LXI triggering
You can choose the LXI instrument class that provides the triggering speed and synchronization capability your application demands.
Bob Rennard, LXI Consortium -- Test & Measurement World, 9/1/2006
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Ethernet, the world’s most widely adopted communication standard, transformed the way we work. Now, you can apply the benefits of Ethernet to transform the way you build test systems. Spurred by the belief that Ethernet is the natural successor to GPIB (IEEE 488), test and measurement companies, software companies, and system integrators created the LXI Specification, an Ethernet-based interface that combines the simplicity of GPIB with additional features that will enhance test system speed and capabilities.
LXI devices are defined by three functional classes, which differ primarily in their triggering capabilities (Figure 1):
- Class C, the base class, requires Ethernet connectivity, a Web interface, an Interchangeable Virtual Instrument (IVI) application programming interface (API), discovery, and some basic physical and mechanical features.
- Class B adds IEEE 1588 Precision Time Protocol and peer-to-peer communication to the base class. (See the 1588 Web site, ieee1588.nist.gov, or the LXI Consortium Website, www.lxistandard.org, for more information.)
- Class A requires both the IEEE 1588 Precision Time Protocol and an eight-channel low-voltage differential signalling (LVDS) high-speed trigger bus.
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| Figure 1. LXI instrument classes correlate with use models. |
Uniform triggers
A key feature of the LXI Specification is a uniform triggering model and an API that treats all trigger sources equally. By defining identical procedural calls between a LAN trigger and a hardware trigger, programmers use a single set of API calls and can switch easily between hardware, software, and time-induced triggers.
For example, if you use a driver named 'Arb’ to program an arbitrary waveform generator and want to use the LXI trigger bus line number 2 as the source of a trigger signal, you would write this line of code:
Arb.Trigger.Source = “LXI2”
If you then wish to switch to a LAN trigger with the same ID, you would simply change the command to:
Arb.Trigger.Source = “LAN2”
The LXI Specification recommends that LXI devices be capable of triggering any available action by any available means, although this will not be practical in all cases.
If a measurement can be triggered by a traditional hardware trigger line, then you should also be able to trigger it with a LAN trigger, the LXI trigger bus, or any other trigger input supported by the device. You simply configure the device to accept the appropriate trigger source and route it to the desired action.
The IEEE 1588-based time trigger software layer allows programmers to specify an action and set an “alarm clock” to “go off” at any specified time. This enables programmers and system integrators to execute complex test sequences in near-perfect unison and eliminate wait statements from test code.
For instance, an arb can be programmed to begin at a specified time and sequence through a series of timed steps. A receiver in the same test system could track the digitizer’s operation with near-perfect timing and with no connections between the instruments or controller other than LAN. This is particularly useful in distributed systems, where time-based triggers can eliminate cable-length latencies.
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| Figure 2. The histogram from an IEEE 1588 boundary clock shows the typical clock accuracy available with IEEE 1588. |
Some applications are not sensitive to jitter or will find jitter less problematic than latency. For example, many physical data-acquisition applications do not have tight timing requirements. Also, consider a radar test range where the distance between instruments may be considerable. With cable latencies of roughly 1-ns/ft, significant delays can result with widely separated instruments or test stations. Since IEEE 1588 is a UDP-based system, it works best on subnets built behind a single router. Where very long distances, measured in kilometers or more, are involved, GPS offers a convenient synchronization mechanism to tie subsystems together.
IEEE 1588 derived time-based triggers are not a panacea for all test applications, providing the motivation for alternative trigger mechanisms. For example, time-based triggers are not ideal for very fast asynchronous events and some UUT-induced trigger events, so LXI supports a high-speed eight-channel LXI trigger bus and LAN triggers. LAN-based triggers operate by sending electrical signals over a wire, much like traditional hardware triggers, but LAN-based triggering eliminates trigger cabling in many applications, simplifying system integration and streamlining device upgrades.
LAN-based triggers with IEEE 1588 also go beyond traditional hardware triggers because they can carry time-stamp information that hardware triggers cannot. Time stamps provide a time record of when an event happened, or they coordinate when multiple events occurred, simplifying post-acquisition analysis. Similarly, an LXI device can use a circular buffer, as with a logic analyzer, to “look back” in time to capture an event that happened before the trigger signal was received. (continued)
Peer-to-peer communicationIn addition to IEEE 1588 Precision Timing Protocol, the LXI Specification requires Class A and B devices to support peer-to-peer communication. Prior to LXI, most test-and-measurement system architectures relied on a master-slave configuration using a central controller. With the exception of some basic trigger lines, which carry little more than a trigger edge, there is no instrument-to-instrument communication in these setups. Each instrument communicates directly with the controller, which then sends commands to other instruments. Master-slave systems with high channel counts or controller-based measurement applications can quickly overtax central controllers, creating a processing bottleneck that stalls high bandwidth I/O and degrades performance well below expectations.
With LXI, instruments can communicate with one another independent of the controller, simplifying common applications such as stimulus-response measurements. Also, LXI allows system designers to download executable code or scripts that are triggered by a message from other instruments in the system, further freeing the controller to deal with data rather than control. With less system management, the system is more responsive.
Applying LAN and IEEE 1588The IEEE 1588 Precision Time Protocol provides system integrators with several advantages. Some are easy to envision, such as the ability to synchronize over great distances or to time-stamp large quantities of data. An antenna test range comes to mind as does a high-channel-count data-acquisition system.
Others advantages are more subtle, such as eliminating physical trigger cables in many applications or eliminating the need to calibrate and correct for multiple trigger cable delays. Such advantages simplify the programmer’s task and make software maintenance easier because hardware can be changed without disrupting the test program.
IEEE 1588 and LAN triggering can replace some trigger cables, but not all. The ability to replace cables depends on test-system speed requirements, which are often driven by the device under test. Some test systems, such as data-acquisition applications measuring temperature, pressure, or mechanical stimuli, require millisecond or microsecond resolution. Both resolutions are well within the capabilities of IEEE 1588. Time-critical applications, often seen in RF applications such as high-speed radar or scope and logic-analyzer triggering, require nanosecond and subnanosecond resolution that IEEE 1588 cannot provide. Other applications with fast asynchronous or device-induced triggers are a poor fit for IEEE 1588-based triggers.
Using IEEE 1588 time stamps allows instrument suppliers and system integrators to “go back in time,” using circular buffer techniques to investigate events leading up to a trigger. This technique has been used for years in logic analyzers and oscilloscopes, and IEEE 1588 makes it practical for many other instruments and systems.
Automatic time stamping means controllers and application software no longer have to keep track of when data was taken, simplifying one of the most tedious and fragile aspects of developing large test systems. Many aerospace systems have hundreds or thousands of control and data lines, many of which can be eliminated by distributing a sense of time out to the edges of the tester. The result is simpler cabling and instrument-to-UUT interfaces, shorter reconfiguration times, and elimination of some fragility in the test program set, or TPS.
Imagine instrumenting a jet engine with thousands of sensors to monitor complex stimulus and response channels. Or consider a modal-analysis system with thousands of strain gages on an airframe, where tight timing interrelationships are critical to maintain phase relationships between adjacent points. With time stamping and parallel execution, data interpretation and fault trees become simpler, tests can execute faster, and many difficult channel-synchronization tasks become trivially simple.
The LXI Web interface and Ethernet connectivity enables widely distributed applications. For example, an engineer in Finland can work with a contract manufacturer in China, looking at the same instrument screen to troubleshoot a problem or monitor a process without either leaving home. This has profound implications on a company’s need to replicate specialized expertise in multiple locations.
By leveraging the efforts of thousands of engineers in the Ethernet industry, LXI gives system designers the familiarity of GPIB with the power of new capabilities. The IEEE 1588 Precision Time Protocol is a core enabler for LXI, providing a low-cost network-centric time base. LXI builds on IEEE 1588 by defining how to apply it to a variety of test and measurement situations. It offers significant value to test system design. While today’s IEEE 1588 speed and resolution are insufficient to meet the most demanding test applications, they are sufficient for many applications and offer numerous possibilities for the future.
| Author Information |
| Bob Rennard is president of the LXI Consortium and program manager at Agilent Technologies in Santa Rosa, CA. |
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