subject: Lxi Vs. Pxi In Switching Applications Part 1 [print this page] A switching system is the glue that connects a UUT to a systems test equipment. But often, it gets overlooked in the overall system design process. Switching hardware may involve more potential solutions and compromises than any other part of the test setup.
The switching system often is the part of the test system that challenges engineers to do what they have to do well: Evaluate the options and come up with the most effective strategy that permits making meaningful measurements to show product compliance without spending more capital or labor in developing it than required.
Switches are a classic example of the need to make engineering compromises, including density, voltage and current ratings, path loss, bandwidth, cost, and configuration choices. Every aspect of the decisions made regarding the switching system has an influence on the test system.
One of the first choices the engineer typically makes is the platform to control the switching. Although this is not the first choice by necessity, for some it helps simplify product choices.
Pickering Interfaces supports the two most common platforms for switching systems: PCI eXtensions for Instrumentation (PXI) and LAN eXtensions for Instrumentation (LXI). There are more common reasons for selecting one of these standards over the other. While there is no single answer to every switching need, there are questions to ask once you understand the options.
Comparing the Platforms
The PXI modular platform has been around for more than a decade. Customers frequently ask which platform is best. There is only one answer: It depends.
The two platforms are simply different. The promoters of the two standards used different assumptions and starting points in creating the standards, but both have their place in the test and measurement industry.
PXI Standard
The PXI Standard describes a mechanically modular platform that uses the PCI bus, both standard and PCI-e, on a defined backplane to control the modules. The modules have a fixed mechanical form factor with the most common, the 3U.
The PXI chassis provides power from a common power supply with a baseline standard of performance. It has shared cooling and a backplane trigger bus system to exchange triggers between modules, a feature switching applications rarely need to exploit because due to slow operation of relays.
A PXI-based system generally is Windows-centric. Use of other operating systems is possible, but the PXI specification only defines WIN32 drivers. Depending on the vendor, this may or may not be possible.
Most switching modules usually are simple register-based designs with little built-in intelligence. The control functions are dictated from the single central controller where all the software resides. This centralized architecture requires a fast backplane interconnection system to ensure that communications bandwidth does not slow down the central controller.
The LXI Standard
LXI products are designed to be linked via Ethernet connections. There are no constraints on the physical size of these devices. Although most are designed to be mounted in racks, some may use a smaller format.
Each LXI device has its own embedded controller, power supply, and cooling system. The LXI standard defines the way an LXI device behaves on the network so all LXI devices share a consistent look and feel. The Web server built into each device must provide at least a minimum set of configuration management support capabilities. The standard also requires LXI devices to be good neighbors in the presence of LXI devices and non-LXI devices from other vendors.
LXI systems can be small or very large. Their Ethernet connectivity permits the components of LXI-based systems to be widely distributed even over intercontinental distances. The IEEE 1588 timing protocol provides a common framework for systems in which timing synchronization is important.
Triggers can be LAN-based or the system can use the wired trigger bus. Triggers can be controller-centric or operate peer-to-peer. Communications between LXI devices take place via a fast interconnect sucha s Ethernet, but in most applications, the distributed nature of the data processing means that a fast interconnect is not essential to ensure faster operation.
PXI vs. LXI
The two standards use a different control model so they have different strengths and weaknesses. PXI focuses on a central processing model based on a PC, using products that are modular and reliant on a high-speed data bus for communicating with the central processor that provides the computing power needed to perform system functions. LXI uses a more distributed control and data-processing system that communicates via a reasonably fast interfacea model that has some similarities to GPIB.
For switching, speed is rarely an issue. Mechanical relays may take hundreds of microseconds to operate or they can take many tens of milliseconds. When designing new modules for specific applications, the platform architecture chosen can impact switch system design.
For example, the physical constraints can make a difference. Modular structures are great for supporting relatively small switching systems or systems that require a broad mix of switch types. But for high-density requirements, even the cleverest design engineer will run out of space. At the other extreme, if a small and diverse switching system is needed, the controller and supporting hardware in LXI could be costly.
