www.ck365.cn From:LXI Alliance Chairman Bob Renn Time:2006-5-28
LXI incorporates a number of achievements of the test and measurement industry in recent years: The first is a clear shift from proprietary technology centered on T&M to the computer industry. The first is a clear shift from T&M-centered proprietary technology to computer industry technology, with increasing use of Ethernet, PCs, and PC-standard software in test applications. Ethernet has become increasingly popular among test and measurement users because of its low cost, adaptability, high-bandwidth LAN interface, and increasingly powerful discovery and network management capabilities; the second is the publication of IEEE 1588, which introduced deterministic timing synchronization to the LAN, and has become commonplace in the telecommunications and industrial automation industries. The second is the release of IEEE 1588, which introduces deterministic timing synchronization to the LAN, a standard that has been widely embraced by the telecommunications and industrial automation industries, and the nanosecond timing capability of IEEE 1588, which adapts well to T&M applications; and the third is the fact that users have come to recognize the value of the network interface, which is now so popular that users can use it to access instrumentation and application information, and to enjoy the instrumentation screen with their colleagues around the world ****.
Identifying real-world needs
Before the release of LXI, the creators met with a number of system integrators and users to hear their positive and negative opinions of the current offering, and the results were not surprising. System integrators and test users made it clear that they were looking for simpler methods of system integration and lower overall costs. They like the size, convenience and neatness of card-box instruments, but also need the versatility and high performance of benchtop instruments. The main reasons people are buying PXIs are compact size and ease of integration; it's easy to change plug-in cards by pulling out the old one and plugging in the new one, and there are no cables or complicated connections. These users also want multiple high-speed triggers. Most card-box instrument users don't like the large and expensive chassis, they don't like the outdated and expensive zero-slot controllers, and finally, they find that card-box instruments have very limited error-checking capabilities.
System integrators find benchtop instruments to be easy to use, high performing, inexpensive, and very adaptable. Benchtop instrument features include a user-friendly interface that minimizes the need to transmit large data streams through distributed processing within the system. But system integrators also point to the large size of benchtop instruments, excessive functionality beyond what is needed, and the need for messy cable connections between instruments.
Test system developers, on the other hand, want a simpler approach to integration. Their ideal is to have open systems that can use product-based software to connect to popular applications and reporting tools like Excel and browsers, so it's easy to set up and check for errors. They want to mix instruments from different vendors without knowing the manufacturer of the hardware, and they also want to use low-cost PC industry IO rather than expensive T&M interfaces like GPIB and MXI.
LAN-based instruments and modules are already on the market. The overall response has been very positive, with all users welcoming it, and many saying they have been looking forward to it for a long time.
Setting the Direction
The founders of the LXI Alliance also considered many of the computer industry's IOs, especially LAN, to provide the open interfaces that unite thousands of engineers innovating in Ethernet technology. And there are far more engineers working on Ethernet every day than there are engineers in the entire T&M industry. Ethernet is everywhere, so to speak, and the cost of a physical connection is much lower compared to MXI and GPIB in T&M. Ethernet allows for a wide range of communication media - - Coaxial cable, Cat-5, fiber optic, and wireless - it's fast and OS-independent and symmetrical - without the master-slave mode of operation found in USB. The biggest need in the T&M industry is stability. Ethernet has been around as long as GPIB, but speeds have increased three orders of magnitude from the days when it was first used and show no signs of slowing down. ethernet was accepted as IEEE 802.3 in 1985, and practice has shown that it has been stable for the past 20 years or so and will continue to be so for many more years. ethernet provides the stability that T&M engineers need; it is faster and more symmetrical than other technologies. Ethernet provides the stability that T&M engineers need; it is also inexpensive, stable, ubiquitous, and easy to use compared to other card-box systems.
Figure 1 Measurement Bus Evolution
Defining LXI
LXI basically replaces GPIB and MXI with LAN. simply replacing the GPIB and MXI cables with CAT-5 allows for lower price and greater convenience, as well as taking advantage of the benefits of LAN. Over the past 20 years, Ethernet has expanded its capabilities far beyond T&M interfaces such as GPIB and MXI. Ethernet has evolved from point-to-point technology for connecting PCs and printers to powerful peer-to-peer technology using DHCP addressing, with network management capabilities and diagnostics available at no cost. The LXI Alliance standard is essentially a best practice for LAN implementation in test and measurement.
The LXI standard will ensure that all LXI-compliant devices can exist on the same network as equals and are vendor-independent.
The LXI Rules and Recommendations require all LXI devices to support IEEE 802.3 and TCP/IP, providing a consistent user experience that is easy to use. For example, since most networks have DHCP servers, LXI eliminates the need for cumbersome and error-prone manual IP address assignments (although static IP addressing is supported if needed or requested by the user). Similarly, by defining peer-to-peer messaging, triggering, and scripting capabilities, LXI provides system integrators and test developers with new architectural choices so that legacy controllers can be dispensed with, as well as inefficient instrument-controller traffic. LXI's scripting handles instrumentation runs much faster and with a much simpler program than many controller-based systems.
Users also told consortium members that they were frustrated by how expensive and poorly adapted card-box instruments were, and that they couldn't find the features and performance they needed, especially for high-performance RF instruments. In an effort to improve the fit and performance of instruments while lowering the selling price, the LXI Alliance has minimized unnecessary mechanical devices, allowing vendors to leverage their R&D investment in one form of construction to another. The test and measurement industry invests hundreds of millions of dollars in R&D each year - this step by LXI will allow that investment to be shifted to modular system components in this way. LXI eliminates the structural constraints of card-box systems that have always forced vendors to specialize in plug-in cards rather than borrow boards, components, and technology from larger, desktop instruments of the same type.
Each LXI device must support an HTTP server that provides a Web page for host operation and control, which works with any WC-3-compliant browser. Because modular instruments do not have front panels or displays, a tightly defined web interface is essential for system-ready LXI devices. Because it allows for extended diagnostics and application information in addition to providing a robust user interface, Alliance members report that the web interface is very popular among customers and system integrators. With the web interface, asset management is simplified by the network's ability to query the firmware version and calibration date of an instrument or module. The final point is that web interfaces enable engineers to work with colleagues in other locations.
One of the features of LXI devices is a consistent API, which is helpful for system development engineers who need to integrate devices from multiple vendors. Writing an entire test program from a single "syntax" will greatly simplify the programming effort for most test systems, and the LXI programming interface supports the work of the IVI Foundation, which promises to follow sound programming practices and minimize resource constraints. Most system developers are familiar with the IVI interface. It is, of course, up to the company to include any other drivers or APIs to meet specific market needs.
A question needs to be asked about IVI - why not introduce over-the-wire protocols, simple C scripts, or WebServices, since simple scripted drivers have certain requirements. Because simple script drivers have certain requirements, IVI needs to ensure that drivers do not compete with resources or interfere with each other for compatibility.
ebServices is attractive and I believe we will move to it in due course. And at the moment, we don't think it's mature and stable enough, including requirements.
Triggering and synchronization
An important feature of the LXI specification is the public **** trigger definition. The ability of an LXI device to send a trigger to one or more other LXI devices on the LAN provides synchronization capabilities similar to triggering, but without the wires.The LXI specification defines a unified trigger model that increases the availability of LAN triggering by allowing actions to be initialized by one of several different trigger events, and also reduces the integration effort. Of course, this unified trigger model becomes even more powerful when combined with the embedded scripting and peering capabilities described above. For applications that require the low jitter characteristics of traditional trigger lines, LXI has defined a public*** hardware trigger bus trigger to simplify the system integrator's task. The trigger bus is a 25-pin, 100MHz differential LVDS interface. It has eight independently assignable channels that can form a star or daisy-chain configuration. The Trigger Bus specifies the basic requirements, and a variety of other connectors are available on the module, depending on the specific application or market requirements.
Finally, one of the most important features of the LXI specification is IEEE 1588 synchronization, which allows different devices on a LAN to automatically and transparently synchronize their system clocks to a highly accurate clock; 100ns accuracy is common, and 10ns or more can be achieved in the lab. Synchronization using IEEE 1588 does not require any user intervention. When IEEE 1588 was defined as a standard for synchronized clocks, it did not specify how to use clocks that had already been synchronized; the LXI standard solves this problem with its LAN-based and time-based triggering feature, which specifies how to apply synchronized timing to real-world test and measurement applications. With LXI, IEEE 1588 provides deterministic timing that is passed back and has been lost to the LAN. It also allows different devices in a test system to run complex sequences of events autonomously without the intervention of the system controller. In practice, it also allows data to be time-stamped to simplify synchronization issues, a capability especially needed for high channel counts and distributed systems. It also allows deterministic triggering via software at the LAN port to simplify cable and instrument upgrades.
Three Types of Instruments
LXI defines three basic device types that differ primarily in their triggering capabilities. LXI was originally envisioned as a modular specification, using 1/2 rack width and 1U or 2U height. Discussions with system integrators soon turned into a consistent, LAN-based instrument specification that goes well beyond these modules. Full-size benchtop instruments, for example, benefit from the consistent LAN, network, and trigger definitions in the LXI specification. Similarly, many data acquisition and sensor applications require devices much smaller than any benchtop instrument, which can utilize Power over Ethernet (POE), and cables that are simplified to a single Ethernet connection. To accommodate the variety of real-world usage models described by system integrators, the LXI specification specifies three basic device types: Class A, B, and C. The LXI specification is based on the LXI standard, and the LXI specification is based on the LXI standard.
Class C is the base class, a consistent LAN implementation that allows LAN to be attached to instruments and modules, ensuring good compatibility with other vendors' LXI products. class C defines the network interface UI and IVI APIs that are common to all LXI devices. products that meet this baseline are labeled with the LXI logo. class B includes all of the capabilities of class C plus the IEEE 1588 timing standard. IEEE 1588 timing standards. class A has a hardware trigger bus in addition to all the capabilities of classes B and C. Of course, any device can incorporate more than just the IEEE 1588 timing standards. Of course, any device can add more capabilities to its base type.
Summary
Looking ahead to the future of test systems, Ethernet will be a major player, and so will LXI. The reality of the industry is that systems will continue for many years, and integrators and system developers will see what are called "hybrid systems". Benchtop, VXI, LXI, and PXI will ****exist in systems that are typically just a node on the LAN. the LXI specification defines scalable features and performance, allowing customers to use full-featured benchtop instruments in the development and design verification phases, and modular instruments in space-constrained manufacturing environments and applications, where the software developed during the design phase can be ported throughout the process.
LAN-based systems are easier to configure and troubleshoot, using simpler cabling and auto-discovery and addressing in the same way as for printers and other peripherals.LXI also leverages Ethernet's many diagnostic network management tools, providing unprecedented capabilities for T&M users. To learn more, visit the LXI Web site at www.lxistandard.org.