By Richard A. Quinnell, Contributing Editor
WHAT IS LXI?
When you want to coordinate the operation of an array of test instruments, they need a standard means of inter-communicating. In the past that link has been the General Purpose Interface Bus (GPIB), but GPIB performance lags behind today's test technology. The technology targeting the replacement of GPIB is the Local Area Network (LAN) Extensions for Instrumentation (LXI).
LXI leverages the ubiquitous Ethernet networking standard and augments it for the needs of test and measurement. The approach offers many immediate improvements over GPIB. For one, using Ethernet eliminates the need for a special system controller like the GPIB Slot 0 device. A standard PC, nearly all of which now come with Ethernet builtin, serves as a remote interface to every instrument on the network. Once configured, the network works without the need for centralized control and allows peer-to-peer communications rather than needing to pass data through a central clearinghouse.
Ethernet also provides performance advantages over GPIB. Today's Gigabit Ethernet LANs run at least 125x faster than GPIB, provide automatic discovery of instruments on the network, have no restrictions on the number of nodes that can be connected, and allow instruments to be separated by virtually any distance. This separation can be across a factory floor or across the world. As long as equipment can communicate across a network, it can form a coordinated test system.
IEEE 1588 TIMING AND SYNCHRONIZATION
In order to provide coordinated and precision timing for instruments operating across the non-deterministic Ethernet, the LXI specification applies the IEEE 1588 Clock Synchronization Protocol. The protocol standard, published in 2002, provides a masterslave mechanism for ensuring that clocks scattered across a network are all synchronized to a common time base. A grandmaster clock provides the common time base for the network. The specific performance characteristics of each module's clock, however, can vary with the type of clock and timing control it offers.
Clock performance has three key parameters: precision, accuracy, and epoch. Precision measures the variations in clock time as compared to the grandmaster reference, and is a function of each individual slave clock's precision (Figure 1) as well as the precision that the network is able to achieve. Accuracy measures the system clock's deviation from true time, such as by comparing the system clock's second to the international standard for a second, and the grandmaster clock's accuracy controls all system clock accuracies. Epoch, the definition of a reference point in time, is also set by the grandmaster clock.
For most test and measurement systems, which typically utilize only relative time, precision and accuracy are the key parameters. Epoch becomes important when events or data must be linked with coordinated universal time (UTC), such as for making correlations with other events or archiving data for later analysis. In such applications, designers can synchronize the grandmaster clock to a recognized UTC-traceable source such as a Global Positioning Satellite (GPS) system or a Network Timing Protocol (NTP) timeserver.
The grandmaster clock solely determines system clock accuracy, but clock precision is still a factor that network and module designers can influence. One of the key elements to control is module placement. The IEEE 1588 protocol automatically establishes its master-slave hierarchy, resulting in some clocks being slaved to the grandmaster clock then becoming masters to clocks further down in the network. Understanding the protocol will allow system developers to position clocks to prevent low-precision clocks from becoming high-level masters, thus eroding the accuracy of other clocks in the system.
A tree structure, such as shown in Figure 2, can serve to isolate low-precision clocks in their own trees, preventing their contamination of other system clocks. Implementing this structure, however, requires the use of network switches. Switches introduce timing fluctuations into the protocol's communications efforts, which can introduce errors as large as 400 nanoseconds in system clock precision and accuracy. The IEEE 1588 standard addresses this problem by defining switch modifications that allow creation of boundary clocks to preserve timing accuracy across the switch. Such IEEE 1588-enabled switches have started becoming available.
Compliance with the specification occurs at several levels. A Class C device follows the LAN interface specifications,-discovery behavior, the Web interface, and the physical requirements of the LXI standard. Class B devices also implement the IEEE 1588 protocol, which allows time and frequency synchronization of instruments on the network. The Class A devices add a standard hardware-based triggering bus to the Class B functions. This multi-layer approach allows the creation of hybrid systems that include instruments based on other structures, such as PXI and GPIB, through the use of bridges and adapters. The LXI standard defines adapters as providing data and control communications between structures while making the foreign structure look and behave as an LXI device. Bridges link structures without requiring LXI behavior from the foreign structure.
At its most basic, the LXI standard--administered by the LXI Consortium--defines a series of small, modular instruments that uses Ethernet as the system backbone. The interface specifications describe an interoperable implementation for the Ethernet portal that vendors can build into their instruments. This description includes: 1) content and layout guidelines for web-based user interfaces; 2) requirements for IVI (Interchangeable Virtual Instrumentation) drivers and programming interfaces; and 3) specifications for networking behaviors such as addressing and error responses. The standard's physical specifications define the physical characteristics that allow LXI modules to provide the size and integration advantages of modular instruments without the need for a card cage. The standard defines such things as module sizes and dimensions for rack-mounting, module cooling methods, and the type and placement of power and I/O connections.
In addition to specifying the configuration and data exchange operations of instruments on the Ethernet backbone, the LXI standard defines timing and triggering methods. To offer coordinated, precision timing of sampling and events, LXI modules can use the IEEE 1588 timing protocol to synchronize their clocks. For triggering, modules can exchange software-based triggering commands, use scheduled triggering, or hardware triggering. The standard defines an 8-channel hardware trigger bus that uses differential signaling and operates in either a driven or wired-OR mode. The driven mode is similar to the PXI trigger bus, and the wired-OR mode is similar to VXI.
One of the advantages for both vendors and users is that the LXI standard does not impose any restrictions on the instrument's behavior other than its web and LAN interface. This allows vendors to create bench-top instruments and then repackage the same design to conform to the LXI physical specifications.
STATUS OF LXI Products and Specifications
The LXI Consortium now has nearly 50 member companies offering more than 100 instruments that conform to the standards. The specification was released as version 1.0 in 2005, and ratification of version 1.1—which makes some clarifications and language changes—is expected in mid-2006. Work is also underway to define version 2.0, which will include enhanced security features as well as improved discovery behavior. Six working groups are making continual refinements to the specifications in the areas of LAN interfaces, Web interfaces, driver behavior, physical considerations, trigger and synchronization capabilities, and hardware-based triggering. Up-to-date information can be found at the LXI Consortium web page, www.lxistandard.org.
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