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National Instruments digitizers are available in a wide variety of form factors including USB, PCI, PXI, and PXI Express. Each bus has unique features and advantages, so determining the bus that is appropriate for your application is important. This paper explores important considerations when choosing a bus such as cost constraints, performance, and future expandability.
Table of Contents
USB
Virtually all PCs, laptops, and industrial computing platforms provide several USB ports, driving USB to become synonymous with portability and ease of use. USB offers very fast setup times and, after installation of the appropriate driver software, requires no action or configuration besides plugging the connector into the USB port when connecting a USB device to a host computer. Because USB is hot-pluggable, you do not have to turn off the computer to add or remove a device. These features simplify setup and mobility.
The characteristics described above have made USB desirable for digitizers and other instruments. With bus-powered operation, you can use a USB digitizer such as the NI USB-5133 with a laptop to take oscilloscope measurements just about anywhere, and, with plug-and-play operation, you can achieve quick measurements and easy visualization. Overall, USB is best suited for digitizer applications that require portability or quick setup. However, many applications require features beyond the capabilities of USB. High measurement throughput (including the ability to sustain a high sample rate acquisition), instrument integration, and higher resolution are a few features that are better served by another bus.
PCI and PCI Express
PCI and PCI Express provide better performance for automated test. Like USB, almost every computer has PCI or PCI Express slots. One of the primary benefits of PCI is high data throughput, theoretically up to 132 MB/s. While the achievable throughput depends on the application, PCI throughput is still much greater than that of USB. PCI Express, the next generation of PCI, offers backward compatibility with PCI and increases data throughput by an order of magnitude – up to 1 GB/s for a x4 PCI Express lane.
Engineers often select the PCI bus for digitizer applications because of its throughput, especially if the application requires a single digitizer. As the use of PCI instruments has grown, the number of available instruments has also expanded to accommodate applications that require higher sample rates, greater resolution, or more channels. In general, the PCI form factor offers a greater variety of measurements than USB, and PCI lets you take advantage of higher measurement throughput for automated testing or extended digitizer data captures. While PCI delivers many benefits, applications often need even more digitizer features including multi-instrument integration, synchronization for more accurate measurements, and greater system reliability.
PXI and PXI Express
PXI and PXI Express provide the best performance and features for automated test applications. PXI (PCI extensions for instrumentation) is a PC-based platform that combines a modular architecture with the high throughput of the PCI and PCI Express buses. PXI Express expands the PXI platform to take advantage of the higher throughput of PCI Express. A modular solution based on PXI offers benefits including reduced cost and size through a shared chassis, backplane, and processor; faster throughput with a high-speed connection to the host processor; and greater flexibility and longevity through user-defined software.
High Throughput
PXI is the most widely used bus for digitizer applications. One of the primary reasons for its breadth of use is that it leverages the benefits of the PCI bus. Using a high-bandwidth, low-latency bus to connect the digitizer to the processor of the controlling computer means you can quickly take common or even custom measurements quickly, and the dead time between measurements is short, which translates to shorter test times and lower test cost per unit.
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PCI and PCI Express provide the highest bandwidth and lowest latency, decreasing test time and delivering flexibility and longevity through user-defined software.
A high-bandwidth bus also delivers benefits beyond automated test. The PXI platform, using PCI and PCI Express, enables streaming applications as well. Since a x4 PXI Express slot can pass data at close to 1 GB/s, the combination of a PXI Express digitizer and a RAID hard drive can help you sustain the maximum sample rate of the digitizer for extended periods of time. The benefit of this is effectively infinite onboard memory. For example, using the NI PXIe-5122 100 MS/s digitizer with the NI HDD-8264 3 TB RAID array, you can capture data at the maximum sample rate for hours, instead of acquisitions of a few hundred milliseconds possible with other buses such as GPIB, LXI, or USB.
Timing and Synchronization
Many measurement and automation applications require advanced timing and synchronization capabilities that you cannot implement directly across buses such as PCI/PCI Express, Ethernet/LAN, or USB. PXI offers advanced timing and synchronization features for these applications:
- 100 MHz differential system reference clock
- 10 MHz reference clock signal
- Differential star trigger
- Star trigger bus with matched-length trigger traces to minimize intermodule delay and skew
- Trigger bus to send and receive high-speed timing and triggering signals
- Differential signals for multichassis synchronization
With these features, you can synchronize multiple NI digitizers and other mixed-signal instruments (arbitrary waveform generators, digital waveform generators and analyzers) with picosecond-level accuracy for high-channel-count and mixed-signal applications.
System Reliability
The PXI specification defines requirements that make PXI digitizers well-suited for harsh environments. PXI features the high-performance IEC (International Electrotechnical Commission) connectors and rugged Eurocard packaging system used by CompactPCI. The PXI specification also defines specific cooling and environmental requirements to ensure proper operation in industrial environments. Modularity makes it easy for you to configure, reconfigure, and repair PXI systems, resulting in very low mean time to repair (MTTR). Because PXI is modular, you can update individual modules and components without replacing the entire system. For example, if an application expands and requires a higher acquisition sample rate, you can add a new digitizer within the same software framework instead of updating the entire system.
Conclusion
Different buses provide unique advantages for digitizer applications. USB is great for bus-powered, portable measurements, and PXI offers high-performance features including high measurement throughput, high resolution, and instrument integration and synchronization. When you are setting up a system, thinking ahead about application requirements, cost constraints, performance, and future expandability can help you choose the architecture that best meets all your needs.
Additional Resources:
PXI: The Industry Standard Platform for Instrumentation
Digitizer/Oscilloscope Fundamentals
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