The Killer Bs Are Coming

Undoubtedly, there are many compelling reasons for working with Class C LXI instruments. But the real excitement in the industry today comes with Class A and B instruments, particularly with the time-aware nature due to the implementation of the IEEE 1588 Precision Timing Protocol (PTP).

Roughly 95% of the LXI products currently on the market are Class C. Only a small number of Class A products are available, and just two Class B products, the Keithley Instruments Model 3706 Switch/DMM System and the Agilent Technologies E5818A Trigger Box, have begun shipping.

These time-aware instruments are opening up totally new test and measurement scenarios and application possibilities. When test engineers see these killer B products, they will wonder how they ever got along without this functionality.

Class A, B, and C Overview

Here's a CliffsNotes™ summary of the LXI instrument classes. Class C is the baseline with LAN capabilities, a Web interface, and IVI drivers. To that, Class B adds expanded triggering, such as multicast and peer-to-peer communications between instruments and time-based trigger events. The time-based triggering is made possible by implementing the PTP, which can distribute a precision timing source across many Class A and B devices over a LAN. Class A devices build upon Class B devices by adding a wired trigger bus for precision triggering.

Time-Aware Instruments

Remember action movies where a group of commandos gathers to synchronize their watches so each knows exactly when another makes a particular move even if miles away? That's the basic principle behind 1588.

“Never has there been a test technology that automatically synchronizes time across instruments and distances to this level with theoretical synchronization levels of single-digit nanoseconds,” said Chuck Cimino, marketing director for multi-application products at Keithley Instruments.

Class B (as well as Class A) provides a mechanism for allowing multiple distributed instruments to have a fairly accurate notion of absolute time even if they don't share a common clock source. The instruments are time aware: Each knows what the other is doing and when it is doing it and with high time resolution.

After a group of Class B instruments synchronizes themselves in time with PTP, they can timestamp local events and know that these records correspond very closely to timestamps on other instruments. Next, when one instrument detects an event, it can send a LAN message to other instruments within milliseconds or even hundreds of microseconds so they can act accordingly.

One instrument also can instruct others to perform an action at a future time. Instrument-to-instrument communications happen considerably faster and more accurately than is possible with a PC using software running under operating systems such as Windows.

Given the fact that the LXI spec is now four years old, it might seem it's been a slow process in getting Class A and B products onto the market. Conrad Proft, technology product planner at Agilent, noted that Class C is purely a software change to any existing instrument that already has a LAN port. In fact, Agilent has an internal mandate for Class C compliance for all new products with a LAN port and already has updated some older products to Class C such as the 33220A and the N5700.

Mr. Proft feels that it is more likely that Class B with submicrosecond PTP accuracy will be introduced with new instruments rather than through upgrades, and LXI vendors are requesting and validating input from users on how precise and accurate the PTP timing must be for various applications. PTP accuracy below 1 ??s requires hardware assistance to monitor PTP time packets while submillisecond accuracy can be performed in firmware.

Most data acquisition equipment might need only 1-ms time accuracy, and those products would likely be updated from Class C to Class B by using a software implementation. However, any existing instrument requiring submicrosecond accuracy will need a hardware modification, and it's often not practical for suppliers to upgrade existing instruments.

Whether a software or hardware upgrade, in either case, it represents a significant R&D effort. However, once the Class B technique is established for a particular instrument platform, it tends to migrate to the other products using that same platform.

There's another reason why many companies have been holding back on developing Class B products: There will be a major change to the LXI spec with regard to IEEE 1588 time synchronization. LXI Version 1.2 products based on 1588-2002 will not be backwards compatible with Version 1.3 products based on the latest 1588-2008. At the PlugFest scheduled for October in Munich, there likely will be the first trickle leading eventually to a flood of certified Class B products going to market.

Class B Applications

There are a number of applications where Class B instruments shine. An obvious case is when you need to synchronize triggers or measurements across long distances. This could be on a physically large item like a bridge or a large test chamber or when test instruments need to be hundreds of feet, miles, or even continents apart.

Here all the instruments are running with a synchronized clock established over the net. Each instrument can operate its own scan/measure sequence at prescheduled times with full confidence that it is in sync with other instruments in the test system.

Consider an antenna test where a signal generator sends a signal through one antenna, and a spectrum analyzer on the receiving antenna is several hundred meters away (Figure 1). Running trigger lines to coordinate the instruments would be a challenge, but LAN lines are no problem.

Figure 1. Class B Instruments Using LAN Messaging to Coordinate the Measurement of Satellite Signals

You even could implement longer distances with wireless LANs and directional antennas. Then, the instruments can pass LAN messages back and forth. The signal generator could tell the spectrum analyzer when to start taking data, and the spectrum analyzer would tell the signal generator that it has completed its measurement and is ready for the next frequency.

What happens when the antennas are sending signals via satellite whether the signal generator and the spectrum analyzer are next to each other or miles apart? You know there's a delay of several hundred milliseconds before the signal reaches its destination. If both instruments start at exactly the same moment, you not only acquire unnecessary data before the signal actually arrives, but it also takes some postprocessing to find the data of interest.

Accordingly, a Class B-enabled signal generator could send a LAN message to the spectrum analyzer saying, “I'm going to send my test signal at t = T; you start collecting data at t = T + 300 ms.” Then, when the spectrum analyzer successfully collects and stores its data, it can send a LAN message to let the function generator know that it can move to the next frequency in the sweep and continue the test procedure.

Another application is the sequencing of instruments such as in a controlled power shutdown. In some cases, very expensive DUTs receive power from multiple supplies that must be applied and disengaged in a specific order or the DUT will be damaged or destroyed. If one Class B instrument receives a signal indicating a system error, it can issue LAN messages to other instruments controlling the power supplies so that each shuts down at a particular time.

Sequencing takes on another meaning when you work with two Class B data acquisition units and synchronize their A/Ds to effectively double the sampling rate. You could align the sampling clocks and then shift one of them as needed to acquire interleaved data that you later reconstruct in software.

Besides allowing instruments to talk to each other quickly if something goes wrong, Class B timestamps can help engineers analyze what happened in a given test sequence, a tool extremely valuable for troubleshooting. Each instrument can have its own event log of triggers and other activities.

If a system malfunctions, an engineer can read time logs from various boxes into a single spreadsheet, sort the data by time, and see the sequence of events throughout the system at a glance. In addition, it doesn't matter if events are microseconds, minutes, or hours apart; the resolution of the timestamp remains in the range of nanoseconds.

And if you do need the timestamps on distributed Class B instruments to correspond to real clock time, it is possible to synchronize them to a GPS receiver rather than to a local master Class B instrument. GPS then provides an absolute time reference that can be obtained globally, and many applications use it as a reference for testing and data stamping.

Introducing the First Bs

The first true Class B instrument is from Keithley Instruments with the Model 3706, a six-slot system switch with an integrated 7??-digit DMM (Figure 2). The slots accommodate a variety of mux cards designed for different speeds, currents, and voltages.

Figure 2. The Model 3706 Switch/DMM From Keithley Instruments

Fully loaded, the compact mainframe can support 576 two-wire mux channels. These feed into a DMM that features a single-channel reading rate that ranges from >10,000 DCV or two-wire Ohms readings/s at 3??-digit resolution to 60 readings/s at 7??-digit, 26-bit resolution.

Another feature is the embedded Test Script Processor. Users can write scripts with complete test routines including complex decision-making and control so the instrument can perform autonomously.

Meanwhile, anyone can use Agilent's Class B E5818A Trigger Box to make their products Class B to preserve legacy investments (Figure 3). It comes with a 5,000-entry event log with timestamp resolution of 20 ns.

Figure 3. The E5818A Trigger Box From Agilent Technologies

To interface to traditional instruments, the E5818A provides two TTL trigger output signals and two digital timestamp input lines, and each channel can generate LAN messages intended for other LXI instruments. With this Trigger Box, you get most of the capabilities of a true Class B instrument, which can timestamp virtually any condition and respond to a LAN message to initiate some testing activity.

When 1588 Just Isn't Enough

In some applications, even the synchronization capabilities that Class B delivers aren't precise enough, and the Class A wired trigger bus is needed for the ultimate in phase alignment. That was the experience of VXI Technology when a major commercial aircraft company wanted to measure stresses on the wings of its most recent aircraft design. More than 10,000 channels were distributed over the structure, and it was very important that minimal skew occurred in the arming/initialization/triggering process. Otherwise, it would be difficult to correlate data and achieve dependable results.

While Class B instruments could start the clocks of all the units at the same time, that's not sufficient according to Tom Sarfi, business development manager at VXI Technology. The 55-MHz sample clocks that drive the sigma-delta A/D converters also must be tightly correlated to single-digit nanoseconds throughout the acquisition run so there is virtually no skew, which is not feasible with the free-running clocks on distributed Class B instruments.

Instead, the application uses the Class A wired trigger bus so each EX1629 remote strain-gage measurement shares a master sample clock. In this way, a single clock edge can govern the sampling of all the distributed A/Ds with very minimal skew. To keep skew sufficiently low, the company uses equidistant cables to all the boxes.

To handle long distances, VXI Technology also employs the EX2116 LXI Trigger Bus Extender. The EX1629 accepts 48 channels and produces sampled data at rates to 25 kHz using a 24-bit A/D on each channel. Software-configured strain and voltage conditioning and excitation allow the connection of quarter, half, or full bridge or voltage inputs to any channel.

About the Author

Paul G. Schreier, editor of LXI ConneXion, 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 since written articles in countless technical magazines. 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]