Understanding LXI and PXI for Switching

The use of switching systems to route instrumentation and stimulus signals to appropriate test points on a unit under test has a crucial role in most electronics test systems. The switching system manages the sharing of test resources, connection of calibration references, load management, and many other functions; the switching system acts as the interface between the unit under test (UUT) and the test equipment.

You have a choice in choosing the platform used to control switching. For us, the principal platforms are PXI and LXI. This page explains some of the background reasons for these platforms. 

Jump to the comparison table here >>

Brief History

We have been developing complete switching systems since our first products were introduced in 1988. Using the knowledge base of reed relays offered by our relay division, Pickering Electronics, we developed a range of switching systems based around a proprietary platform that provided GPIB or RS232 control of the relays. 

Even at that time, there was a demand for switching systems based on open modular standards, so we introduced switching products based on both the VXI and the PXI standards. The VXI standard is now not commonly used on new systems (VXI Future), but the PXI standard has proven to be an excellent basis for modular switching products and instrumentation. The development costs for the PXI form factor are relatively low once the initial PXI entry barriers have been crossed. The speed of product development can be high, enabling a large variety of standard and custom switching solutions to be made available in response to user demand. More information on PXI can be found in our PXIMate book or on the PXI Systems Alliance website.

Over the years, we have become a market leader in PXI switching, with over 1000 modules available. The PXI standard is mature and widely adopted in the test industry. While it offers a fast communication interface to support applications requiring the exchange of large data blocks that may be needed for analysis by the system controller, it also imposes mechanical and power restrictions on the modules. Not all users want their instrumentation or switching products to be extensions of their PC's PCI bus or for the data analysis to be performed in the PC rather than processing in the instrument. Although it has been hugely successful, PXI has not always displaced the majority of rack or bench instruments historically based largely, but not entirely, on GPIB interfaces.

GPIB instruments have their control interfaces either supplemented or replaced by a LAN/Ethernet connection. The de-facto standard for control via the Ethernet interface is LXI. Ethernet is everywhere on system controllers, and the connection cables are easily managed, have a latching mechanism and have virtually no restrictions on distance or the number of instruments supported. The LXI standard enforces compliant instruments to implement the Ethernet control in a standardized way, eliminating the concerns about implementation conflicts on the Ethernet connection that could arise on proprietary implementations. More information on LXI can be found in our free LXImate book or on the LXI Consortium website.

Standardization of LAN connections for instrumentation was a significant leap forward following the introduction of the LXI standard's first revision in 2005. The adoption of LXI has been rapid; the value of LXI-enabled products shipped is, for example, much higher than that of PXI. Like PXI, the LXI standard is mature and stable, reaching version 1.5 in 2016, and LXI devices conforming to the first version of the standard still work with the devices conforming to the latest version.

General Comparison of PXI and LXI

PXI and LXI are not competing standards, though they will have similar characteristics in some applications. In others, there are apparent differences, which can mean that one platform is more suitable.

The table below clearly illustrates some of these differences; the relevance of any factor in this table depends on what product the user/vendor wants to create.

	PXI	LXI
Power	Central	AC/DC to each box
Programming	Register-based (mainly)	Message-based (mainly) with its embedded controller providing the LAN interface
Controller	PC-based, either an external general purpose PC with a link to chassis or a proprietary format PC in chassis controller slot	Agnostic, typically Linux-based embedded controller or embedded PC for high-end products within the LXI Device that provides local intelligence and is managed from a network-based controller
Operating System	Mainly Windows	Agnostic
Hot Connection	Not supported; adding parts requires restarting the system controller	Supported, the network assigns an IP address without a restart
Web Interface	Not supported	Supported
Cooling	Chassis based	Individual device-based Not constrained by the standard.
Mechanical Size	3U or 6U (3U being by far the most common)  Defined module size (pitch, height and depth), which has a relatively small footprint	Typically based on half or full-rack width formats with whatever height and depth are required.  Can be modular based either on other standards (including PXI) or proprietary standards.  Devices can be added to the network.
Multi-vendor	Chassis can support modules from different vendors	On modular systems, the format is proprietary and can only be from a single vendor. This is true even if the mechanical format is an open standard (PXI)
Bus Interface	PCI or PCIe connecting the module to the controller bus with limited connection distance to the controller.	Ethernet with almost unlimited connected distances, including inter-continental distances over VPNs.
Trigger	Hardware backplane trigger or software-based trigger from the controller (including IVI)	External cabled wired trigger bus  IEEE1588-based event timing  Peer-to-peer triggering  Software triggers over Ethernet

LXI instruments are largely platform agnostic, whereas PXI is dependent on the PC architecture (and, in many implementations Windows). LXI Devices do not have many mechanical or electrical constraints, but PXI has to conform to the PXI standard to benefit from the multi-vendor chassis platform. They can also have quite different data speeds as well. Although PXI has faster connection speeds, it relies on processing data in the controlling PC—so it inherently needs a high-speed interconnect to perform data processing functions. An LXI system might be expected to process data within the LXI Devices and simply has to report the results; it can also dump blocks of data for controller processing.

It is a fundamental objective of the PXI standard that it has to permit products from different vendors to co-exist in a chassis; otherwise, as a standard, it fails to meet the market expectation and the objectives stated in the standard. That means that a chassis, and indeed the software infrastructure, has to operate with potentially many different types of modules (instrumentation or switching) in the same chassis. The chassis design has to meet the expectations of all PXI vendors, which requires the defined minimum requirements set out in the PXI standard.

The PXI chassis choice can get a little more complex since the PXI standard has evolved into two distinct backplane control interfaces, PCI and PCIe, which are not interchangeable. PXI chassis provide slots that support PCI-based modules, PXIe chassis can provide slots that are either exclusively for PCIe-based modules or support both PCIe and PCI-based modules (but not at the same time) and PCI based only slots. The increased complexity of the PXIe chassis means they are more expensive to manufacture when they support both PCIe and PCI interfaces. Therefore you need to carefully consider all options when choosing their chassis to ensure that all slots can be used. Still, otherwise, inter-changeability is assured mainly by compliance with the standard. Care should be taken to ensure that the modules you require are available in the interface available on the chassis backplane; the vast majority have PCI interfaces rather than PCIe interferences.

For LXI, since there are no chassis constraints to concern you, simply complying with the minimum set of requirements set out in the standard is enough unless specific support is required for the optional extended features, such as IEEE1588 timing.

Applying to Switching

For switching, speed differences are of little consequence because, in practice, the speed of change of switching systems is constrained by mechanical components (relays)—even solid state switching does not need the multi-gigabit speeds of PXIe. The mechanical and electrical constraints of PXI can influence what can be cost-effectively supported in PXI. Still, on the other hand, the overheads of an LXI device can limit the minimum functionality that can be cost-effectively supported.

As ever, one standard does not fit all. LXI offers the greatest design freedom, while PXI provides a means to integrate small modules into an open standard chassis. Not surprisingly, that means both have their place, and both standards can claim circumstances under which they might perform better than the other for switching and other applications.

In our case, we have an extensive range of PXI switching products developed over the years. Many users have found that PXI is the natural answer to their switching needs, and it forms most of our switching business. 

There is a clear distinction based on size. Where, for example, a large structured matrix is required, LXI will often be the better choice. We offer a range of large LXI switching matrices for this application. PXI can be used to implement these large systems, but the PXI chassis cost, cable interconnect cost and software complication of implementing a large switching function over many modules increases the system cost, decreases its performance and adds complexity to the system control and programming time. A matrix created from multiple PXI modules that are cable connected is controlled as separate modules, not as a single matrix.

LXI products like our 60-554 integrate multiple sub-assemblies into a single matrix in preference to using separate matrices with connecting cables with the user having to control each sub-matrix independently.

Multi-slot PXI Switching Modules