PCI eXtensions for Instrumentation (PXI) 101

April 30, 2009
PCI eXtensions for Instrumentation (PXI) is a rugged PC-based platform that offers a high-performance, low-cost means of deploying measurement and automation systems.

PCI eXtensions for Instrumentation (PXI) is a rugged PC-based platform that offers a high-performance, low-cost means of deploying measurement and automation systems. PXI combines the Peripheral Component Interconnect (PCI) electrical bus with the rugged, modular Eurocard mechanical packaging of CompactPCI and adds specialized synchronization buses and key software features. PXI also adds mechanical, electrical, and software features that define complete systems for test and measurement, data acquisition, and manufacturing applications. These systems serve applications such as manufacturing test, military and aerospace, machine monitoring, automotive, and industrial test.

National Instruments developed and announced the PXI specification in 1997 and launched it in 1998 as an open industry specification to meet the increasing demand of complex instrumentation systems. Currently, PXI is governed by the PXI Systems Alliance (PXISA), a group of more than 50 companies chartered to promote the standard, ensure interoperability, and maintain the PXI specification. Because PXI is an open specification, any vendor is able to build PXI products. CompactPCI, the standard regulated by the PCI Industrial Computer Manufacturers Group (PICMG), and PXI modules can reside in the same PXI system without any conflict because interoperability between CompactPCI and PXI is a key feature of the PXI specification.

Much like the commercial PC industry drastically improved the available bus bandwidth by evolving from PCI to PCI Express in late 2005, PXI has also incorporated higher bus bandwidth capabilities with the introduction of PXI Express. PXI has the ability to meet even more application needs by integrating PCI Express into the PXI standard. PCI Express technology can be integrated into the backplane while preserving backward compatibility with the large install base of existing systems. The system controller slot is capable of supporting up to x16 PCI Express links in addition to x1, x4, and x8 links, which provide up to 6 GB/s bandwidth to the PXI Express backplane. By taking advantage of PCI Express technology, PXI Express increases the available bandwidth from 132 MB/s with PXI to 6 GB/s for a more than 45X improvement in bandwidth while still maintaining software and hardware compatibility with PXI modules. With this enhanced performance, PXI can reach many new application areas, many of which were previously served only by expensive and proprietary hardware.

PXI systems are composed of three basic components — chassis, system controller, and peripheral modules.

The chassis provides the rugged and modular packaging for the system. Chassis generally are available in 4-, 6-, 8-, 14-, and 18-slot 3U and 6U sizes. A rack unit (U) is a unit of measure used to describe the height of the device intended for mounting in a 19 or 23 in. rack (refers to width of rack). One rack unit is 44.45 mm (1.75 in.) high. The size of a piece of rack-mounted equipment is usually described as a number in “U.” Options for specific chassis include AC and DC power supplies and integrated signal conditioning. Many PXI Express chassis accommodate PXI and PXI Express peripheral modules and some have hybrid and PXI Express peripheral slots so you can use PXI Express and hybrid-compatible PXI peripheral modules. These chassis allow for multiple PXI system configurations to meet many application needs.

The chassis contains the high-performance PXI backplane, which includes the PCI bus and timing and triggering buses. PXI modular instrumentation adds a dedicated 10-MHz system reference clock, PXI trigger bus, star trigger bus, and slot-to-slot local bus to address the need for advanced timing, synchronization, and sideband communication while not losing any PCI advantages.

Building on PXI capabilities, PXI Express provides the additional timing and synchronization features of a 100 MHz differential system clock, differential signaling, and differential star triggers. By using differential clocking and synchronization, PXI Express systems benefit from increased noise immunity for instrumentation clocks and the ability to transmit at higher-frequency rates.

Most PXI chassis contain a system controller slot in the leftmost slot of the chassis (slot 1). You can choose from a few options when determining the best system controller for an application, including remote controllers from a desktop, workstation, server, or laptop computer and high-performance embedded controllers with either a Microsoft OS (Windows Vista/XP) or a real-time OS (LabVIEW Real-Time). The two types of controller options are laptop control of PXI and PC control of PXI.

PXI Embedded Controllers - Embedded controllers eliminate the need for an external PC, therefore providing a complete system contained within the PXI chassis. These embedded controllers come with standard features such as an integrated CPU, hard drive, RAM, Ethernet, video, keyboard/mouse, serial, USB, and other peripherals, as well as Microsoft Windows and all device drivers already installed. They are available for systems based on PXI or PXI Express, and you have your choice of OSs, including Windows Vista/XP or National Instruments’ LabVIEW Real-Time.

PXI Peripheral Modules - Because PXI is an open industry standard, more than 1500 modules are available from more than 70 vendors.

(With permission of National Instruments; http://zone.ni.com/devzone/cda/tut/p/id/4811)

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