LXI: A Technology Leap for Test Instrumentation

Since the 1970s, the measurement industry has relied on the GPIB bus as its preferred method of instruments communicating with computers. GPIB was developed to meet the specific needs of test automation that were not addressed by the prevailing computer bus architectures. That it remained the instrument automation I/O standard of choice for over three decades is a testament to its effectiveness and popularity.



A majority of instruments are designed to be used on the bench or in a system – they therefore have the displays and controls needed by users in a stand-alone mode, and the GPIB interface to enable them to be used in a system. This provides users the flexibility to choose the appropriate instrument from a wide range of manufacturers. GPIB-based test systems have two significant considerations – size and speed. In applications where such instruments function exclusively in test system racks, the available-but-hardly-used front panels and displays add to the size.



Modular instruments have been one approach to address the need for test systems of smaller footprints and higher speeds. The VXI and PXI architectures available in the market today are example of this – computer I/O architectures adapted to instrumentation needs. The instruments themselves take a smaller, modular (and being display and panel free) form, housed in card-cages.



Complexity of product design creates more complex test, and promotes an insatiable appetite for data as more complex experiments call for more channels of data, taken at everincreasing speeds.



Measurement system architectures have been created based upon the criticality of this data pipeline. For example, where slow to moderate speeds are prevalent, GPIB rack and stack instruments are the architecture chosen for their exceptional measurement capability, comfortable user interface and adequate I/O speed, combined with excellent triggering. Where frequency domain measurements are important, self-contained GPIB instruments again take the prize for being able to internally process massive amounts of data in real time. But whenever real-time control is involved, GPIB falls short, and a card cage backplane such as VXI or PXI is necessary to move the data required upon execution of an asynchronous trigger.



While GPIB is a reliable means of communicating between an instrument and a computer, it has been around over 30 years and in the meantime the PC industry has caught up with the needs of the measurement industry. Ethernet-based I/O is now sufficient in latency and block transfer speeds to be used in all but the most stringent applications.





Objections to LAN

As LAN has progressed from 100 Megabit to 1 Gigabit, block transfer speeds are now over 100 times faster than GPIB.



However, the bulk transfer speed of LAN is not the objection. For the engineer or scientist interested in synchronizing or capturing data, it is the non-deterministic latency of LAN that creates headaches. Part of the problem can be solved by putting all the instruments and the PC on a private LAN, effectively preventing any enterprise LAN traffic from invoking a collision detection that might interrupt service at a critical time.



The private LAN can return determinism to the system, and ever-increasing processor speeds reduce the absolute delay. Our measurements with newer PC processors have shown LAN latencies that approach or better those of GPIB. For cases where absolute delay must be under the 100 microseconds available to the best LAN systems, there is a need for a better solution. Enter LXI.





LXI

LAN Extensions for Instruments (LXI) is a new architecture for test instruments. In the process of adoption by the LXI Consortium, the LXI specification addresses LAN triggering, both for synchronization and for absolute delay, as well as a

number of other characteristics:









LXI Specifies:

• IEEE1588, a synchronization standard for LAN

• Local trigger bus, based on the VXI trigger bus

• Discovery via DHCP, so instruments can be recognized on the LAN

• Local reset, in the event of system lockup

• Mechanical standards based on EIA standard rack sizes

• IVI-COM drivers for software transportability



NOT Specified:

The LXI specification does not include PC board size. This is counter to the stringent requirements of VXI or PXI card cages, and gives the instrument designer a valuable commodity: an extra degree of freedom.





How is LXI Different?

At first glance, LXI appears to be just another I/O standard for test instruments; however, closer examination reveals significant differences in the approach to mechanical, I/O, electrical and software parameters.





Mechanical

As mentioned previously, there is no specific PC board layout. The only restriction is that boards fit into standard EIA halfrack or full-rack width modules. The modules can range from one EIA unit (1U) to six EIA units (6U) tall.



The units must have a local LAN reset capability, in the event of a network lockup. That capability can be in the form of a soft key or a hardware switch.



Instruments can have any user interface, varying from a full display with knobs and keys, to a faceless instrument. Connections should be on the front of the instrument, and the LAN, power and trigger cables must be on the rear.





I/O

LAN is, of course, the I/O of choice for LXI instruments. The PC industry has long recognized the advantages of LAN for moving large amounts of data and being able to accommodate many devices on the same network. Longevity of protocol is another aspect of LAN. It can mean not having to change software just because the PC backplane has suddenly been updated.







Figure 2: As the PC progressed, its backplane changed dramatically every few years, but the Ethernet standard has been relatively constant for many years. This longevity of protocol makes LAN an attractive communications method for instrumentation.



Figure 2 shows the evolution of the PC backplane, instrument backplanes and Ethernet. When the instrumentation backplane is based solely upon the PC backplane, frequent updates become necessary on the instrument side. This can be costly, especially during the transition stage. LXI escapes this issue by relying upon LAN to essentially replace the backplane.





Electrical

By eliminating the need for a card cage in most measurement applications, LXI instruments can be built as self-contained products. That means, unlike universal card cages, LXI power supplies and shielding can be optimized for the instrument—a major advantage for reducing EMI and noise.













Software

Since LXI allows for the same instrument to be built with or without a front panel, the implication is the firmware is very nearly identical in both circumstances. This sets up the possibility to use one type of instrument in R&D, with the full front panel, user display and controls, and a second type of instrument in manufacturing, with no front panel display or knobs. Since footprint is a major issue for manufacturing, and most of the instrument interaction with the test engineer is done under PC control, the ability to have a faceless version of the standard instrument gives the test engineer another degree of freedom to design the system.





Synchronous Triggering

While there are a number of advantages for LXI, as cited above, the non-deterministic nature of LAN presents a challenge when the application calls for synchronizing a number of instruments. To meet this challenge, Agilent Labs created a synchronization mechanism now known as IEEE 1588.





IEEE 1588

If every instrument in the system is equipped with a stable clock, one of the instruments can act as a master to which all the others are synchronized Our tests indicate the synchronization possible in such a setup is better than 100 nanoseconds. The presence of the internal clock also affords the instrument the ability to time-stamp every piece of data.



This function of LXI is useful in real world examples such as distributed monitoring, where the experimenter might be trying to measure the pressure wave as it travels down a pipeline. Every A/D converter in the measurement chain would need to be synchronized to a known start time.





Asynchronous Triggers

If the initiation of the event is a predetermined time, the IEEE 1588 method works well, but if the trigger is asynchronous, the experimenter is still at the mercy of LAN latency. Most of these cases can be mitigated by using local LAN and a high-speed PC processor. This results in trigger delays commensurate with GPIB instrumentation. However, in the few instances where this delay is too long, the LXI Consortium has allowed for an alternative trigger mechanism.





Consortium

An LXI Consortium was formed in November, 2004 to develop the specifications and underlying principles of LXI. Started initially by Agilent Technologies and VXI Technology, the Consortium has grown to include most of the major test & measurement instrument manufacturers. Final release of the LXI specification was scheduled for September 26th, 2005.





Conclusions:

The advantages of the new LXI architecture are legion, for example: distributed measurements, wide-area synchronization, smaller footprint, more trigger flexibility, long-lived protocol and no universal card cage needed. Where real time control is necessary, the card cage may still be the best alternative, but for virtually all other applications, LXI will be the new architecture of choice for many of the leading test & measurement manufacturers around the world.





To learn more, go to

www.lxistandard.org or

www.agilent.com/find/open