Synthetic instrumentation: The time is now
Greg Caesar, National Instruments -- Test & Measurement World, 4/1/2005
To reduce the cost of ownership of test systems and increase reuse, the DoD, through the Navy's NxTest program, has specified that future ATE use an architecture built on modular hardware and that makes use of reconfigurable software called synthetic instrumentation. The adoption of synthetic instrumentation represents a significant development in the specification of future military ATE systems. Successful implementation of software-based test systems requires an understanding of the hardware platforms and software tools in the market, as well as an understanding of the distinction between system-level architectures and instrument-level architectures.
Software-based systemsThe Synthetic Instrument Working Group—which includes the Department of Defense, defense prime contractors, and suppliers—defines a synthetic instrument as a reconfigurable system that links a series of hardware and software components with standardized interfaces to generate signals or make measurements using numeric processing techniques. This description shares many properties with virtual instrumentation, in which software defines the functionality of generic measurement hardware. Both technologies help users achieve greater flexibility, which in turn increases performance capabilities while reducing cost.
Fig. 1 Traditional instruments (left) and software-based virtual instruments (right) largely share the same architectural components but are based on radically different philosophies.
Figure 1 shows the architectures of two instrumentation systems. On the left is the traditional architecture found in a stand-alone instrument. On the right is the modular virtual instrumentation architecture, such as VXI or PXI. The basic hardware components of each system are nearly identical. Each has a power supply, timing and control circuits, and a measurement system that is connected to a processor via a high-speed, low-latency bus such as PCI.
Yet, the systems have two main differences. First, the traditional instrument system is closed, with no provision for user access into the internal firmware or measurement systems; the virtual instrumentation system lets the user access and control each modular component. Second, the traditional architecture implements measurements and the user interface in firmware, which is defined by the instrument manufacturer and closed to the user. In the virtual instrumentation system, the user implements measurements and interfaces in user-defined software.
Performance factorsThe performance capabilities of a modular system are defined largely by the following factors:
• Processing power. With measurement calculations implemented on a processor, the performance of that processor is a major factor in measurement throughput. With modular systems, the processor can be upgraded over time at the pace of the PC industry, therefore allowing customers such as the DoD to maintain flexibility and reconfigurability while avoiding the obsolescence of built-in processors.
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| Fig. 2 The combination of low latency and high bandwidth maximizes system performance for all measurement types, ranging from configuration, switching, and measurement tasks to the transfer of large blocks of waveforms. |
• Driver and analysis software. In virtual instrumentation systems, a critical component is the availability of powerful, easy-to-use driver software and measurement and analysis functions.
System-level viewCompanies such as PhaseMatrix, Teradyne, and Aeroflex are presenting synthetic instrumentation systems based on PXI and VXI architectures. The PXI and VXI modular architectures provide a good framework for synthetic instrumentation systems. In both architectures, instrumentation modules can be connected via a high-performance, low-latency backplane to an embedded controller for processing. The test system designer then implements measurements on the embedded controller, thereby providing for the software modularity and reconfigurability of the system.
Both VXI and PXI have mainstream acceptance in the military ATE market. The market research firm Frost & Sullivan reports that the VXI market peaked in 1999 at approximately 3100 systems sold per year, but has since dropped slightly to hold steady at 2500 to 2900 systems per year. PXI has seen tremendous adoption in military ATE applications as well, with Frost & Sullivan reporting 5400 PXI systems in 2004, with continued growth of 44%. PXI growth is focused in the industries of communications, military/aerospace, automotive, and industrial systems.
Future architecturesThroughout 2005, the test and measurement industry will continue to see more work on specifications for system-level architectures. The LXI specification (LAN eXtensions for Instrumentation), which is being proposed by the LXI Consortium, defines a stand-alone architecture that can be used for synthetic instruments. Unlike PXI and VXI modular architectures, each LXI instrument will have its own power-supply and packaging. While the specification is not yet complete, it looks likely that LXI instruments will appear in either a 1U or 2U rack-mount package, with Ethernet connectivity to a PC.
Specification work is also underway on integrating PCI Express into PXI. PCI Express provides up to 24 times the bandwidth of PCI and Gigabit Ethernet architectures today, while maintaining the very low latency of PCI. As a result, instrumentation systems based on PCI Express will be able to address a wide range of new applications, from very high-channel-system applications to the implementation of new high-resolution and bandwidth instrumentation.
Work is nearing completion within the PCI Industrial Computer Manufacturers Group (PICMG) on CompactPCI Express (EXP.0), which brings PCI Express communication to the CompactPCI standard. The PXI Systems Alliance (PXISA) will then use CompactPCI Express as the base specification for PCI Express integration into PXI. The new CompactPCI Express specification defines a new hybrid slot that maintains backward compatibility by giving users the ability to install modules with either a PCI or PCI Express signaling in a single slot.
When designing medium-to-large ATE systems, it is likely that one platform is not likely to meet all requirements. For example, certain measurement tasks may deliver your required performance only on the PXI or VXI platforms, or may only be available in stand-alone instruments with a GPIB or Ethernet interface.
Hardware interconnectivity is provided on all platforms. For example, a PXI system is easily configured with USB 2.0, GPIB, Gigabit Ethernet, and VXI MXI interfaces to other systems (MXI is a PC-to-VXI link). Additionally, software interface standards, such as VISA (virtual instrumentation system architecture) and IVI (interchangeable instrument drivers), provide a common interface to GPIB, USB, Ethernet, PXI, VXI, and other platforms.
With the DoD now actively adopting software-based test systems through the synthetic instrument initiative, software-based architectures are now mainstream. Virtual instrumentation, which extends user configurability from the system level all the way down to hardware compatibility, is suited to meet the needs of future ATE applications.
| Author Information |
| Greg Caesar is the PXI Marketing Group Manager at National Instruments, Austin, TX. greg.caesar@ni.com. |
