Configuring PXI Systems Using MXI-Express


This article provides an overview of PXI, including the PXI hardware architecture, software architecture, and an introduction to configuring PXI systems.


PCI eXtensions for Instrumentation (PXI) is a rugged PC-based platform that offers a high-performance, low-cost deployment solution for 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.

NI 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.

Hardware Architecture

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


Figure 1. High Performance 18-Slot PXIe Chassis Containing an Embedded System Controller and Seventeen Peripheral Modules

PXI Chassis:

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, such as the NI PXIe-1085, 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.

Figure 2. Standard chassis options include the NI PXIe-1071 (left) and the NI PXIe-1085 (right), which has 18 slots for the system controller and peripheral modules.

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.

Figure 3. PXI Timing and Triggering Buses – PXI combines industry-standard PC components, such as the PCI bus, with advanced triggering and synchronization extensions on the backplane.

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.

Figure 4. PXI Express timing and triggering buses expand the capabilities of PXI timing and triggering buses and add a differential system clock, differential signaling, and differential star triggers on the backplane.

Using these timing and triggering buses, you can develop systems for applications requiring precise synchronization. To learn more about PXI specifications for timing and triggering buses, refer to the NI PXI Specification article.

For more information on in-depth timing and synchronization concepts, refer to Configuring IEEE 1588 Synchronization With the NI PXI-668x Timing and Synchronization Module.

PXI Controllers

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. Options include embedded and remote controllers.

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 or Linux.

PXI embedded controllers are typically built using standard PC components in a small, PXI package. For example, the NI PXIe-8135 controller has a 1.3 GHz quad-core Intel Core i7-3610QE processor (3.3 GHz maximum in single-core, Turbo Boost mode), up to 16 GB of DDR3 RAM, the option of hard-disk drive or solid-state drive, two Gigabit Ethernet ports,  and standard PC peripherals such as Hi-Speed USB, serial, and parallel ports.

Figure 5. The NI PXIe-8135 embedded controller features a 2.3 GHz quad-core Intel Core i7-3610QE processor.

PXI Remote Controllers:

Remote controllers allow you to control the system from a desktop, workstation, server, or laptop.

With NI MXI-Express and MXI-4 interface kits, you can control PXI systems directly from desktop, workstation, or server computers. You can control PXI systems from PCs through a software- and driver-transparent link. During boot-up, the computer recognizes all peripheral modules in the PXI system as PCI boards, and you can then work with these devices through the controller. PC control of PXI consists of a PCI/PCI Express board in your computer and a PXI/PXI Express module in slot 1 of your PXI system, connected by a copper or fiber-optic cable.

The MXI-Express interface kit provides a 110 MB/s PCI Express-to-PCI bridge from the PC to the PXI chassis. With the NI PXI-PCIe8362 two-port interface kit, you can control two PXI systems simultaneously from a single PC. Using a MXI-Express interface, you can create multichassis PCI systems using the star topology configuration, with either 3 or 7 m copper cables. This interface is compatible with x1, x4, x8, and x16 PCI Express slots, so you can use this option in a wide variety of desktop computers. For more information on MXI-Express, refer to the MXI-Express Series User Manual.

Figure 6. Remote control with MXI-4 provides PC control of PXI as well as multichassis PXI systems.

The MXI-4 interface kit offers a 78 MB/s PCI-to-PCI bridge from a PC to the PXI system. The MXI-4 interface kit comes with low-cost copper links or fiber-optic links for both extended distances and electrical isolation. As shown in Figure 7b, you can build multichassis PXI systems with MXI-4 as well. Using a MXI-4 link, you can implement either a daisy chain or a star topology to build multichassis systems. For more information on topologies for multichassis configurations, refer to the MXI-4 Series User Manual.

With PXI remote controllers, you can maximize processor performance with minimal cost by using a desktop computer or laptop to remotely control a PXI system. Because all remote control products are software-transparent, no additional programming is required. You can purchase many desktops, workstations, or server computers and then remotely control your PXI system using either a MXI-Express or copper/fiber-optic MXI-4 serial link. For more information, refer to the NI PC Control of PXI page.

With NI ExpressCard MXI (Measurement eXtensions for Instrumentation, you can control PXI systems directly from laptop computers through a software-transparent link. During boot-up, the laptop computer recognizes all peripheral modules in the PXI system as PCI boards, and you can then control these devices through the laptop computer. Laptop control of PXI consists of an ExpressCard in the laptop and a PXI/PXI Express module in slot 1 of your PXI system, connected by a copper cable.

Figure 7. NI ExpressCard MXI Interface Kit

The NI ExpressCard MXI interface kit provides a 110 MB/s PCI Express-to-PCI bridge from the laptop computer to the PXI chassis. You now have the advantage of mobile/portable PXI systems with laptop control of PXI. You can purchase any laptop compatible to remotely control your PXI system. For more information, please refer to the NI Laptop Control of PXI page.

PXI Peripheral Modules

NI offers more than 200 different PXI modules and because PXI is an open industry standard, more than 1,500 modules are available from more than 70 vendors.

To learn more about NI PXI modules, refer to the NI PXI Modules resource page. A categorized list of modules offered by NI and NI PXI product partners is available at the following PXI page.

PXI provides the industry’s highest-bandwidth and lowest-latency bus with modular I/O for applications ranging from high-resolution DC to 6 GHz RF. Because PXI is mostly compatible with CompactPCI, you can use many 3U or 6U CompactPCI modules in a PXI or PXI Express system. Additionally, you can install CardBus/PCMCIA and PMC (PCI Mezzanine Card) cards in PXI systems using carrier modules. For example, with the NI PXI-8221 PC Card carrier, you can connect CardBus and PCMCIA devices to PXI systems. NI offers PXI modules for instrumentation, data acquisition, switching, motion, vision, advanced synchronization, and interfacing with other buses so you can create a wide variety of PXI systems.

PXI also preserves your investment in stand-alone instruments or VXI systems by offering standard hardware and software for communication between multiple buses. For example, interfacing a PXI system to GPIB-based instrumentation is no different when using a PXI-GPIB module than it is using a PCI-GPIB card. The software is identical. Additionally, you can choose from several methods to interface PXI and VXI. To learn more about hybrid systems, refer to Integrating LXI, USB, PXI Express, and Other Standards into a Hybrid Test System.

Software Architecture

The development and operation of Windows-based PXI systems is no different from that of a standard Windows-based PC. Additionally, because the PXI backplane uses the industry-standard PCI bus, writing software to communicate with PXI modules is, in most cases, identical to that of PCI boards. Therefore, you do not have to rewrite existing application software, example code, and programming techniques when moving software between PC-based and PXI-based systems.

PXI Express systems also provide software compatibility to help you preserve your investment in existing software. Because PCI Express uses the same driver and OS model as PCI, the specification guarantees that you have complete software compatibility among PCI-based systems. As a result, both vendors and customers do not need to change driver or application software for PCI Express-based systems.

Because PXI and PXI Express systems can use the same drivers as PCI, software to communicate with an NI PXI-6251 multifunction data acquisition module is identical to software that communicates with an NI PCI-6251 board in a PC.

Figure 8. Two different packages – one software standard. In software, communication with a PXI module (bottom) is identical to that with a PCI board (top).

As an alternative to Windows-based systems, you can use a real-time software architecture for time-critical applications requiring deterministic loop rates and headless operation (no keyboard, mouse, or monitor). Real-time operating systems help you prioritize tasks so that the most critical task always takes control of the processor when needed. With this feature, you can program an application with predictable results and reduced jitter. 

The PXI specification presents software frameworks for PXI systems based on Microsoft Windows operating systems. As a result, the controller can use industry-standard application programming interfaces, such as NI LabVIEW, LabWindows™/CVI, and Measurement Studio; Visual Basic; and Visual C/C++. Initialization files that define system configuration and system capabilities are required for PXI components. Finally, implementation of the Virtual Instrument Software Architecture (VISA), which has been widely adopted in the instrumentation field, is required by PXI for configuration and control of VXI, GPIB, serial, and PXI instruments.


PXI modular instrumentation defines a rugged computing platform for measurement and automation that takes advantage of the technology advancements in the mainstream PC industry. By taking advantage of the standard PCI bus, PXI modular instrumentation systems can benefit from widely available software and hardware components. The software applications and OSs that run on PXI systems are already familiar because they are already in use on common desktop computers. PXI meets your needs by adding rugged industrial packaging, plenty of slots for I/O, and features that provide advanced timing and triggering capabilities.

Additional Resources

NI PXI Slot Blockers - NI also offers slot blockers, which are plastic modular filler PXI cards, to populate unused slots in the chassis. This enhances the airflow in the populated slots by reducing airflow bypass in the empty slots. The use of slot blockers can reduce the temperature rise of electronic components on installed modules by up to 20 percent, improving the cooling performance of your chassis. For more information about the installation of NI PXI slot blockers, refer to the NI PXI Slot Blocker Installation Guide .

NI PXI System Monitor - You can observe the operating parameters of many NI PXI chassis and controllers. The API is available in both ANSI C and LabVIEW software, so you can programmatically collect and process those values to diagnose system health based on individual use cases.

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