Electronic Design

PXI And LXI Set The Table For A Tasty Test Buffet

With today's tighter customer deadlines and ruthlessly competitive industry, designers can't delay in getting their products to market. Fortunately, new testing options are accelerating the process. The PXI modular instruments and the new LXI instruments let engineers assemble economical test systems optimized for their needs.

But don't let that similar "XI" ending fool you. These instruments aren't necessarily competitive. Actually, they're different systems that address different situations, giving test engineers a bountiful choice of test options that will keep them in the race to market.

PXI Came First
PXI is a modular instrument platform based on the Compact PCI bus. Test engineers can plug over 1200 test instrument modules from many different vendors into its standard chassis to build the perfect test system for their needs.

These systems combine hardware, software, and communications to produce a system that not only meets target needs but also provides similar flexibility, throughput, and synchronization previously available only with more expensive traditional bench instruments.

Standards set by the PXI Systems Alliance (www.pxisa.org) ensure compatibility and interoperability. National Instruments first introduced PXI in 1997. Now, over 65 other companies who are members of PXISA support the standard as well. Instruments that meet the standards are allowed to use the PXI logo.

The typical PXI test chassis consists of a built-in system controller, which is typically a powerful dual-core embedded PC running at 2 GHz or more (Fig. 1). Versions that use an external PC or laptop are also available. The chassis contains a power supply and a PXI backplane that comprises the PCI bus and a special trigger bus for timing and triggering operations. The chassis has slots for seven instrument modules chosen to meet the desired test needs.

PXI instruments are virtual instruments. The plug-in modules serve as signal conditioning, data capture, and storage media. Using software like National Instruments' LabVIEW, the internal or external PC does the actual instrument processing and display.

The newer PXI systems use the PCI Express (PCIe) super-fast serial version of PCI to boost throughput and cut latency. PCIe uses two or more unidirectional pairs or lanes to transmit data across the bus at speeds from 2.5 to 5 Gbits/s in the newer versions. With x4, x8, or x16 lanes, the total throughput can be as much as 45 times greater than the basic PXI parallel bus. These faster versions of PXI suit the systems for some of the more challenging test situations, such as wireless and RF.

The PXI instrument business is doing well, with wide use in the communications, military, aerospace, and automotive industries. Nine of the top 10 contract manufacturers use PXI systems. Furthermore, 17 of the top 20 electronic companies use PXI. Research firm Frost & Sullivan forecasts a 23% compound annual growth rate for PXI through 2012.

LXI: The Newcomer
LXI (LAN eXtension for Instrumentation) is the newest system in test and measurement. The LXI Consortium (www.lxistandard.org) first developed it in 2004 and released the first version of it in September 2005. Since then, over 150 LXI instruments have been developed and standardized.

LXI is based on Ethernet, the ubiquitous local-area network (LAN) technology, including the wireless versions. It uses TCP/IP and Web-based interface and program control. All LXI instruments have a Web page that users can access with any browser to view instrument outputs and change instrument parameters. Also, LXI instruments are designed for rack mounts. They don't have a traditional front panel. And, an external PC controls them via Ethernet.

One way to view the new LXI instruments is to see them as standalone measuring equipment that can be linked to other equipment and PCs as well as into complete systems using Ethernet as the replacement for the General Purpose Interface Bus (GPIB).

Hewlett-Packard (now Agilent) developed the first programmable instruments and interconnection platform in the late 1960s. Known originally as the HP Interface Bus (HPIB), it was later renamed GPIB and supported by almost all test instrument companies to some degree. GPIB was subsequently standardized by the IEEE as IEEE 488 in 1975.

The standard has been maintained and updated with the additions of codes, formats, protocols, and common instrument commands that come together in the Standard Commands for Programmable Instrumentation (SCPI). Most instruments comply with SCPI.

The IEEE 488 interface is an 8-bit parallel bus with a maximum speed of 1 Mbyte/s. An 8-Mbyte/s version is also available, but length limitations (20 m max) restrict its use in some applications. While the IEEE 488 interface is still around and widely used, it's gradually giving way to faster and more flexible and powerful systems based on LXI.

The choice of ubiquitous Ethernet LAN as LXI's communications medium makes it inexpensive and fast. Also, it can take advantage of the ever-increasing Ethernet speeds. While 10/100-Mbit/s systems are common, 1-Gbit Ethernet is now widespread, and there is growing use of 10-Gbit Ethernet. 100-Gbit/s Ethernet is in the planning stages, giving LXI a throughput roadmap to follow.

Timing and synchronization are key issues in LXI instrumentation. LXI has its own Trigger Bus, which uses a high-speed, low-voltage differential signaling (LVDS) interface. Users can send a trigger signal over the bus to initiate an event or perform some synchronization task. LXI also uses the IEEE 1588 Precision Timing Protocol (PTP) to provide nanosecond resolution timing and time stamping, which has become a must in modern test systems.

The standard defines three LXI instrument classes. Class C, which includes the simplest devices, supports all of the standard's LAN, programming, and Web browser elements. These instruments mainly use Ethernet to replace GPIB. Class B devices have all of the Class C features while supporting IEEE1588 timing as well. Designers use Class B when they need to sync or time an operation over the LAN. Class A instruments include the Class B and C characteristics while also supporting the separate highspeed trigger interface for high-speed performance.

Agilent's 6000L LXI Class C rack-mount digital oscilloscopes (DSOs) are identical to and hardware/software compatible with the company's 6000A scopes, which are widely used for design, though the 6000Ls lack the display. The 1U high (43.6 mm or 1.75 in.) by 19-in. rack enclosures let designers build high-density test systems in the smallest available space.

The 6000L DSOs have four channels and come in models with bandwidths of 100 MHz, 500 MHz, or 1 GHz. The sampling rate is up to 4 Gsamples/s, and the standard memory is 8 Mpoints deep. Using an XGA video interface, the 6000L can provide an update rate of 100,000 waveforms/s. The standard connectivity is 10/100 Ethernet, but USB and GPIB interfaces are also provided. The USB port lets users plug in a flash thumb drive to store screen images or test data.

For more, see "The Future Of Modular Systems."

LXI/PXI Trends

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