CompactPCI and PXI

CompactPCI and PXI Enable Advanced Measurement and Control in Embedded Applications

The instrumentation extensions to PCI are embodied in PXI modules, which can coexist with a wide world of CompactPCI modules on a combination backplane.


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CompactPCI and PXI have become increasingly popular standards in the industrial and test and measurement spaces. Because of the significant adoption of these standards, a wide variety of interchangeable modules have become available over the past decade. A growing number of embedded applications have found the standards as suitable platforms for high-performance, reliability and rugged installations.

CompactPCI was created by the PCI Industrial Computers Manufacturers Group (PICMG) in the late 1990s by taking the desktop PCI specification and designing an industrial Eurocard-based form factor around it. This enabled the benefit of the PCI bus while providing a form factor that is suitable for industrial computing applications (Figure 1).

Figure 1
PXI combines the PCI electrical bus with rugged, modular Eurocard mechanical packaging of CompactPCI and adds specialized synchronization buses.

The PCI eXtensions for Instrumentation (PXI) combine the Peripheral Component Interconnect (PCI) electrical bus with the rugged, modular Eurocard mechanical packaging of CompactPCI and add specialized synchronization buses and key software features. These systems serve applications such as manufacturing test, military and aerospace, machine monitoring, automotive and industrial test. 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 a superset of CompactPCI, both PXI and CompactPCI 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.

Thousands of Modules Available

From industrial bus interfaces to high-speed measurement I/O, you can find just about any I/O imaginable on the PXI and CompactPCI form factor. Data acquisition, instrumentation, hard drive carriers, motion control interfaces and image acquisition are just a few of the many categories of modules available. 

National Instruments offers enhanced functionality for modules within a given family. For instance, many of the modular instruments NI offers, including digitizers, DMMS and high-speed digital offer tight synchronization and integration through software. NI Data acquisition devices offer similar features, enabling scalable medium to large channel count embedded systems.

If an engineer or scientist is looking for a module that is not available they should not be concerned. Because CompactPCI and PXI are open standards, anyone can design their own module. This is easy because of the availability of ICs to interface to PCI and PCI Express as well as the standardized connector and other I/O components.

Form Factor

CompactPCI and PXI systems come in a variety of sizes. If an engineer or scientist cannot find the form factor they are looking for, they can design the backplane into their own custom mechanical form factor. National Instruments offers all of its CompactPCI/PXI systems as a backplane only without a card cage. Because the backplanes use standard spacings and components for card guides, it becomes relatively straightforward to design CompactPCI/PXI into an embedded application. Backplanes with as few as four slots are available all the way up to eighteen slot configurations. In addition, there are 3U and 6U form factors available (Figure 2).

Figure 2
The NI PXI-1031 backplane provides a system controller slot and three peripheral module slots, which accept PXI and CompactPCI modules.

Depending on their environment, an engineer or scientist can design for the appropriate amount of vibration isolation, cooling, electrical power, etc. Some environments may require enhanced cooling or even pressurization, while others may require little cooling and limited mechanical vibration isolation. Selecting a PXI or CompactPCI backplane provides the flexibility to design in the specifications needed without being burdened by specifications an engineer or scientist may not want in a particular off-the-shelf chassis or card cage (Figure 3).

Figure 3
There is a variety of slot configurations and controller options for PXI and PXI Express.

Many of the backplanes that are available can be powered simply with direct connection to the ATX style power connector on the backplane. Power supplies can be designed to accommodate the power needed for the modules in an individual system. Off-the-shelf supplies exist or engineers and scientists can provide the power rails from an existing power system designed in their application. Modules can consume from a few watts to up to 38.25 watts for high-speed or high-density modules.

CompactPCI and PXI leverage industry standard components. First, standard Eurocard connectors and card guides allow for ease of design of mechanical installation. Second, standard chipsets are offered from a variety of vendors to communicate over PCI and PCI Express.  Additionally, many backplanes offer standard connectivity to ATX power supplies. Finally, leveraging standard components enables fast and low cost development of CompactPCI and PXI embedded systems.

PCI Express and Advanced Timing Features

Both the CompactPCI and PXI specifications have evolved to incorporate PCI Express (PCIe). 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 Gbytes/s bandwidth to the PXI Express backplane. By taking advantage of PCI Express technology, PXI Express increases the available bandwidth from 132 Mbytes/s with PXI to 6 Gbytes/s. This makes for a more than 45X improvement in bandwidth while still maintaining software and hardware compatibility with PXI and CompactPCI modules. 

One of the key features that differentiate PXI from CompactPCI is the timing and synchronization architecture of PXI and PXI Express. PXI offers an onboard 10 MHz crystal oscillator to enable a stable clock reference for modules. In addition, PXI offers a trigger bus that enables modules to send and receive triggers. Finally, PXI offers a star trigger bus, allowing engineers and scientists to send a trigger to all modules with less than 1ns of skew between modules (Figure 4). 

Figure 4
PXI and PXI Express 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.

The small form factor, rugged mechanicals, advanced communication and timing and synchronization architecture make CompactPCI and PXI ideal for high-performance embedded applications. With both standards growing strong in their second decade, an increasing number of modular instruments and I/O vendors are designing a variety of products. If an engineer or scientist is looking for a scalable and flexible backbone for their next embedded application, they should consider CompactPCI and PXI as an attractive option.  

National Instruments
Austin, TX.
(512) 794-0100.