PXI Specification Standards Explained



The PCI eXtensions for Instrumentation (PXI) specification defines a rugged PC-based platform for measurement and automation systems. PXI uses the high-speed Peripheral Component Interconnect (PCI) bus, which is the de facto standard driving today’s desktop computer software and hardware designs. PXI combines the PCI electrical bus with the rugged modular Eurocard mechanical packaging of CompactPCI. PXI then adds mechanical, electrical, and software features (see Figure 1) that define complete systems for test and measurement, data acquisition, and manufacturing applications. National Instruments developed and announced the PXI specification in 1997 as an open industry specification. Today, the PXI specification is managed by the PXI Systems Alliance -- a group of more than 60 company members. Because PXI is an open specification, any vendor can build PXI products. CompactPCI, the standard regulated by PICMG, and PXI modules can reside in the same system without any conflict because interoperability between CompactPCI and PXI is a key feature of the PXI specification. To download a free copy of the full PXI specification, see pxisa.org.

As the commercial PC industry drastically improves the available bus bandwidth by evolving PCI to PCI Express, PXI has the ability to meet even more application needs by integrating PCI Express into the PXI standard. To ensure the success of PXI Express and CompactPCI Express, engineers within the PCI Industrial Manufacturers Group (PICMG), which governs CompactPCI, and the PXI Systems Alliance (PXISA), which governs PXI, have worked to ensure that the PCI Express technology can be integrated into the backplane while still preserving backwards compatibility with the large installed base of existing systems. By taking advantage of PCI Express technology, PXI Express increases the available PXI bandwidth from 132 MB/s to 6GB/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 into many new application areas, many of which were previously only served by expensive and proprietary hardware. 

Mechanical Features

The PXI specification defines requirements that make PXI systems well suited for harsh environments. The high-performance IEC connectors and rugged Eurocard packaging system used by compactPCI are used in PXI. PXI also adds specific cooling and environmental requirements to ensure operation in industrial environments. You can find more specifics on PXI hardware and software specifications in the Specifications section of the PXISA website.

High-Performance Connector System

PXI employs the same advanced pin-in-socket connector system used by CompactPCI. These highly dense (2 mm pitch) impedance-matched connectors are defined by the International Electrotechnical Commission (IEC-1076) and offer the best possible electrical performance under all conditions. These connectors have seen widespread use in high-performance applications, particularly in the telecommunications field. Because of the electrical characteristics of these IEC connectors, PXI systems offer more slots on a single bus segment than desktop PCs.

Eurocard Mechanical Packaging

The mechanical aspects of PXI and CompactPCI are governed by Eurocard specifications (ANSI 310-C, IEC-297, IEEE 1101.1, IEEE 1101.10, and P1101.11), which have a long history of application in industrial environments (for example, VME and VXI). These electronics packaging standards define compact, rugged systems that can withstand harsh industrial environments in rack-mount installations. PXI specifies two module sizes -- a small (3U = 10 by 16 cm) and large (6U = 23.335 by 16 cm). The 3U size is the most popular size for PXI systems. Because of the small size, you benefit from the miniaturization of high-performance electronics. You can use all 3U size modules in 6U systems with an adapter. PXI defines the system slot, the system controller location, on the far left end of the bus segment. This defined arrangement is a subset of the numerous possible configurations with CompactPCI. Defining a single location for the system slot simplifies integration and increases the degree of compatibility between controllers and chassis from different vendors.

Additional Electronic Packaging Specifications

All mechanical specifications defined in the CompactPCI specification apply directly to PXI systems; however, PXI does include additional requirements that simplify systems integration. The PXI specification requires forced-air cooling of all chassis and recommends complete environmental testing, including temperature, humidity, vibration, and shock. All PXI products are required to have documentation of these test results. Operating and storage temperature ratings are required for all PXI products. The PXI specification also requires electromagnetic emissions and susceptibility testing to ensure compliance with international standards.

Electrical Features

Many instrumentation applications require system timing capabilities that cannot be implemented directly across standard ISA, PCI, or CompactPCI backplanes. PXI modular instrumentation adds a dedicated system reference clock, PXI trigger bus, star trigger bus, and slot-to-slot local bus to address the need for advanced timing, synchronization, and side-band communication (see Figure 2). PXI adds these instrumentation features while maintaining all PCI bus advantages.

System Reference Clock

The PXI backplane provides a built-in common reference clock for synchronization of multiple modules in a measurement or control system. Each peripheral slot features a 10 MHz TTL clock, transmitted on equal-length traces, providing skew of < 1 ns between slots. The accuracy of the 10 MHz clock is chassis specific, but is typically less than 25 parts per million (ppm), making it a reliable clock for synchronization based on phase lock looping (PLL) methods. For example, multiple 100 MHz digitizers are easily synchronized by phase-lock-loop of their individual voltage-controlled crystal oscillator (VCXO) 100 MHz clocks to the 10 MHz system reference clock. The accuracy of the 10 MHz clock can be improved by installing a capable board into the star trigger slot (slot 2) of the chassis. For example, placing a NI PXI-6608 into slot 2 will reduce the clock error to less than 75 parts per billion (ppb) by driving a high-accuracy 10 MHz clock to the backplane.

PXI Trigger Bus

PXI defines eight trigger bus lines for synchronization and communication between modules. Trigger, clock, and handshaking signals can be shared using the trigger bus lines. Triggers can be passed from one module to any number of modules, so you can distribute digital trigger signals from master to slave measurement devices. The trigger bus allows transmission of variable frequency sampling clocks, so multiple modules can directly share a sample clock or variable frequency time base. For example, four data acquisition (DAQ) modules using a 44.1 Ksps CD audio sampling rate can directly share a clock that is a multiple of 44.1 KHz over the trigger bus. However, for clock frequencies of approximately 20 MHz or greater, direct transmission of a clock with the trigger bus is not recommended due to signal degradation, and you should use a system reference clock instead.

Star Trigger Bus

The star trigger bus has an independent trigger line for each slot oriented in a star configuration from a special star trigger slot (defined as slot 2 in any PXI chassis). The PXI star line lengths are matched in propagation delay to within 1 ns from the star trigger slot. This feature addresses high-speed synchronization where you can distribute start/stop trigger signals from the master measurement module in the star trigger slot with low delay and skew. Alternately, a variable-frequency clock signal can be transmitted to modules over the star trigger bus with < 1 ns skew.

Local Bus

The PXI local bus is a daisy-chained bus that connects each peripheral slot with its adjacent peripheral slots to the left and right. Thus, a given peripheral slot’s right local bus connects to the adjacent slot’s left local bus, and so on. Each local bus, which is 13 lines wide, can pass analog signals as high as 42 V between cards or provide a high-speed side-band communication path that
does not affect the PCI bandwidth.

PCI Features

PXI offers the same performance features defined by the desktop PCI specification, with one notable exception. PXI and CompactPCI systems can have up to seven peripheral slots per bus segment, whereas most desktop PCI systems offer only three. Otherwise, all PCI features apply to PXI/CompactPCI:

  • 33 MHz performance
  • 32 and 64-bit data transfers
  • 132 Mbytes/s (32-bit) and 264 Mbytes/s (64-bit) peak data rates
  • System expansion via PCI-PCI bridges
  • 3.3 V migration
  • Plug and Play capability


PXI Express

PXI Express not only retains the timing and synchronization features of PXI, but it also adds several new synchronization features by taking advantage of the existing differential connectors required in PXI and technological advances that provide higher performance, low-cost differential signaling. Building on existing PXI capabilities, PXI Express provides the additional timing and synchronization features of a 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 clocks. In addition to allowing engineers to improve the performance of the system, high-frequency clocks also match well with modern processes and allow lower cost products to remove clock multiplication circuits.

Software Features

PXI defines software requirements in addition to electrical requirements to further simplify systems integration. These requirements include the use of standard operating system frameworks. Appropriate configuration information and software drivers for all peripheral devices are also required.

Common Software Requirements

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 National Instruments LabVIEW, Measurement Studio, Visual Basic, and Visual C/C++. PXI also requires certain software components to be made available by module and chassis vendors. 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 specified by PXI for configuration and control of VXI, GPIB, serial, and PXI instruments.

PXI Express

PXI Express systems also provide software compatibility so that engineers can preserve their investment in existing software. 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, such as PXI Express. As a result, both vendors and customers do not need to change driver or application software for PCI Express-based systems.


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

For more information on the PXI hardware architecture, software architecture, and an introduction to configuring PXI systems, see the Introduction to the PXI Architecture.

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