Paul Schreier examines LAN eXtensions for Instruments, a PC-instrument interface standard that should eventually put GPIB out to pasture
We’ve long known of the benefits of connecting instruments to computers. Over the years, though, the equation has changed with the rapid development of the PC.In early days, most instruments connected over the serial port, but that communications protocol is slow, works with only one instrument per port, and the RS232 standard is so loose in spots that exchanging commands and data can take some extra work. The USB and FireWire ports look interesting, but their cables are not industrially graded, are sensitive to noise, have limited distances, andthe connectors don’t lock firmly into place and cancomeloose. Meanwhile, Ethernet ports are standard on virtually every PC to provide an interesting alternative for instrument connectivity – and a new standard, LXI, makes it easy to reap the benefits of Ethernet technology while meeting the unique needs of test systems and ensuring compatibility with a wide range of instruments.
The previous dominant standard
Historically, though, the dominant scheme for interfacing instruments to a computer has been IEEE-488, which started out as the Hewlett Packard Interface Bus (HPIB), and today is also well known as GPIB (General Purpose Interface Bus). HPIB was developed in the late 60s, and in 1975 the IEEE published its first version of that bus standard. In 1977 National Instruments unveiled a GPIB card for the Digital Equipment PDP-11, but minicomputers such as that were soon pushed out of the market by PCs. Thus, in 1983, National Instruments started selling an IEEE-488 card for the IBM PC, and other firms followed with their own models. The PC became the platform of choice for instrument control.
What made IEEE-488 so popular were industrial-grade cabling, high transfer rates compared to the serial port, along with sophisticated triggering from the PC to instruments as well as among instruments. On the downside, IEEE-488 requires an adapter, whether a plug-in card or external box, while the physical distancesand number of instruments on a bus are limited, and the cabling is expensive.
However, users are increasingly attracted to Ethernet as an instrument interface. As mentioned, it is essentially ‘free’ because it comes standard with everyPC.Cables are inexpensive, and distances are much greater. It is continually evolving, with speeds today reaching 10 Gbps, and the large installed base makes it economically worthwhile to develop the spec to even higher speeds, all while maintaining backwards compatibility. It’s also possible to setup a wireless instrument network using inexpensive commercial routers. As a result, in the past few years many instruments started to ship with Ethernet interfaces as standard or options.
The instrument community, though, wants certain functions that are not typically available with ‘office’ Ethernets. They want device discovery, determinism, synchronisation, and predictable software driver interoperability. They also want these things to be standardised so they can learn them once and know that the same knowledge is transferable.
The benefits of LXI
These are things that they gain when they purchase an LXI-based instrument. The LXI Consortium was incorporated in the autumn of 2004, and the first certified products were released in early 2006. There are currently 420 such instruments in 53 product families (a list is kept at www.lxistandard.org/applications/products/).
Indeed, annual sales of LXI test and measurement equipment now exceeds $US200m according to the LXI Consortium, which adds that this is the fastest ramp-up in sales of any communications standard in the history of the test industry.
Distributed measurement capability at a reasonable price is one key attraction of LXI. By using Ethernet, it allows scientists to place their measurement devices long distances from the host computer. This is an advantage in dangerous environments or in sensitive environments (such as a wafer fab, where operators can sit in a control room and read measurements without having to wear a bunny suit and go into the fab).
Three classes of LXI instruments
In examining LXI instruments, it’s essential to know that there are three classes of instruments (A, B, and C, where A is the most powerful) and to understand the differences among them. The vast majority of LXI instruments today fall into Class C, which fulfills the baseline requirements. In fact, because the spec is so new, most vendors are ‘getting their feet wet’ with Class C instruments, and many are taking existing units and upgrading them to Class C. If a unit already has an Ethernet interface, the remaining work is mostly in the software area.
What can you expect with Class C? All device classes use Ethernet as their communications interface and comply with IEEE 802.3 for both physical and functional compliance. Other key features include device discovery, an integrated web page, and software standards.
In setting up a system, it helps if the host can automatically detect instruments as they are added to the network. LXI mandates the use of the VXI-11 standard for identification of instruments. With this scheme, which uses RPC (remote procedure calls), the host sends a broadcast RPC, then requests information from each instrument found and builds a resource table that other applications and utilities can use.
Each LXI instrument has its own welcome page and a LAN configuration page that users can easily access with a standard web browser. The specification’s requirements are relatively modest, mandating such items as host name, instrument model, manufacture, serial number, LXI class, LXI version, TCP/IP address, MAC (medium access control) address, and software/firmware version. However, suppliers are expanding on these requirements in imaginative ways. For example, some reproduce the instrument’s front panel in software so that users can manipulate an instrument with a mouse while the actual unit is metres or kilometres away (Fig. 1).
Fig. 1: The front panel on Keithley Instruments' Model 2810 LXI RF signal analyser (left) is duplicated on the instruments built-in web page (right) so users from remote locations can control operations and view results
In another case, LXI switching boxes often have visual displays of the switch matrix where users can click on the image to set upa configuration (Fig. 2). In fact, some vendors feel that sophisticated web pages will eliminate the need to run separate programs for many sequential and analytical applications. For example, an interesting application is in the works at power-supply vendor Kepko. While their current instrument web pages let you set all power-supply operating parameters, the company’s plan is to implement a list command to program waveforms on a point-by-point basis as a basic function generator.
Fig. 2: The soft front panel for Pickering Interface's Model 60-711 dual 24 x 8 video matrix lets users set up the switching functions from the web page without writing a program
It’s also possible to have multiple users access a web page at the same time. A scientist might run an experiment, have the results appear on the web page, and then ask some colleagues to log on and comment on the results. Or a professor teaching physics could run an experiment and, rather than have a number of students crowd around an instrument’s front panel, have each student view the results on the instrument’s web page in real time while sitting at their own PC. This could also make distance-learning more interesting.
For applications that require more programming than is possible on a web page, all LXI classes mandate that suppliers provide an IVI (Interchangeable Virtual Instrument) driver, either or both IVI-C (based on ANSI C) or IVI-COM (for compatibility with Component Object Model technology). Thus, users can program their systems using traditional programming languages including C and Visual BASIC as well as popular application development environments such as National Instruments’ LabView, Matlab from The MathWorks, Data Translation’s Measure Foundry, and Agilent’s VEE.
Triggering is the thing
Recognising that many instrument systems require triggering from external events and between different units, the next two LXI classes address that requirement: one in software (Class B) and one in hardware (Class A). Note that at this time there are very few Class B and A instruments, because suppliers are climbing up the learning curve. But the features they offer will be beneficial when they become widely available. Class B expands on Class C by supporting direct LAN messaging in the form of peer-to-peer communications without sending signals over a wired bus, for module-to-module triggering as well as supporting time-based trigger events. The addition of LAN-based triggering is made possible through support for IEEE 1588-2002 PTP, which provides the means to distribute a precision timing source across many LXI devices. Timing accuracy of tens of microseconds to tens of nanoseconds is possible.
LAN messaging would be useful, for instance, to have a signal generator send a stimulus to a device under test and then notify a spectrum analyser directly to take a measurement without having to involve a central controller. A small program running inside each box could send messages back and forth as the two instruments step through a series of levels or frequencies. Users could download a list of values, have each instrument run on its own time, and tell the central controller when that task is done. This approach would take traffic off the bus and speed up communications.
Hardware triggering through the WTB (wired trigger bus) is the key feature of Class A devices. Three trigger configurations are specified: daisy chain, star, and hybrid star, all implemented using an 8-twisted pair bus and MLVDS (multipoint low voltage differential signalling) technology. In addition, two trigger bus modes are supported: driven and wired-OR. Class A devices provide the capability to create instrument subsystems linked by hardware triggers. A typical application might be the creation of synthetic instruments that consist of several source/measurement components with coherent intermodule clocking and triggering over the LXI trigger lines.
A good illustration of this approach comes from Agilent, which offers a number of ClassA instruments that allow you to build synthetic instruments. These modules, which are building blocks with no front panels rather than being complete traditional instruments, include among other things a digitiser, down-converter, waveform generator, and an upconverter. By combining these modules in various ways, users can emulate traditional instruments or create specialised instruments. The reusability of modules in different scenarios reduces the total lifetime costs of a test system by extending its longevity and provides obsolescence protection.
Fig.3: The EX2500 from VXI Technology is an LXI-VXI Slot0 interface that allows a rack of VXI instruments to work with LXI instruments
TheWTB will also play a large role in letting engineers integrate LXI into existing test setups and make LXI compatible with legacy hardware while allowing people to ease into these new technologies. This is part of the philosophy behind VXI Technology’s EX2500, an LXI-VXI Gigabit Ethernet Slot0 interface (Fig. 3). While Class A instruments must include both the WTB and IEEE 1588 triggering, several vendors have developed Class C instruments that add the WTB but not IEEE 1588 because some vendors note that adding 1588 can be the most expensive and toughest part of this business. These instruments are more than Class C but don’t qualify as Class A, so some vendors informally refer to them as ClassCPlus.
Leveraging previous architectures
A key aspect that will make LXI viable is the ability to work with legacy instruments. This was just mentioned in connection with ClassA devices and the WTB, but a new product from Agilent moves interoperability into Class B devices. Specifically, the Model E5818A LXI trigger box (Fig. 4) is itself a Class B unit, but when an LXI Class C or GPIB instrument is connected to it, that older unit gains the timing capabilities of an LXI Class B instrument. The E5818A can achieve a synchronisation accuracy of up to 13ns (standard deviation over a direct connection) and provide time stamping for as many as 5,000 events.
The number of LXI instruments is starting to grow. Many instrument vendors are starting to upgrade existing products and make LXI an option, sometimes removing an RS-232 interface to make room. In newer designs, you will likely see LXI become standard with GPIB as an option instead of the reverse situation today.
Fig. 4: At the top of each of these three racks you can see an Agilent E5818A LXI trigger box (the thin unit with the yellow-green display), which gives exisiting GPIB instruments the timing capabilities of an LXI Class B instrument.