New LXI products and capabilities unleash the power of Ethernet (Guest Commentary)
Bob Rennard, LXI Consortium -- Test & Measurement World, 3/12/2008 11:56:00 AM
The recent LXI PlugFest, hosted by VXI Technology in Irvine, CA, demonstrated how LXI and network technologies are changing test and measurement. With nearly 500 products certified as LXI-compliant, and a growing list of Class B and Class A products on the market and under development, the meeting was alive with application stories demonstrating impressive systems results through the application of LXI. The power of LXI really becomes clear with Class B- or Class A-compliant devices.
As easy as configuring a printer
In today’s fast paced, competitive environment, nobody has time to waste on unnecessary system setup tasks. The cost of configuring a test system today can easily rival the cost of several instruments, making fast configuration more important than ever. LXI addresses this with impressive instrument discovery capabilities that enable instruments to configure themselves by pulling necessary information from the instrument’s web server or vendor site. Engineers associated with LXI believe setting up instrument systems should be as easy as connecting a printer to a PC, so they applied open-source software, such as the familiar Zero Configuration Networking (Zeroconf) discovery protocol or Apple’s open-source Bonjour, to LXI test instrumentation. The result is simpler system integration, lower integration costs, and faster first measurement.
At the recent LXI PlugFest this was demonstrated in the multi-vendor test system, hosted by The MathWorks, where instruments from a diverse group of vendors were assembled, self configured, and up and running within minutes with minimal human interaction.
No more WAIT statements
Test speed is the greatest imperative in instruments. We all talk about test speed and the need to optimize throughput, but how many times do we use WAIT statements to just get the system to work? Nearly every test system uses WAIT statements to deal with timing irregularities, but they are typically hand tuned and far from optimal. For example, we use WAIT statements to compensate for settling times or device interactions, often arbitrarily dropping values in until the system works. Rarely do we go back to optimize system performance or check whether that 50-ms WAIT statement could have been cut to 5 ms. Time-aware instruments and systems give integrators unprecedented transparency for optimization and troubleshooting. Today, when a new test system fails to yield the desired results, we have few clues to guide us. How do we know whether a switch was in the wrong position, a source was settling, or the DUT was not producing the expected signal? Time aware instruments produce an event log with time-stamped events, allowing the integrator to reconstruct a time sequence to aid troubleshooting. Event logs also aid optimization by identifying idle time, settling times, and delays where parallel operations can occur.
The value of time-aware instruments can be seen in an example of a common stimulus-response measurement where a source steps through a series of values and the analyzer records the results, triggering the source to increment after each acquisition. This may be an RF application using a source and spectrum analyzer or it could be a low frequency application using an ARB and DMM. In a typical system, the source steps to the desired value and then sends a trigger to the analyzer which sets up to receive the expected signal. The analyzer receives the signal, processes it to make a decision, and sends a trigger back to the source to move to the next step. The process is repeated as it steps through the series of values. In this case, the source setup and trigger, the analyzer trigger receipt, setup, acquisition, analysis, and step trigger are done serially. If we know, directly or through logs, how long each step takes, we can use time triggers to overlap operations. For example, we might increment the source while the analyzer is processing data from the last measurement, saving setup delays and completely eliminating over-the-wire transit latencies.
Eliminating the PC bottleneck
In most test systems today, the PC acts as a traffic cop, monitoring and directing traffic to instruments on the system bus. The problem with this model is that it creates a lot of unnecessary traffic over the IO channels and consumes PC performance with low value tasks. Further, the PC becomes a bottleneck to system performance, often compounded by unnecessary delays from Windows’ inability to prioritize tasks. The computer world proved that decentralized computation is key to improved system performance because the host computer is freed from the critical path. LXI enables this with downloadable scripts, state recall, and peer-to-peer communication.
Studies of test system I/O traffic, whether over GPIB, MXI, or LAN, show most of the bandwidth is consumed by small, chatty commands between instruments. For example, when one instrument measures a value that triggers action from another instrument, the controller wastes time and bandwidth passing that data. LXI recommends, and Class B and Class A require, peer-to-peer communications to reduce unnecessary controller traffic. Another source of controller congestion comes from instrument setup. In a typical SCPI environment, chatty controller-to-instrument communications set up instrument states or execute test sequences.
LXI Class B and Class A instruments require the ability to accept downloadable scripts and recall states to reduce controller-to-instrument traffic, cut latency, and speed system performance. While these techniques have been around for years and have a solid record speeding test system performance, the advent of LXI with networked peer-to-peer communications significantly enhances their capabilities. By downloading scripts, also known as macros, subroutines, and applications to the instrument, long command strings are reduced to a single trigger command, which reduces I/O traffic and PC loading. Similarly, state management allows integrators to recall a known instrument state rather than rebuilding it each time it is needed. Going forward, LXI engineers will augment the utility of state recall by defining a fingerprint or hash that tells an application whether the instrument has changed from its last known state. If the instrument has not changed, there is no need to rebuild the state, which further reduces I/O and controller traffic.
Already the fastest growing T&M I/O interface in history, as measured by both product availability and shipments, LXI benefits are accelerating as new Class B and Class A instruments become available. In addition to fast setup, which is enabled by tools developed by the PC industry for printers and other peripherals, time aware instruments can speed troubleshooting and optimize system performance with the unprecedented transparency offered by time stamps and event logs. Eliminating controller bottlenecks with peer-to-peer communication, downloadable scripts, and state recall capabilities accelerates system operation. These capabilities, coupled with familiar benefits such as low cost commercial Ethernet infrastructure, unlimited nodes and distance, and web page interfaces are reinforcing LXI’s lead as the future of test.
Bob Rennard is president of the LXI Consortium.
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