PXI: The Industry Standard Platform for Instrumentation

Publish Date: Feb 24, 2014 | 14 Ratings | 4.14 out of 5 |  PDF

Table of Contents

  1. Chassis
  2. Controller
  3. Device/Module
  4. Software

PXI (PCI eXtensions for Instrumentation) is a rugged PC-based platform for measurement and automation systems. PXI combines PCI electrical-bus features with the rugged, modular, Eurocard packaging of CompactPCI and then adds specialized synchronization buses and key software features. PXI is both a high-performance and low-cost deployment platform for measurement and automation systems. These systems serve applications such as manufacturing test, military and aerospace, machine monitoring, automotive, and industrial test.

Developed in 1997 and launched in 1998, PXI was introduced as an open industry standard to meet the increasing demands of complex instrumentation systems. Today, PXI is governed by the PXI Systems Alliance (PXISA), a group of more than 70 companies chartered to promote the PXI standard, ensure interoperability, and maintain the PXI specification.


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


To learn more about PXI, download resources in the PXI Technical E-Kit.


1. Chassis

The PXI chassis is the backbone of your PXI system. It provides the power, cooling, and communication buses of PCI and PCI Express for your PXI controller and modules. PXI chassis are available in a variety of configurations such as low noise, high temperature, and low- to high-slot count. They also offer a range of I/O module slot types, integrated peripherals such as LCD displays, and more. Below are some of the key aspects of a chassis.


Figure 1. NI PXI chassis come in many different sizes from four to 18 slots.

PCI and PCI Express Communication

The PCI bus gained adoption as a mainstream computer bus in the mid-1990s. The most common implementation of the PCI bus operates at 33 MHz and 32 bits with a peak theoretical bandwidth of 132 MB/s and is the implementation in the majority of PXI systems. It uses a shared bus topology, where bus bandwidth is divided among multiple devices, to enable communication among the different devices on the bus. Over time, some devices have become more bandwidth-hungry. As a result, PCI Express was created to overcome the limitations due to these bandwidth-hungry devices starving other devices on the same shared bus.


The most notable PCI Express advancement over PCI is its point-to-point bus topology. The shared bus used for PCI is replaced with a shared switch, which gives each device its own direct access to the bus. Unlike PCI, which divides bandwidth between all devices on the bus, PCI Express provides 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 MB/s bandwidth per direction, per lane for PCI Express 1.0. Multiple lanes can be grouped together into x1 (“by one”), x2, x4, x8, x12, and x16 lane widths to increase bandwidth to the slot and achieve up to 4 GB/s total throughput. Since the introduction of PCI Express, the standard has evolved to allow faster data rates while maintaining backward compatibility. For instance, PCI Express 2.0 doubles the per-lane bandwidth from 250 to 500 MB/s per direction.


Figure 2. PCI Express is the highest bandwidth (up to 4 GB/s) data bus.


Timing and Synchronization

One of the key advantages of a PXI system is the integrated timing and synchronization. A PXI chassis incorporates 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 and synchronization. These timing signals are dedicated signals in addition to the communication architecture. The 10 MHz clock within the chassis can be exported or replaced with a higher-stability reference. This allows the sharing of the 10 MHz reference clock between multiple chassis and other instruments that can accept a 10 MHz reference. By sharing this 10 MHz reference, higher-sample-rate clocks can PLL (phase-lock loop) to the stable reference, improving the sample alignment of multiple instruments. In addition to the reference clock, PXI provides eight TTL lines as a trigger bus. This allows any module in the system to set a trigger that can be seen from any other module. Finally, the local bus provides a means to establish dedicated communication between adjacent modules.


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. PXI Express chassis provide these more advanced timing and synchronization capabilities in addition to all of the standard PXI timing and synchronization signaling.


Figure 3. The timing and synchronization capabilities of PXI and PXI Express chassis provide the best-in-class integration of instrumentation and I/O modules.


In addition to the signal-based methods of synchronizing PXI and PXI Express, these systems have many synchronization methods using absolute time. A variety of sources including GPS, IEEE 1588, or IRIG can provide absolute time with the use of the additional timing module. These protocols transmit time information in a packet so systems can correlate their time. PXI systems have been deployed over large distances without sharing physical clocks or triggers. Instead, they rely on sources such as GPS to synchronize their measurements.


Power and Cooling

With PXI chassis, you can populate a variety of I/O and instrumentation modules. Power provided to these modules is critical for reliable operation. PXI modules may require up to 25 W per slot. National Instruments PXI Express modules may require up to 38.25 W per slot. Best-in-class chassis deliver enough power for you to populate every slot. In addition to providing this power, PXI chassis offer proper cooling so you can operate the system at high temperature ratings.


When you are designing your PXI system, select a chassis that can provide plenty of power for your configuration. You may want to make sure there is enough power available if you plan to add modules in the future. Look for a system that can deliver this power across the entire operating temperature range.


Shop by Standard Chassis Types
Integrated PXI Chassis and Controllers PXI Chassis PXI Express Chassis


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2. Controller

As defined by the PXI Hardware Specification, all PXI chassis contain a system controller slot located in the leftmost slot of the chassis (slot 1). Controller options include remote controllers from a desktop, workstation, server, or laptop computer as well as high-performance embedded controllers with either a Microsoft OS (Windows 7/Vista/XP) or a real-time OS (LabVIEW Real-Time).


PXI Embedded Controllers – Embedded controllers eliminate the need for an external PC, therefore providing a complete system contained within the PXI chassis. NI has a large offering of embedded controllers ranging from high-performance to high-value.  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 7/Vista/XP or LabVIEW Real-Time.


PXI embedded controllers are typically built using standard PC components in a small, PXI package. For example, the NI PXIe-8135 controller has a 2.3 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 (2 SuperSpeed USB, 4 Hi-Speed USB), ExpressCard/34, serial, and parallel ports.

Figure 4. The NI PXIe-8135 controller has a 2.3 GHz quad-core Intel Core i7-3610QE processor
(3.3 GHz maximum in single-core, Turbo Boost mode) embedded controller. 


NI embedded controllers are designed for demanding conditions such as 24/7 operation and extreme operating temperature ranges. They are ideal for portable systems and contained “single box” applications where the chassis is moved from one location to the next. To learn more about NI embedded controller options, visit the PXI Embedded Controller resource.


Laptop Control of PXI – Using NI ExpressCard MXI (Measurement eXtensions for Instrumentation) remote control kits, you can control PXI systems directly from laptop computers through a software-transparent link. With the help of this software-transparent link, 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 card in the laptop and a PXI/PXI Express module in slot 1 of your PXI system, connected by a copper cable.


NI ExpressCard MXI Interface Kit

Figure 5. Laptop Control of PXI via ExpressCard Remote Control Kits


You can purchase any laptop compatible with NI ExpressCard MXI to remotely control your PXI system. For more information, refer to the NI Laptop Control of PXI resource page.


PC Control of PXI – With NI MXI-Express and MXI-4 remote control kits, you can control PXI systems directly from desktop, workstation, or server computers. Similar to how the products for laptop control of PXI operate, 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.

Figure 6. PC Control of PXI with MXI-Express Remote Control Kit


NI offers MXI-Express remote control kits that have either one or two ports for interfacing to PXI chassis. With the two-port remote control kit, you can control two PXI systems simultaneously from a single PC, so you can create a multichassis star topology. With a combination of these MXI products, you can also daisy chain multiple chassis to a single controller, creating a line topology. For more information, refer to the NI PC Control of PXI resource page.


Figure 7. Remote control with MXI provides PC control of PXI as well as multichassis PXI systems.


With PXI remote controllers, you can optimize processor performance and cost by using a desktop computer or laptop to remotely control a PXI system. In addition, you can use these devices to create physical and electrical isolation between the System/Host controller and the instrumentation.


Rack-Mount Controllers – To provide an alternative computing and control option, National Instruments offers external 1U rack-mount controllers. They feature high-performance multicore processors for intensive computation and multiple removable hard drives for high data storage capacity and high-speed streaming to disk. These controllers are designed to be used with MXI-Express and MXI-4 remote controllers for interfacing to PXI or PXI Express chassis. In this configuration, the PXI/PXI Express devices in the PXI system appear as local PCI/PCI Express devices in the rack-mount controller.


Figure 8. Rack-mount controllers with MXI-Express or MXI-4 remote controllers can be used to control PXI or PXI Express chassis.


Shop by Standard Controller Types
Embedded Controllers Remote Controllers Rack Mount Controllers


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3. Device/Module

Standard Module Types

National Instruments offers more than 300 different PXI modules. Because PXI is an open industry standard, nearly 1,500 products are available from more than 70 different instrument vendors.


  • Analog input and output
  • Boundary scan
  • Bus interface and communication
  • Carrier products
  • Digital input and output
  • Digital signal processing
  • Functional test and diagnostics
  • Image acquisition
  • Prototyping boards
  • Instruments
  • Motion control
  • Power supplies
  • Receiver interconnect devices
  • Switching
  • Timing input and output
  • RF and communications


Because PXI is directly compatible with CompactPCI, you can use any 3U CompactPCI module in a PXI system. Browse a list of third-party products in the NI PXI Community and the full list of NI PXI modules.


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4. Software

The development and operation of a Windows-based PXI or PXI Express system is no different from that of a standard Windows-based PC. Therefore, you do not have to rewrite existing application software or learn new programming techniques when moving between PC-based and PXI-based systems. Engineers who select PXI can reduce their development time and quickly automate their instruments by using NI LabVIEW, an intuitive graphical programming language that is the industry-standard for test, or NI LabWindows/CVI for C development.  Engineers can also use other programming languages such as those that are part of  Visual Studio .NET, Visual Basic, and C/C++. In addition, PXI controllers can run applications developed with test management software such as NI TestStand. For more information on developing test architectures for PXI, read this white paper on developing a modular software architecture.


Figure 9. Reduce development time with LabVIEW graphical programming.


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 OSs help you prioritize tasks so that the most critical task always takes control of the processor, reducing jitter. You can simplify the development of real-time systems by using real-time versions of industry-standard development environments such as the LabVIEW Real-Time and LabWindows™/CVI Real-Time modules. Engineers building dynamic or hardware-in-the-loop test PXI systems can use real-time testing software such as NI VeriStand to further reduce development time. Visit the real-time measurements portal to learn more about deterministic test.


The mark LabWindows is used under a license from Microsoft Corporation. Windows is a registered trademark of Microsoft Corporation in the United States and other countries.

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