New LXI standards in development: It's about your time
July 01, 2008
The Consortium is now witnessing the emergence of Class B instruments.
The LXI standard identifies three functional certification classes: Class C, Class B, and Class A. The class succession from C to A provides progressively more functionality and gives instrument designers the ability to incorporate the precise functionality required for their instruments and intended applications.
Classes of LXI products
Functional Class C LXI Devices provide a standardized LAN and Web browser interface conformant with the LXI standard. These devices, which are not required to support either the wired trigger or IEEE 1588 timing aspects, are particularly suited to applications where non-LXI products have been adapted to the standard. This class includes physically small products (such as sensors) that use battery power or Power over Ethernet and devices with key attributes including a simple architecture, low cost, and small size.
Functional Class B LXI Devices provide a standardized LAN interface, synchronization API, and IEEE 1588 timing support. The Class B interface allows devices to execute triggered functions equivalent to those available over the General-Purpose Interface Bus (GPIB) with similar or better timing accuracy.
Functional Class A LXI Devices provide a standardized LAN interface, synchronization API, IEEE 1588 operation, and a wired trigger bus interface. The wired trigger bus provides a standardized capability for supporting trigger events between devices whose timing accuracy is limited by cables and LXI Device hardware. The trigger functionality is broadly equivalent to the backplane triggers of modular instruments in card cages (though cable lengths might be longer than backplane trigger lengths) and the ad hoc point-to-point trigger systems used on bench instruments.
Since the release of the LXI standard, most instruments have been certified as Class C. These instruments incorporate the key features attributed to LXI-compliant instruments, built-in Web servers so that users can monitor and control the system using Ethernet, and any standard Web browser. All major instrument functionalities are accessible through the Web server, which simplifies software development, system commissioning, troubleshooting, and maintenance. Some of these Class C instruments include features associated with Class B or Class A LXI Devices but not the complete set of features required to declare class compliance.
Web services are used to gain access to LXI Devices and achieve immediate control over their configuration, making them easy to set up, configure, and debug. These LXI Devices allow users to build quick and easy-to-manage test systems that can be remotely controlled, as shown in Figure 1.
Class B to the front
The Consortium is now witnessing the emergence of Class B instruments. Class B instruments provide LAN and Web server functionality with the addition of a synchronization API and support for the IEEE 1588 precision time protocol. These features allow devices to execute trigger functions and, for the first time, standardize a method for executing triggers in a system based on knowledge of common system time reference to an accuracy not achievable before the advent of IEEE 1588.
Class B LXI Devices provide timing and synchronization between the various components based on a common reference clock without requiring a controller (PC) to be the origin of time information. The IEEE 1588 scheme ensures that instruments agree on the correct current time, leveraging a master clock located within the system. Even if the time "known" by the system is wrong relative to the physical world, instruments still agree on the timing and perform the various functions required in the correct sequence at the correct interval.
In addition to this feature, the LXI standard introduces time-based triggers. Prior to LXI, test and measurement instruments typically relied on PCs and an active Internet connection for the time check, which created opportunities for inaccuracy. IEEE 1588 timing is more accurate and consistent and can avoid the message-based triggers issued from a controller at time-sensitive moments. This controller and its operating system can cause variable delay in the trigger because of latency and variable delays. Windows-based controllers commonly cause several milliseconds of variable delay. Though Real-Time Operating Systems (RTOSs) can improve timing variability by limiting latency, they still introduce uncertainty, and delays are still present.
LXI Devices can circumvent network and controller latency. A typical local LXI test system with IEEE 1588 capability using a hardware PHY is accurate to approximately 50 nanoseconds. The test system has access to additional capabilities including peer-to-peer triggering, which enables the instrument to respond to an instruction from another instrument, also circumventing any controllers or other components in the system.
Certifying a product
To certify an LXI product, the manufacturer must be a member of the LXI Consortium, which ensures access to the documentation and support needed to guarantee the device meets all the requirements for conformance. The documents include forms that must be filled out and submitted to the Conformance Working Group. The Consortium also supplies a suite of conformance pre-test software that allows manufacturers to check if products conform to the LXI standard.
Members ready to submit instruments for testing can attend a PlugFest or hire one of several approved private testing services. Either way, a representative from the manufacturer who is familiar with the instrument must be present during testing. Test results are forwarded to the LXI Conformance Working Group Chairperson, who then forwards the application to the Board of Directors with a recommendation. After the Board votes, the Consortium notifies the manufacturer and, if the application is denied, explains the reasons why and recommends ways to resolve the issues.
During a PlugFest, the Consortium pays an independent lab to perform conformance testing on members‚Äô early designs. If problems are found, experts are available to help vendors improve their implementations. Applications and tutorial sessions at PlugFests help new members and integrators become familiar with LXI.
LAN is ubiquitous, and more test systems are using it. LXI puts the power of Ethernet and the Web inside those systems, allowing users to create the system needed today and quickly move on to the next one. Figure 2 shows the many application options for LXI.
The next revision (1.3) will incorporate IEEE 1588-2008 into the LXI standard. Future revisions will concentrate on enhancing how LXI products communicate directly with each other and the user. New feature sets will include:
- Improved Web support for trigger operations, allowing users to initiate and test trigger operation through Web pages without using the programmatic interface
- Resource management, which allows users to more easily manage multicontroller systems
- State management, which handles common functions like store and recall
- Event log and schema, which improve how LXI devices log events and report them so that logs from different devices can be combined more easily
- Peer-to-peer communication that does not rely on a controller
This work will likely take a year to prototype, test, and document before it is included in Revision 2. In the next six months, the Consortium will take major steps toward improving compliance test procedures for the standard.
More member companies are commenting on how customers will not accept a non-LXI version of an Ethernet-enabled product, insisting instead on an LXI version. This is helping boost LXI instrument sales and encouraging new vendors to adopt the standard.
Furthermore, many customers are insisting on LXI as more systems are developed with LXI compliance as a key component. LXI-enabled instrument sales are increasing rapidly, making LXI the fastest adoption ramp of any I/O architecture. While the numbers are still small as a percentage of the overall multibillion dollar test and measurement industry, this is largely dictated by the amount of legacy equipment available and the long lead times for military programs. Nevertheless, LXI system sales are rising rapidly as more products are being introduced, and the Consortium expects that growth to continue.
Face to face in Toronto
This year‚Äôs PlugFest held May 21-23 in Toronto concentrated on testing the latest version of IEEE 1588-2008, slated to be incorporated in the specification later this year. During this PlugFest, the Consortium developed formal test procedures and tested proposed rules that expand standards coverage for system-level aspects of LXI test systems. These issues included resource and state management in a multicontroller environment, event logs, and peer-to-peer communication issues. Face-to-face meetings with experts in each area complemented the test work.
Also in May, the Consortium hosted its first LXI Day open to the general test and measurement community. This special event provided working LXI product demonstrations and showcased technical application presentations that explained LXI‚Äôs benefits, its importance to the future of test and measurement, and ways it can be implemented in various applications.
The PlugFest held in Newport Beach earlier this year made progress in testing IEEE 1588 and starting the process to incorporate 1588-2008 into the standard.
The LXI Consortium has an active Development Group that reviews developing LXI products. The Consortium invites speakers to address the group, which includes many vendors who are not members of the Consortium, to provide those defining the LXI standard with comprehensive, objective viewpoints. As with any network, the value of the network increases with the number of nodes. LXI recognizes this and invests resources in helping members, integrators, and product vendors ascend the learning curve.
The LXI Consortium provides a venue for LXI vendors to work together, even though the companies involved might be competitors. This benefits end users by allowing systems integrators, engineers, and developers to influence the standard‚Äôs development. LXI is a true multivendor standard in the sense that a variety of vendors are motivated to grow the standard for the test and measurement industry.