Data streaming in terabytes with PXI Express
18 March 2008
Scott Savage demonstrates why the PCI Express bus gives PXI Express the performance needed for the most data intensive applications.
Data streaming refers to the ability to move data to or from an instrument at rates high enough to sustain continuous acquisition or generation. Complex applications, such as noise mapping and signal intelligence, push the limits of current PC bus technologies to be able to move data throughout a test system. In the past, an instrument’s memory may have provided enough storage for common measurements, but with more applications requiring higher resolution and increased data rates, data streaming provides a way for engineers to capture or generate essentially “infinite” test patterns through the latest advancements in bus technology and high-capacity data storage solutions.
The ability of a PC-based test system to stream data at high rates is primarily a function of its internal bus architecture. PCI is capable of providing up to 132 MB/s of shared bandwidth across all devices on the bus at any one time. PCI Express, the next generation PC bus, extends the capabilities of PCI by offering dedicated bandwidth to each instrument in the system, allowing for higher aggregate transfer rates. PXI Express uses the PCI Express bus, to deliver up to 6 GB/s of total system bandwidth and up to 2 GB/s of dedicated bi-directional bandwidth per-slot, providing the highest data streaming rate of all standard instrumentation platforms; including GPIB, USB and Ethernet (LXI).
One of the major benefits of increased bandwidth is the ability to perform faster parallel test. Instruments for PXI Express are optimised to take advantage of the bus throughput to stream data at the full rate of the instrument. Instrument memory can be continuously updated during a test, or in some cases may require almost no onboard memory by taking advantage of the data rates offered by the bus. The modular architecture of PXI Express allows you to mix and match instruments to adjust for higher channel count, or perform mixed-signal testing, all on the same system. Where a single test may have been limited by memory or bus throughput in the past, higher system bandwidth allows engineers to run multiple tests on a single device in parallel, or even test multiple devices with these instruments simultaneously.
Recording large sets of data continuously for long periods of time also requires a storage medium that can keep up with the high data rates from the instrument. A typical hard disk can stream data continuously at rates varying between 30 and 60 MB/s. For higher transfer rates, solid state drives or PC RAM could be a solution, but they are limited by storage capacity as well as cost. An ideal storage solution that is both high-speed and high-capacity is a Redundant Array of Independent Disks (RAID), which is available on PXI Express. The NI 8264 (Figure 1), for example, provides 3 TB of storage with record/playback rates up to 600 MB/s. At these rates and capacity, PXI Express allows engineers to easily solve the record and playback needs of data intensive application.
Scott Savage is the Product Marketing Manager for High-Speed Digital I/O Devices, in National Instruments Modular Instrumentation Group.
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