The evolving PXI standard: expanding the capabilities of the PXI architecture

Author : Michael Dewey | Product Specialist | Marvin Test Solutions

01 May 2020

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The PXI standard, now over 20 years old, has evolved over time, driven by advancements in card module technology & end-users’ requirements. In late 2018, the PXISA’s (PXI Systems Alliance) technical committee completed an update of the various technical specifications associated with the standard.

This article was originally featured in EPDT's 2020 PXI for T&M supplement, included in the May 2020 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.

As Michael Dewey, Product Specialist at vertically integrated aerospace T&M company, Marvin Test Solutions tells us, these updates, which affected both software & hardware specifications, were the result of both supplier and user requests for the purpose of supporting evolving future test needs. Of particular significance were specification updates that addressed support for the Linux® operating system, and the capability to support higher power PXI modules and chassis.

The PXI (CompactPCI eXtensions for Instrumentation) standard was established in 1998, with specifications PXI-1 and PXI-2 defining the hardware and software requirements for a card modular instrumentation standard. In 2005, the PXI standard was updated to address the use of the PCI Express bus as the underlying architecture for the PXI Express specification. Specifications for the PXI Express architecture are detailed in the PXI-5 PXI Express Hardware Specification and the PXI-6 PXI Express Software Specification.

More recently, the PXI standard was updated in 2018, which affected both the hardware and software specifications. These updates increased PXI module and chassis power capabilities/consumption, defined a consistent method for the use of multi-slot PXI modules within a PXI system, and established a framework and requirements for PXI systems that employ the Linux operating system.

Figure 1. PXI Express hybrid slot with eHM backplane connector

Increased power capabilities/consumption for PXI & PXIe chassis & modules

Ongoing technology advancements in instrumentation and the use of high-performance components employed by modular instrumentation suppliers prompted the PXISA to review and update the specification to accommodate higher power consumption by PXI and PXI Express modules. Based on investigative work done by the PXISA technical committee, it was determined that the current-carrying capacity of the eHM connector which is used on PXI Express modules and backplanes, and supplies 12 V and 3.3 V power (Figure 1), could sustain 3 A per pin on the +3.3 V and +12 V rails in  a PXI Express chassis, without exceeding the 125 °C contact temperature limit of the connector. Similar results were found for applications employing the +12 V and -12 V pins associated with PXI-1 slots.

Subsequently, one of the significant updates made to the PXI standard was the increase in power available to PXI-1 modules, hybrid slot compatible PXI-1 modules, and PXI Express modules. Both the PXI Hardware Specification (PXI-1) and the PXI Express Hardware Specification (PXI-5) were updated to reflect the increased power available for modules.

Key changes and updates to both the PXI-1 and PXI-5 specifications included:

Table 1. PXI peripheral & chassis power capabilities, baseline specifications vs 2018 updates

•  PXI Express modules can now draw up to  3 A per pin from the +12 V and +3.3 V rails.

•  PXI-1 modules and Hybrid Slot Compatible PXI-1 modules can now draw up to 3 A per pin from the +12 V and -12 V rails.

•  Clarification that PXI and PXI Express chassis can supply additional power per slot, beyond the stated minimum power per slot requirement.

•  Recommendation that PXI-5 modules incorporate a thermal sensing mechanism to protect modules from overheating.

Note that these updates to the specification did not change the minimum power requirements for a PXI or PXI Express chassis. Rather, these updates offer suppliers and users of modules and chassis the flexibility to create higher performance modules and systems that require more power than the baseline standard of 26.5 or 30 watts per slot. In addition, the PXI standard provides chassis suppliers the flexibility to offer chassis that can supply much higher power per slot, as long as the current per pin does not exceed the new specified limits. Table 1 summarises how these specification updates affect both PXI chassis and modules.

Figure 2. 1600W PXI chassis and 32-channel, timing per pin PXI digital module

As detailed in Table 1, with the latest updates to the specifications, the PXI standard now allows for higher power capabilities for chassis and higher power consumption by PXI-1 and PXI Express modules. Figure 2 depicts an example of a high power/high performance PXI chassis that offers 1600 watts of system power, and a 3U PXI high performance digital test module which features timing per pin and high channel density. 

Support for the Linux operating system

The PXI standard was originally based on the Windows® operating system, with all instrument vendors required to supply Windows-based instrument drivers. However, several PXI vendors also supply Linux drivers for their instrumentation, and there are numerous end users today employing the Linux environment for their test systems. This growing interest in the use of Linux prompted the PXISA to update both the PXI-2 PXI Software Specification and the PXI-6 PXI Express Software Specification to include support for the Linux OS. The support for Linux is strictly optional for instrument suppliers. The primary goal associated with this effort was to standardise how the Linux OS can be supported in a defined and interoperable way for PXI and PXIe systems. The guiding principles associated with incorporating Linux into the PXI standard included:

•  Maintain parity and consistency with the methods and requirements associated with the Windows environment for supporting PXI.

•  Avoid creating requirements that are unique to Linux or that don’t exist for the Windows environment.

•  Support for both 32 and 64 bit versions of Linux.

Figure 3. INI file listing for a PXI peripheral module

Support for the Linux OS involved the re-definition and specification of “services trees” and the software services and interfaces for the Linux OS.

For the Windows environment, PXI file locations and services are located or named via Windows standard locations or via the Windows registry. However, for the Linux environment, support for these PXI software components is based upon a file system structure. Top-level and vendor-level keys associated with the Windows environment are replaced by directories, and INI file(s) are used at the model level. Figure 3 provides an example of an INI file for a PXI peripheral module.

System Description and Configuration files are located in the /etc/pxisa/directory and the Chassis Description files are located in the /usr/share/pxisa/chassis/ directory.

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Software services for PXI modules operating in a Linux environment were also redefined. For the Windows environment, a PXI driver is implemented as a 32-bit Windows DLL with each operation corresponding to an exported symbol of the DLL. For the Linux environment, drivers are implemented as shared objects (SO), matching the bitness of the OS, with each operation corresponding to an exported symbol of the SO.  The resulting driver is called a PXI driver SO that employs the ANSI C calling convention and uses the same C data types as Windows driver DLLs. 

With the definition and standardisation of services trees and software services for the Linux OS, integrating a PXI system that employs the Linux environment requires much less effort and time.

Summary

The 2018 updates to the PXI standard addressed several other technical areas, including the management and configuration of multi-slot modules. However, the addition of increased system and module power capabilities, and the standardisation of Linux for use with PXI systems represented the most significant updates to the standard. These changes reinforce the PXISA’s commitment to continually evolve and update the standard in order to leverage the latest technologies and capabilities for PXI systems – providing end-users with a test platform that can address both current and future test needs.


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