PXI express

One of the major strengths of the PXI platform is its ability to address requirements for a system that meets changing needs and maximises investments. Whether a developer expects to add channels or functionality to the system, needs to meet longer lifecycles, or wants to reuse existing equipment, how to maximise the system investment is a driving concern.

PXI solves these needs and fully interoperates with other buses to enable hybrid systems. Most of all, PXI makes compatibility a key feature with the release of new specifications within the standard; for instance, PXI Express increases maximum system bandwidth by 45X but still maintains backward hardware and software compatibility.

Features and compatibility

Fully integrated in the PXI platform with software and hardware compatibility, PXI Express incorporates 2.5 Gbits/s PCI Express data and control signalling and the highest-performance timing and synchronisation capabilities. PCI Express was designed to extend the PCI bus.

Unlike PCI, which divides bandwidth between all devices on the bus, PCI Express uses a shared switch to provide each device with its own dedicated data pipeline. Data is sent serially in packets through pairs of transmit and receive signals called lanes, which enable 250 MBytes/s bandwidth per direction, per lane.

Multiple lanes can be grouped together into x1 (“by-one”), x2, x4, x8, x12, x16, and x32 lane widths to increase bandwidth to the slot.

PXI Express specifies each slot to support up to a x8 connection, giving users a maximum 2 GB/s per slot dedicated bandwidth. Also, PXI Express provides timing and synchronisation features built-into the backplane including a 100 MHz differential reference clock and differential star triggers in addition to the standard timing and synchronisation features provided in PXI.

Finally, PXI Express ensures both software and hardware compatibility. Hardware compatibility enables developers to use any of the existing 1000+ PXI peripheral modules in their PXI Express system.

PXI hardware compatibility is provided through hybrid chassis which include both PXI and PXI Express slots as well as through PXI Express hybrid slots.

These hybrid slots can accept both PXI and PXI Express peripheral modules. By taking advantage of the available pins on the high-density PXI backplane, the PXI Express hybrid slots are capable of delivering signals for both PCI and PCI Express.

In doing so, these PXI Express hybrid slots provide backward compatibility that is not available with desktop PC card-edge connectors, where a single slot cannot support both PCI and PCI Express signalling. Thus, the hybrid slot allows you to install a PXI module that uses PCI signalling or a high-performance PXI Express module that uses PCI Express signalling.

In Figure 1, the diagram of the PXI Express hybrid slot demonstrates how it provides compatibility to both PXI and PXI Express. The hybrid slot can accept PXI, PXI Express, CompactPCI, and CompactPCI Express modules. Thus, with this hardware compatibility, developers can use both PXI and PXI Express peripheral modules side by side in the same chassis.

PXI Express systems also provide software compatibility so that engineers can preserve their investment in existing software. PCI Express software compatibility is guaranteed through the PCI Special Interest Group (PCI-SIG). Because PCI Express uses the same driver and OS model as PCI, the specification guarantees that engineers have complete software compatibility among PCI-based systems, for example PXI, and PCI Express-based systems.

As a result, neither vendors nor end-users require driver or application software changes for PCI Express-based systems. By maintaining software compatibility between PCI and PCI Express technology, the specification drastically reduces cost for vendors and integrators to take advantage of new PCI Express technology in test systems.

PXI Express initial system cost (hybrid chassis and controller) is less than ten percent more than existing PXI systems while delivering high performance capability. Thus, developers can take advantage of PXI Express to improve performance or solve new applications at an incremental cost difference.

New applications

PXI Express builds on the flexibility, scalability, and interoperability that have made PXI successful. Providing high, dedicated per slot bandwidth in PXI Express, PXI can solve new applications that require multiple high bandwidth modules.

Previously, these modules shared a maximum system bandwidth of 132 MB/s thereby limiting the achievable performance. For instance, PXI Express can enable more high-speed imaging applications by supporting multiple cameras and/or additional I/O in one system.

For areas like RF design validation and digital test, users will be able take advantage of higher data rates to solve applications that previously could not be met through COTS based systems. In addition, PXI can better support high speed streaming applications which previously depended on deep on-board memory and other data streaming options to account for the high bandwidth needs.

Lastly, high channel count systems will also benefit from the higher overall system bandwidth available as this bandwidth will enable larger systems with greater throughput.

High-speed core for multiplatform systems

In addition, users can also take advantage of PXI interoperability to help extend system life. Oftentimes developers keep a stock of these test components, pay a premium for end-of-life test components, or redesign a whole test system in an effort to fight obsolescence.

While these processes help extend system life, they are not only cumbersome and expensive, but they also force engineers to trade longevity for scalability, flexibility, and possibly cost or performance.

Instead, developers can maximise longevity by using multiplatform test systems. Hybrid multiplatform systems combine test and measurement components from modular instrumentation platforms such as PXI and VXI and stand-alone instruments that connect externally across GPIB, USB, and Ethernet/LAN. Multiplatform systems simplify technology refreshment by easing the process of combining existing equipment with newer technologies. This way you maximise use out of both existing hardware and software but can still take advantage of other technologies.

Connecting to other buses via USB, Ethernet/LAN, GPIB, MXI, serial, and parallel, PXI enables hybrid multiplatform systems so that existing test equipment can be integrated into the system. Because multiplatform systems give you the freedom to use a variety of buses and platforms together, you are not limited to a particular bus.

The hybrid multiplatform system diagram of Figure 2 gives one example of a multiplatform topology. Using PXI as the computing centre is advantageous because it uses the high performance PCI bus to connect to other buses. The internal PCI bus provides the highest bandwidth and lowest latency, providing the best connectivity to other buses.

The key to integrating multiple buses or platforms is not only the bus interconnectivity, but also the software framework that ensures a multiplatform system can be properly architected.

The test system architecture shown in Figure 3 shows the framework of a multiplatform system to enable bus integration, code reuse, and system flexibility.

With hybrid multiplatform systems, the flexibility to combine components allows developers to optimise their resources, extend longevity, and minimise cost as well as create a system to meet their unique needs.

Summary

PXI Express enhances the capabilities of the PXI platform with higher dedicated per slot bandwidth and additional timing and synchronisation features while still maintaining hardware and software compatibility. Since PXI Express is an extension of the PXI platform, both PXI and PXI Express based systems provide a flexible and scalable design and enable interoperability with other buses. By taking a modular, software-defined approach, developers can build a PXI system according to their specific needs and expand as necessary. With the breadth of modules available from data acquisition to bus interfaces to instrumentation modules, engineers can build a system customised to their measurement requirements. PXI modularity also easily enables expansion as additional modules can be added to increase channel count or to add new functionality while still maintaining timing and synchronisation requirements.

Most of all, the software-defined approach gives users the flexibility to define their measurements through PC processing as opposed to relying on vendor-defined firmware.

With a higher data bus available in PXI Express, the number of applications that can take advantage of flexible PC processing will increase. Through a modular, software-defined platform, developers can cost-effectively change and upgrade a system by adding components instead of purchasing a complete new system. With the ability to add channels or functionality to the system, help meet longer lifecycles, or enable reuse of existing equipment, the PXI platform is able to meet the needs of developers. PXI Express enhances these capabilities and builds on the success of PXI to open up whole new arena of potential applications.