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 >>
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.
|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
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
with connecting cables with the user having to control each sub-matrix independently.
Multi-slot PXI Switching Modules
In PXI, some of these issues can be partly addressed by using modules that extend over more than one slot, an example being our BRIC™ large PXI matrix. These products integrate the switching system over 2, 4, 8 and 12 PXI slots using only one connection to the backplane. They then use their backplane to provide connectivity between the sub-assemblies in the BRIC. This reduces cabling and interfacing costs that would otherwise be incurred and make a PXI solution much more competitive, though still not as competitive as can be created in LXI platforms with even larger switching systems.
More information on BRIC PXI matrix designs can be found here, and general topics about the BRICs can be found here.
PXI modules in an LXI Chassis
Our PXI modules do not have to be confined to a PXI chassis. PXI designs may suit your application for those with diverse switching needs, but you do not necessarily want to spread your computer PCI bus to external modules. For example, you may need to have a longer distance between the controller and the switching system chassis, or maybe you want to get a more web-like feel to the system for remote access or have the ability to make changes without powering down the system controller.
40-923A PXI modular and 60-103B LXI modular chassis.
Both of these chassis host PXI modules, the PXI chassis (left) shows a remote control interface fitted for connection to the PC1 bus in the left slots, and the LXI chassis (right) shows our proprietary LXI controller fitted, which provides control of the chassis and the modules its hosts.
We have introduced our LXI chassis for these applications that
support our PXI modules. The restriction to our modules is a software issue rather than a hardware
issue. The LXI chassis has an embedded Linux-based controller that requires access to source code for
the modules supported. The same controller gives the user a barrier between the PXI modules and the system
controller that gives it all the features of Ethernet connectivity, including web-based management of
the PXI switching modules.
Proprietary Modularity in LXI
The LXI standard does not disallow modularity; it is simply not a requirement of the standard. So vendors can also create proprietary modular formats to solve particular problems. We offer a couple of different options that are examples of this.
65-110 wideband modular LXI chassis and 65-200 modular LXI chassis
Our 65-110 wideband modular LXI chassis solves a user requirement for a very broadband (usable to 500MHz) matrix with excellent crosstalk ( see our application story, 65-110 for CERN). The chassis provides a proprietary cable-less interconnect between modules, with each plugin module having a much greater footprint than PXI.
The other is our 65-200 modular LXI chassis, which offers you a solution that can be easily re-configured by the insertion and removal of up to six LXI matrix plug-in modules. We also provide a solution that includes this chassis with some high-density Reed Relay plug-in matrix modules that provide access to all signal connections on 200-pin connectors. It can support matrices with a Y-axis size of 4 and is expandable in the X-axis range up to 1,536 in increments of 128. Another essential feature is that over 1,500 relays can be closed simultaneously for specific conditions for parametric testing.
There is no "best " platform for signal switching in automated test systems; the LXI and PXI standards bring different advantages and disadvantages depending on what you may be trying to do. In this respect, instruments are also not that different—there are always examples of where PXI and LXI implementations will have various compromises on cost, performance and size. Vendor-designed enclosures will usually convey performance advantages to LXI solutions; a modular chassis system can offer benefits in cost and size for diverse systems, particularly where the functions required are diverse and high levels of multi-vendor integration are required.
So there is a broad spectrum of possibilities open to you. For companies like us who embrace both the LXI and PXI standards for our core products, it is very much a question of letting you, the user, decide the most appropriate solution for your application rather than forcing you to use a particular platform.
That is the nature of standards; they use a set of assumptions when they are created that have consequences when products are made that use them. For our company, those consequences of switching systems create different ways of tackling problems that depend on component size, switch system size and switching system diversity. That is why we embrace both the LXI and PXI standards.