Selecting Your Optimum LXI Feature Set

More than a year ago, a few suppliers of LXI instruments were starting with LXI Class C to develop enhanced instruments.1 Class C makes up the core LXI features on which Class B and A instruments are built.2 Now the LXI Consortium has formalized such expanded feature sets in Version 1.3 of the specification, which recently was approved.

Integrating Additional Features to a Class C Instrument

Why is it desirable to add features to a Class C LXI instrument in this way? Here’s the situation: Suppose a supplier of a Class C instrument believes that its customers would benefit greatly from more but not all features that fall under the other classes. The capability to add selected functions is a boon to both manufacturers and users.

Users don’t have to pay for features they don’t necessarily need. This aspect will be especially important for instruments at the lower end of the market such as portable or other low-cost units that use microcontrollers with limited resources to save cost or power consumption. Users also can benefit from a broader selection of instrument products with added features that can be brought to market sooner.

Manufacturers can use such a partial implementation of Class A or B as a steppingstone to a higher class instrument without holding back on the instrument’s market launch. Designing a Class C instrument isn’t as major an undertaking as developing a Class B or Class A instrument, especially now that so many instruments already have Ethernet ports.

However, and this is especially true for companies new to the LXI market, it can take significant time to develop a full Class A or B instrument and go through the conformance testing. If a manufacturer can select only certain features, it can get its products to market faster and at a lower cost. For example, a user may find the LXI LAN messaging feature of Class A and Class B is useful, but the addition of IEEE 1588 and the triggering API is less useful and creates trade-offs elsewhere.

There is nothing in the current or past LXI specifications that prohibits LXI Consortium members from introducing these additional features. In fact, some Class C instruments have included the wired trigger bus (WTB) from Class A for some time.

But because such an instrument doesn’t include all Class A features, it’s not eligible for Class A certification. And even though the LXI Consortium may have tested the WTB implementation and declared it compliant to that section of the specification, until now there has been no official way to tell users about this fact.

Such certification is important for users to gain full confidence in the interoperability of a non-Class A instrument’s WTB in an LXI system. Interoperability testing is one of the strengths of LXI. It has been very effective. Among the LXI-certified instruments on the market, there have been no interoperability problems, which is a great asset to anyone assembling a test system.

With Version 1.3, vendors of Class C instruments now can submit selected features from Class A/B for testing: trigger bus, event messaging, clock synchronization using IEEE 1588, timestamped data including clock synchronization, and event logs including clock synchronization. So how will users know that these extra features exist?

There are no plans for a special label on the front of the instrument, but these extra capabilities can be indicated on the instrument’s welcome page on the Web. Then a product description will include a phrase such as Class C plus LXI Trigger Bus. Those capabilities also can be listed on the LXI Consortium website in the master list of products and in the company’s marketing materials and corresponding help files and manuals.

Only Certain Features Qualify

One aspect the LXI Consortium must deal with in establishing added features to Class C instruments is conformance testing, which ensures that these features meet all the requirements in the spec. It is important to note that the unit remains a Class C device.

The exception would be if all of the features of another class were added to the device. For example, if a manufacturer were to implement and test all five of the Class A features in a Class C instrument, the result would be a Class A instrument.

Most of the Version 1.3-added Class A/B features are relatively self-explanatory. However, the difference between timestamped data and event logs is worth mentioning. With timestamped data, samples can be correlated among many instruments so that you could, for example, see how a change in a power supply was reflected in a UUT. An event log, in contrast, records actions in an instrument such as trigger received, alarm detected, measurements started, or error occurred.

Adding WTB

A handful of vendors added extra features to Class C instruments even before Version 1.3 allowed them to be promoted as LXI approved. At the moment, these instruments fall into two categories: those that have added the LXI trigger bus and those that have added LXI event messaging.

Figure 1. Rohde & Schwarz FSL Spectrum Analyzer With Class C and LXI Trigger Bus

Rohde & Schwarz was the first to receive conformance certification for adding the WTB to its FSL Spectrum Analyzer (Figure 1). The box has a slot that accepts an option card populated with an FPGA that implements the routing of the trigger signals across the bus.

According to Jochen Wolle, head of software R&D, external triggering is traditional in instruments such as spectrum analyzers, signal generators, and scopes. Customers know how to use it. But because it defines a standard connector, the WTB eliminates problems with incompatible cable connector types. Further, with the WTB there’s no longer a need to go behind the instrument and reconfigure the wires on each setup because the lines on the WTB are software configurable.

At the moment, the FSL is Rohde & Schwarz’s only LXI instrument with the WTB. The LXI trigger bus can connect to almost any other triggered instrument if users add the Model 60-982 LXI Wired Trigger Bus Adapter from Pickering Interfaces. This small external device measuring 30 x 84 x 100 mm converts low-voltage TTL (LVTTL) signals to the M-LVDS signals on the eight-line WTB or vice versa, and it supports all modes of WTB operation.

Two versions are available: the thru-line model provides two WTB connectors along with 16 SMB connectors for the LVTTL inputs and outputs from other instruments, and the other includes just one WTB bus connector and a WTB termination along with the 16 SMBs. The unit can be configured manually or with digital I/O.

Another early adopter of the WTB as an extra feature was C&H Technologies. When it introduced the EM-405-8 LXI Ethernet M-Module Carrier/LXI Bridge two years ago, that company added the WTB to this Class C instrument and has received conformance certification for the WTB functionality (Figure 2). The box, housed in a 19″ rack-mount 1U-high enclosure, provides Ethernet connectivity for up to eight ANSI/VITA 12-1996 single-wide M-modules or a combination of double- and triple-wide modules. An IVI driver supports control of all bridge functions. The -0001 version implements the eight LVDS triggers required for the LXI trigger bus.

Figure 2. C&H Technologies EM-405-8 LXI Bridge With Class C and LXI Trigger Bus

This capability was added because many C&H customers are familiar with VXI and its high-speed backplane triggers, explained Gary Guilbeaux, vice president and CTO. Also, many of the firm’s products are M-modules with clocking features. With this LXI bridge product, not only can eight modules trigger among each other, but using a common clock they also can trigger among multiple bridge boxes.

To make the EM-405-8 a Class A instrument, C&H still needs to add IEEE 1588 clock synchronization, timestamping, event log, and event messaging, but Mr. Guilbeaux noted that doing so is more difficult to implement than the WTB. Besides, he added, until now there has been limited customer demand for 1588, but it is starting to pick up. The company does plan to add the features necessary to allow this instrument to jump directly to a Class A.

Figure 3. ZTEC Instruments ZT 42XX Series Oscilloscope With Class C and LXI Trigger Bus

More recently, ZTEC Instruments has combined Class C with WTB capabilities in several of its digital storage scopes, which are packaged in half-width 1U chassis in the LXI versions (Figure 3). All of them digitize with 8 bits of resolution, offer either two or four channels, and spec interleaved sampling at rates to 1 GHz (ZT 4211/12) or 4 GHz (ZT 4611/12).

WTB enables users to configure multiple instruments to trigger using either point-to-multipoint (driven mode) or multipoint-to-multipoint (wired-OR mode). The beauty of the WTB is that multiple LXI instruments can be daisy-chained together to achieve very low timing uncertainty since there is no software latency, according to Boyd Shaw, director of marketing and product stragegy at ZTEC.

A very simple example would be where an LXI oscilloscope triggers on a pattern of signals such as CH1 – H, CH2 – H, and CH4 – L. Upon detecting this pattern of inputs, the scope could use the WTB to trigger an LXI power supply to decrease its output level and to trigger an LXI switch to change its settings.

LAN Messaging

LAN messaging was a requirement that Kepco found among its users, so that feature was built into the Class C Model KLP-600-4-1.2K Hyperbolic Power Supply from the start (Figure 4). Now with Version 1.3, the only thing holding the company back from getting conformance certification as a Class C and LXI Event Messaging product is an LXIivisync interface, which the company expects to have shortly.

Figure 4. Kepco KLP-600-4-1.2K Power Supply With Class C and LXI Event Messaging

In this lab supply, the voltage/current limits automatically are recalculated, forming a constant-power of up to 1,200 W hyperbolic-shaped boundary between the voltage and current modes. This curve, which replaces the single max-power operating point of conventional power supplies, provides the user with a greatly expanded choice of maximum-power V-A combinations.

With LAN-based events and alarms, multiple power supplies can intercommunicate without the need for a central PC by multicasting messages to all LAN devices and synchronizing the activities of multiple supplies within 100 µs. The KLP can send LAN event messages that are alarms to other devices. The alarms are triggered when a unit reaches its current limit or has a fatal error such as missing sense connections.

Upon receipt of such an alarm, another KLP can take one of three actions: change its status from Off to On after a time period from 10 ms to 1 min, change its status from Output On to Output Off after a similar time span, or shut down the unit within 5 ms in Immediate Output Off. Accordingly, if one supply determines that it has an overcurrent condition, it can send an alarm message to shut down any other supplies in a large test system and prevent the UUT from getting damaged—and again, without any PC intervention.

Attractive Feature Sets

Beyond WTB and LAN messaging, other features such as adding 1588 clock synchronization from Class B will likely turn out to be a big driver in sales. So believes David Owen of Pickering Interfaces, who also is chairman of the LXI Technical Committee. With 1588, users gain the ability to synchronize events and record what has happened across a distributed test system.

Adding 1588 initially was a major task for most companies. Suddenly, though, with the emergence of 1588 chipsets, the barrier now is lower, and it becomes interesting to suppliers of low-end equipment. A lightweight data-acquisition box or process-control system could benefit greatly from such synchronization. However, a vendor might not want to add all Class B functionality because of additional development expense and increased end-product price.

Certain combinations of feature sets will likely prove very popular, added Mr. Owen. Market forces will come to bear, and users will tell suppliers which combinations they want. For instance, he sees that the combination of 1588 and event logging makes a lot of sense because it will be great for debugging distributed systems. It will be very interesting to follow developments along these lines and see what innovative new instruments come on the market.

References

1. Schreier, P. G., “Climbing the LXI Learning Curve,” LXI ConneXion, September 2007, page 25.
2. Schreier, P. G., “The Killer Bs Are Coming,” LXI ConneXion, September 2008, page 50.
3. LXI Consortium, www.lxistandard.org

About the Author

Paul G. Schreier is a technical journalist and marketing consultant working in Zurich, Switzerland. He was the founding editor of Personal Engineering & Instrumentation News, served as chief editor of EDN Magazine, and has written articles for countless technical magazines. Currently, he is the editor for LXI ConneXion at EE-Evaluation Engineering. Mr. Schreier earned a B.S.E.E. and a B.A. in humanities from the University of Notre Dame and an M.S. in engineering management from Northeastern University. e-mail: [email protected]

January 2009

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