which Nesnas has worked with? “We no longer compare CompactPCI to VMEbus, we compare to PCI,” Nesnas says. “If you can buy PCI parts at a certain price, you would like CompactPCI to be comparable.”

Cost is also an issue for Dennis Erickson, a senior electronics engineer at Bonneville Power Administration (Vancouver, WA), a US government agency. Erickson has a system that monitors voltage and current at dozens of substations from Canada to Mexico, and from the Pacific Ocean to Colorado, Montana, New Mexico, and Wyoming. A key circuit card in each system is a CompactPCI time and frequency processor that uses global position signals to time-stamp data. Erickson is currently converting the system to PCs from Macintoshes “for political reasons.” Erickson has built a prototype CompactPCI system for evaluation between desktop PC systems and CompactPCI systems.

Figure 2. PXI adds mechanical, electrical, and software enhancements to CompactPCI that make the bus more attractive for instrumentation applications.

Erickson likes the CompactPCI prototype because of its compatibility with LabView drivers and its wealth of instrument cards. But he fears the additional $2000 cost of a CompactPCI system over a desktop system will force him to deploy desktop systems at dozens of locations. Of course, if Erickson had his way, he wouldn’t use either—he prefers the Mac.

Because Erikson is deploying many systems, the cost of using PXI chassis compared to the cost of desktop PCs is important to him. A PXI chassis with a 333-MHz embedded controller costs about $4900. A monitor adds an extra $300 or so. A desktop PC with a faster processor and a monitor sells for less than $2000. PXI instrument cards, however, are comparable in price to their PCI cousins. For example, a PCI 100-kHz multifunction data-acquisition card from National Instruments costs $1995. A comparable card for PXI costs $200 more.

PXI systems do cost more than comparable PCI systems, but you don’t have to open a PC to add or remove a card. You also get more card slots in a PXI chassis (seven) than you typically get in a desktop PC (three or four). And both platforms require the same investment in programming. So, you have to decide which is best for your application. CompactPCI makes good sense for cases where ruggedness, accessibility, and board capacity are your priorities. And if you can find the board you need in CompactPCI, you’ll enjoy low-cost PC-based development tools. T&MW

FOOTNOTE
1. PXI Specification, version 1.0, August 1997, National Instruments, Austin, TX, www.pxisa.org/PXISPEC10.PDF.