The architecture of a test and measurement system depends upon the needs and future requirements of its intended applications. While investing in cutting-edge technology is a must to keep pace with today’s innovation, it is not always economically viable to move to an entirely new platform. Here we compare the various test platforms, helping design engineers to make an informed selection.
LAN eXtensions for Instrumentation (LXI) is an Ethernet-enabled instrumentation standard, introduced in 2005, with the aim to ride on the success of the Internet. With the Internet gaining widespread popularity, the Ethernet bus interface was chosen due to its versatility and easy accessibility.
The LXI consortium is an industry consortium of 53 test and measurement companies that maintains the LXI specification, promotes the LXI standard and ensures interoperability. It continues to keep pace with advancements in its ecosystem. For instance, it recently announced the adoption of the IPv6 extended function, which ensures that LXI vendors approach IPv6 in a consistent way before its use in test systems becomes widespread.
LXI-based systems are built on a backbone of small modular instruments that utilise the low-cost, open-standard local-area network (LAN) or Ethernet. Of course, there are other modular instruments available that work on competing platforms, but the advantage of LXI systems is that these do away with the cost and complexity of card-cage architectures. Furthermore, the integration of instruments is a breeze.
Binoy Johnson, a hardware design engineer, says, “The difficulties faced with Ethernet-based systems also make their way here. LXI-based systems would require you to rope in your IT department for instances such as discovering the instruments and then setting them up.”
“While there have been many claims that LAN (or other buses) are ideal for all applications, the reality is that each bus has different strengths and real-world systems take advantage of multiple busses in a unified software framework. In particular, LAN is well suited for distributed applications, but not necessarily for desktop measurements or automated test”—notes the National Instruments’ whitepaper ‘Understanding LAN/LXI for Instrument Control.’
Another feature built into this platform is interoperability. LXI devices can communicate with PXI and VXI instruments by utilising the interchangeable virtual instrument (IVI) driver that the standard mandates for just this purpose.
Sadaf Arif Siddiqui, marketing programme manager, Agilent Technologies, says, “While the platforms are many, we maintain common measurement science and algorithms across all portfolios so that users can easily mix-and-match the best of products and don’t face any compatibility issue.”
VME eXtensions for Instrumentation (VXI) is the oldest platform discussed here. Launched way back in 1988, it is an open standard platform for automated test. This platform was established by Hewlett Packard (now Agilent Technologies) and Tektronix.
Although an old technology, VXI does have its plus points. “A good thing about VXI instruments is that they can be power cycled independently of the PC. This allows you to keep away from the frustrating task of rebooting the PC,” says Johnson.
VXI is based on the older VersaModule Eurocard (VME) bus developed for the Motorola 68000 line of CPUs, which is not a part of the latest computer architectures and leaves VXI unable to take complete advantage of the advances in PC technology and software. The engineers need to be trained to understand the VXI programming.
“To modernise an existing VXI system, it is more cost-effective to invest in a PXI chassis and controller to control the VXI system using Mxi-2 technology rather than purchase a new VXI controller”—notes the NI whitepaper ‘Working with VXI Platforms.’
However, as a PXI- or VXI-based system can be heavily dependent on the performance of the host PC, in order to get higher performance, you might have to spend a bigger amount in the initial stage itself.
Mike Gooding concludes his paper ‘VXI Plug ‘n Pray’ with some informative remarks: “The test industry has come a long way in providing integrators with instrument hardware that plug together electrically and physically. Their companion efforts in software have not been as simple, nor well-defined. These software efforts have produced significantly easier integration of instruments into test systems. However, we are still far from declaring successful achievement of plug-and-play ease to integration. It is still very much a plug-and-pray activity.”
AdvancedTCA Extensions for Instrumentation and Test (AXIe) is a new modular test instrument standard. It is an open standard that creates a robust ecosystem of components, products and systems targeted at general-purpose instrumentation and semiconductor test.
The AXIe platform allows designers to achieve some critical high-performance instruments like high-speed logic analysers, digitisers, serial bus protocol analysers and high-bandwidth arbitrary waveform generators, which were having some limitation on other modular platforms.
Siddiqui explains, “AXIe has been proposed to the industry by three founding members: Test Evolution Corp, Aeroflex and Agilent. Since its introduction in November, there have been four additional companies that have joined the consortium.”
“Like VXI was built on the VME standard and PXI on the PCI standard, the AXIe standard is built on an existing standard, AdvancedTCA. This standard is primarily used as a computation chassis standard by networking companies such as Cisco. It provides the capability modules that require higher power or density than can be supported by PXI,” he adds.
The ATCA specifications leveraged by this platform were developed to meet the high data rates and processing performance of the telecom industry. ATCA backplanes provide protocol-agnostic, multilane, switched-serial links among boards, and the backplanes support transfer speeds as high as 10 Gbps per lane (see the table for comparison).
Developed in 1997, PCI eXtensions for Instrumentation (PXI) was introduced in 1998 as an open industry standard to meet the increasing demands of complex instrumentation systems. Today, PXI is governed by the PXI Systems Alliance (PXISA)—a group of more than 70 companies chartered to promote the PXI standard, ensure interoperability and maintain the PXI specifications.
The PXI modular platform has been around for more than a decade and currently enjoys the largest product base. PXI describes a mechanically modular platform that uses the PCISIG-governed peripheral component interconnect (PCI) or PCI Express to control the modules. PXI-based instruments also depend heavily on a PC, and are generally Windows-centric as the specification defines only WIN32 drivers. Most PXI instruments are register-based products that require software drivers hosted on a PC for control.
“One of the main difficulties with PXI-based instruments comes from their tight integration to the PC—frequent rebooting of the PC is forced on the user during PXI development. This complicates the software installation and instrument start-up,” shares Johnson.
PXI focuses on a central processing model using products that are modular and rely on a high-speed data bus for communicating with the CPU that does the computing to perform system functions. Utilising CompactPCI, this platform gains on performance and industry adoption. Data bandwidth performance of PXI systems easily exceeds the performance of the older VXI test standard. Perhaps this is one of the reasons for the huge adoption of PXI by test managers.
“Over the last few years, the industry reached a tipping point in automated test and is now making a large-scale switch to PXI. In a recent survey of test managers from around the world conducted by NI, over 70 per cent of test managers indicated they will use PXI as the core of at least one of their next automated test systems. This is in contrast to only 30 per cent of test managers who will continue to use rack-and-stack instrumentation,” says Denver Dsouza, senior technical consultant, National Instruments.
Perhaps the best thing about PXI is that, because it is based on off-the-shelf PC technology, the performance gains come faster and at a much lower cost with each advancement in that arena.
Is there any competition between PXI and the newly released AXIe platform? Adesh Jain, applications consultant with Agilent Technologies, doesn’t think so. “AXIe is seen more like an extension of the PXI test-development model rather than as a competitor to PXI. It leverages existing standards from Advanced Telecom Computing Architecture (ATCA), PXI, LXI and IVI. The standard is broadly applicable to general-purpose instrumentation and semiconductor test,” he explains.
The author is a tech correspondent at EFY