PXI Express raises throughput, adds differential clocking
Rick Nelson, Chief Editor -- Test & Measurement World, 2/1/2006
The PXI Systems Alliance (PXISA) published the PXI Express specification last August, and you can expect to see PXI Express instruments coming to market this year. With those instruments, you will be able to take advantage of a 45-fold increase in bandwidth—to 6 Gbytes/s, vs. 132 Mbytes/s for what's now called PXI-1.
PXI Express leverages the widely adopted PCI Express specification, and PXI Express modules will comply with the CompactPCI Express standard, which combines the PCI Express electrical specification with rugged Eurocard mechanical packaging and differential connectors (see "PXI Express spec paces PICMG effort"). PXI Express offers mechanical and electrical interoperability with CompactPCI Express products, so as new CompactPCI Express products are introduced, systems will support both PXI Express and CompactPCI Express modules.
Hybrid slotsIn addition, you will be able to use PXI Express modules along with more than 1000 existing PXI modules. Chassis that support PXI Express will continue to have 32-bit PXI-1 slots. The PXI Express standard also defines hybrid module slots that will accommodate existing modules as well as emerging Express-class instruments.
For example, the standard defines 3U hybrid peripheral slots that have three connectors: P1, XP3, and XP4, where P1 carries 32-bit PCI signals; XP3 is dedicated for PCI Express data lines, differential triggers, and timing signals; and XP4 carries the timing and synchronization signals defined in the PXI-1 specification. The 6U hybrid peripheral slot defined in the PXI Express standard adds an XP8 connector to provide additional power to 6U modules.
As you add PXI Express modules to your instrument systems, you'll be able to take advantage of new PXI Express instrumentation features including a new differential 100-MHz system clock, differential synchronization, and differential star triggering. The new differential functions will offer enhanced noise immunity and low jitter at higher clock frequencies.
Furthermore, the higher-speed clocks can eliminate the need for costly clock multiplication circuits in emerging PXI Express instruments while maintaining compatibility with existing PXI-1 modules, because the new 100-MHz clock is phase-aligned with the 10-MHz clock of the existing PXI-1 spec, according to the specification (Ref. 1).
Dedicated bandwidth
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| Fig. 1 Traditional PCI implementations share the available 32-bit, 33-MHz bandwidth among all connected devices; PCI Express, and hence PXI Express, offer dedicated bandwidth per device. |
Who needs all that bandwidth? High-bandwidth IF instruments for communications test systems (see "A PXI horse for the RF course," p. 61); high-channel-count, high-sample-rate data-acquisition systems; and high-speed image-acquisition systems are among the applications often cited as ones that can benefit from PCI Express and PXI Express.
Richard McDonell, PXI group manager at National Instruments, commented, "Since the PXI Express spec was approved back in August, we've been contacted by a number of instrument vendors as well as non-instrument vendors who have been building their own ruggedized proprietary architectures, and they are seriously considering PXI Express. So, it's not only opened up opportunities for existing applications to run even faster, but it has also opened up applications that we would never have dreamed could move to a COTS environment," adding that he can't disclose specifics at this time.
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| Fig. 2 Hinting at PXI Express capabilities, a demo at NIWeek 2005 involved acquisition of a table saw’s load, speed, and two-axis vibration data for synchronization with image data, acquired over a Camera Link interface. This demo was implemented using desktop-PC-resident PCI Express cards. |
Dement cited as a specific example a demonstration at NIWeek 2005 in which a table-saw monitoring application (Ref. 2) involved the real-time synchronization of a saw-blade image with acquired saw load, rotational speed, and two-axes vibration data (Figure 2). In this application, the 320-Mbytes/s image-data transfer far outstrips the capacity of the traditional PCI bus. Although the NIWeek demo was implemented in PCI Express modules residing in a desktop PC, the emergence of PXI Express offers the opportunity to extend this or similar applications to take advantage of star triggering and other PXI-specific functions.Preserving software
Dement explained that as you move to PXI Express, you can preserve your software investment. The PCI Express standard, she said, "provides for complete software compatibility between PCI and PCI Express down to a very low level. For example, about a year ago, we released a GPIB interface for PCI Express, and we didn't have to change a single line of code in our own PCI driver to support the PCI Express interface." The PXI Express standard (Ref. 3) affords the same level of compatibility.
As you wait for PXI Express to arrive, Dement said you can take advantage of PXI Express-level bandwidths using already available PCI Express instrument modules, and you can even link them to existing PXI-1 systems. To do that, you can employ a MXI-Express interface in a desktop computer having PCI Express slots, or you can use ExpressCard MXI in conjunction with a laptop.
But the wait shouldn't be long. McDonell won't put a date on the first PXI Express product debuts from National Instruments, but such introductions "are a priority on our part." The main issue, he explained, was just getting the PXISA spec nailed down. Now that that's happened, he said, "We've already got many PCI Express-based cards and PXI modules shipping, so we have in-house expertise in both architectures to get the first wave of PXI Express products out there."
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