PXI Gets the Express Treatment
If engineers responsible for test-and-measurement activities have not already applied the PXI bus to help solve a measurement problem, more than likely, they have heard about it. Since the 1998 introduction of the PXI (PCI Extensions for Instrumentation) bus, companies have introduced over 1200 PXI-related products, such as instrument modules, controllers, chassis, cables, interfaces, power supplies and software.
Although the PXI bus provides a 132-Mbyte/sec bandwidth, data-transfer operations that occur in machine-vision or video-streaming system, for example, require more. To meet those requirements, developers have had to resort to proprietary buses to transfer data, explained Eric Starloff, director of product marketing at National Instruments. That situation will soon change for the better.
To increases the bandwidth of bus communications in desktop PCs, computer manufacturers have adopted the PCI Express bus, an outgrowth of data-intensive graphic applications. The "express" extension adds high-speed signal lines that facilitate communications over a network-like structure. The PCI Express bus uses packets that travel over full-duplex low-voltage differential signaling (LVDS) "lanes" at up to 6 Gbytes/sec. Industrial users of the Compact PCI (cPCI) bus, represented by the PCI Industrial Computer Manufacturers Group (PICMG), will soon follow suit and adopt a cPCI Express standard. Because PXI equipment builds on the cPCI bus specification, the PXI community also will adopt the same standard, labeled PXI Express.
Loofie Gutterman, president of the PXI Systems Alliance (PXISA) explained the organization has worked closely with the PICMG to ensure the latter group addressed the needs of PXI vendors and users in its cPCI Express specification. The PXISA should approve the PXI Express specification by the end of this year, and Gutterman expects vendors will offer PXI Express cards by the middle of 2006.
Current PXI users and vendors need not panic, though. The PXI Express specification will preserve software investments and standard PXI and PXI Express cards can operate in the same system. PXI cards have room for additional bus signals, so new backplanes will accommodate both PXI and PXI Express signals on separate buses. Module manufacturers can supply instrument and I/O controllers that plug into PXI, PXI Express, or both buses.
Individual modules will use either PCI or PCI Express signaling to communicate with a slot-0 controller, but the two buses will not communicate with each other directly. Once the CPU has data, it can transfer it on either bus to link PXI and PXI Express modules, explained Greg Caesar, PXI marketing group manager at National Instruments.
Programmers will notice no difference between PXI and PXI Express modules because the overarching cPCI Express specification provides for backward compatibility with older products. Thus, users of PXI and PXI Express modules will not have to distinguish between them in their software. "One of our first PXI Express modules provides an IEEE 488 interface to bench-top instruments," said NI's Greg Caesar. "The current IEEE 488 PXI driver will immediately work with the new PXI Express module, so developers do not have to face software revisions."
Engineers can expect to see the first PXI Express modules provide high-speed communication ports, RF instruments, oscilloscopes and digitizers--just the types of instruments that require high-bandwidth communications on a data bus. Of course, to use these modules, developers must buy a new system controller and backplane that provide PXI Express signals.
Some instruments, such as lower-bandwidth counters and DMMs, may never require more than the standard PXI bandwidth. Bob Stasonis of Pickering Interfaces noted the switching time of mechanical relays, not the PXI bus bandwidth, limits the operating speeds of switch modules.
Because system developers seek to take advantage of both PXI and PXI Express modules, chassis and backplane vendors must supply a combination of slots that can accept standard PXI, PXI Express, or both module types. The mix-and-match approach raises an interesting question. "Where do vendors put the PXI Express slots?" asked Stasonis of Pickering Interfaces. "Suppose you set aside the last three slots in an 18-slot chassis, but each PXI Express module takes two slots. You end up blocking some of the PXI Express slots." Backplane and chassis vendors will have to offer a variety of card-connector arrangements.
Bigger Might Work Better
Developers might find a way around "blocked slots" by adopting the larger 6U-size PXI (233 mm high) rather than the more popular 3U cards (100 mm high). Both cards have the same depth; 160 mm. Stasonis noted a PXI module that requires three 3U slots could potentially fit on a one-slot 6U module. He noted, though, that customers have not clamored for 6U modules.
"In the military and aerospace markets, we see growing use of 6U modules," said Matt Tracewell, vice president of Tracewell Systems. "And some systems combine 3U and 6U PXI modules." New synthetic instruments require 6U modules and it makes sense to combine them with 3U modules in a common box.
Within the PXI community, a distinct group--Acqiris, SEI, Geotest, and a few others--develops 6U multifunction cards. On the other hand, many suppliers will continue to produce 3U-size modules. "Everything we do uses a 3U-size module," said Dylan McCarthy, sales and marketing executive at PXIT. "Our customers definitely prefer 3U chassis, so if we must expand the functions in a module we enlarge it to two or three slots."
Many suppliers, particularly those in the telecom market, offer 3U and 6U modules that conform to the cPCI specification. Although these cPCI cards lack the special triggering and timing functions that the PXI bus provides they still can run in a PXI system. So, if you need a function that exists as a cPCI module and you do not need the unique PXI functions, you can add 3U and 6U cPCI cards to a PXI system.
Something Old, Something New
In addition to evaluating 6U PXI modules, engineers also must consider non-PXI instruments. The plethora of PXI vendors does not guarantee engineers will find every function they need in a PXI module. Many PXI-based systems rely on a mix of VXI, rack-and-stack or other types of measurement equipment. The mix-and-match approach also makes sense when engineers upgrade a system but cannot afford to replace older, but operational equipment. Older instruments continue to do their jobs well while PXI modules offer new measurement capabilities in a mixed instrument system.
Even IEEE 488-based equipment continues to fill testing needs in PXI systems. In some cases, a PXI module just cannot answer a specific requirement, such as delivering high power to drive external devices. An external power supply controlled through an IEEE 488 port will do the trick, though.
PXI and PXI Express offer engineers a wide variety of instruments to choose from when designing or upgrading a test-and-measurement system. But as always, they pick the instruments that offer the capabilities they need within their budget. We can expect to see mixed test systems that include PXI, PXI Express, VXI, IEEE 488 and benchtop instruments for some time.
For Further Reading
For an overview of PCI Express, see: arstechnica.com/articles/paedia/hardware/pcie.ars/
"What is PXI?" National Instruments. zone.ni.com
18-Jan-2006