Integrating LXI, USB, PXI Express, and Other Standards into a Hybrid Test System

With emerging instrument control standards based on communication buses like USB and LAN including LXI, developers are faced with the question of how to integrate these instruments into their system. Stand-alone instrumentation has continued to develop by taking advantage of newer buses and standards such as USB and LXI, giving users a variety of instrumentation connectivity, each with its own strengths. At the same time, your test system might require modular instrumentation buses like PXI or PXI Express for applications that require software flexibility, low latency, and high throughput. Thus, for various reasons, you might need to combine multiple buses or integrate additional buses into your test system. By taking advantage of hybrid test systems which combine components from multiple platforms, you can easily integrate new buses into an existing test system to help balance design considerations, take advantage of various technologies available, and extend the life of your system. To integrate platforms like PXI, PXI Express, VXI, GPIB, USB, LAN/LXI, and serial into one system, the software framework employed is vital to ensuring successful integration and ease of flexibility to adjust to changing instrumentation buses.

For more detailed technical information, view the Connect to Your Instruments Using the Latest Bus Technology webcast.

Hybrid Test Systems

When designing test systems, developers often have a variety of considerations to balance. In order to meet system needs, you can create a hybrid system which allows you to take advantage of various test platforms. For instance, your system might require the software flexibility and high throughput provided by modular instrumentation buses like PXI, PXI Express, and PCI Express as well as specialized functionality in a stand-alone instrument like those based on USB or LAN including LXI. Hybrid test systems combine components from multiple ATE platforms such as PXI, PCI, GPIB, VXI, USB, or LAN/LXI into one system. In addition, by using hybrid systems, engineers can easily integrate new components into their existing system without a system redesign. The key to creating these hybrid test systems is architecting the system with an extensible layered architecture that helps to streamline maintenance and upgrades.

By designing hybrid multiple platform test systems with the five-layer architecture shown in Figure 1, developers can clearly separate hardware from software. This simplifies the integration of multiple platforms and eases maintenance and upgrades by requiring only minor changes in specific layers as opposed to redesigning the system to accommodate new components. The architecture starts from the bottom with the Device I/O Layer which contains the individual instruments used that can be based on multiple instrumentation buses including PXI, VXI, USB, or LAN/LXI. Moving up the architecture, the Computing Layer includes the embedded and remote controllers used to control modular instrumentation and connect to different stand-alone buses. Above sits the Measurement and Control Services Layer with the hardware and instrument drivers that bridge the hardware to software. The fourth layer, the Application Layer is composed of the individualized test programs like a digital multimeter measurement or a power spectrum. The architecture ends with the System Management Layer that provides a framework to call the test programs as well as log data, generate reports, and manage users.

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A variety of instrumentation buses are available for stand-alone instruments including GPIB, USB, LAN including LXI, and serial. The various instrumentation buses have differing strengths making some more suitable for applications than others. With GPIB, users benefit from a proven instrumentation bus technology and a wide variety of available instrumentation while with USB, developers can take advantage of the wide availability and easy connectivity of USB. With LAN/LXI, users are able to support distributed needs and address distance needs. The variety of instrumentation buses available allows users to select the appropriate instrument for their needs based on factors including measurement functionality, bandwidth, latency, performance, and connectivity.

GPIB
Designed specifically for test and measurement and instrument control applications, the General Purpose Interface Bus (GPIB) has been a robust and reliable communication bus for thirty years and is still going strong. Using a parallel bus, GPIB provides users with an instrumentation bus with low latency. The IEEE 488.2-1987 revision strengthened the standard by defining precisely how controllers and instruments communicate through GPIB. In addition, the IEEE 488.1-2003 revision defined a high-speed data transfer mode increasing the bandwidth by 8 times to give users 8 Mbytes/s bandwidth for GPIB instrument control. Not a PC industry bus, GPIB is rarely natively available on a PC. Instead, users typically use a plug-in board, like the PCI-GPIB, or external converters like the GPIB-USB, to add GPIB instrument control functionality to their PCs. Providing a wide breadth of instrumentation with nearly ten million devices on the market, GPIB has long been a prevalent and trusted communication bus for stand-alone instruments.

USB
The Universal Serial Bus (USB) has become a popular communication bus choice for stand-alone instruments due to its ubiquity on PCs, its plug and play ease of use, as well as its high bandwidth capabilities. With the widespread availability of USB ports on computers, engineers can take advantage of the easy connection and configuration to quickly integrate USB based instruments in their system. The release of USB 2.0 established a new category of high-speed devices that can reach a maximum transfer rate of 480 Mbits/s and reduced the minimum frame latency from 1 ms to 250 us. In addition, the USB Test and Measurement Class (USBTMC) specification addresses the communication requirements of a broad range of test and measurement devices, from simple sensors to mainframes with multiple measurement functions. The USBTMC sets a protocol built on top of USB that allows GPIB-like communication with USB devices so that from the user’s point of view, the USB device behaves just like a GPIB device. For example, you can use the VISA Write function to send the “*IDN?” query and use the VISA Read function to retrieve the response. The USBTMC protocol supports service requests, triggers, and other GPIB specific operations.

LAN including LXI
A mature technology, LAN (Local Area Network) is typically used in test systems in non-measurement capacities including general networking and remote data storage. LAN is ideally suited for distributed systems and remote monitoring. With wide availability on computers today, LAN has grown in popularity as a communication bus for stand-alone instruments. Because LAN can support long cable lengths using switches, routers, and repeaters, users do not have to keep instrumentation local to each other and can distribute stand-alone instruments throughout a system. In addition, the VXI-11 specification provides a standard set of protocols for communication with message-based instruments over TCP/IP. Part of the VXIbus set of specifications, the VXI-11 specification defines a network instrument protocol to be used for controller - device communication over a TCP/IP network. The LXI (LAN eXtensions for Instrumentation) standard defines classes of instruments based on LAN and includes optional synchronization based on IEEE 1588 technology as well as optional triggering specifications with the LXI trigger bus. LXI instruments are a subset of LAN stand-alone instruments. When applying these additional specifications of the LXI standard, users can add a level of timing and synchronization to low frequency distributed and remote monitoring systems.

With all of these instrumentation buses, users still face the same challenges they see with any stand-alone instrument. While stand-alone instrumentation can be beneficial in that they might provide specialized functionality or performance, these instruments are vendor-defined, proprietary instruments. As a result, developers will face limited integration and expandability and are forced to work with a proprietary software model. For more detailed technical information, view the Connect to Your Instruments Using the Latest Bus Technology webcast.

Modular Instrumentation Buses: PCI, PXI, PCI Express, and PXI Express

With modular instrumentation, developers can take advantage of open, multi-vendor standards and software flexibility to create a user-defined solution for their specific application needs. By using a modular architecture and open standards, users can easily integrate components from multiple vendors into one system and scale the system as needed. By providing high throughput, low latency, and software flexibility, developers can create a user-defined test system that is able to meet many application performance needs. Modular instrumentation buses provide better throughput and latency in comparison to stand-alone instrumentation buses. This allows users to meet many application needs like high speed data streaming. By taking advantage of an open software model and PC processing power, users can extract all measurements needed from the data provided by modular instruments. This gives users the flexibility to design a system as needed and only pay for the components required for the application.

PCI and PXI
Introduced in the early 1990s, PCI was first implemented as a chip-to-chip interconnect as a replacement for the fragmented ISA bus. The PCI bus brought a number of advantages over previous bus implementations including processor independence, buffered isolation, bus mastering, and true plug-and-play operation. Typically not used directly for instrument control, the PCI bus serves as a peripheral bus to connect GPIB or serial devices for instrument control. Also, due to its high bandwidth, PCI is used as a carrier bus for modular instruments where the I/O bus is built into the measurement device.

PXI combines PCI electrical-bus features with the rugged, modular, Eurocard mechanical-packaging of CompactPCI, and then adds specialized synchronization buses and key software features. This makes it both a high-performance and low-cost deployment platform for test, measurement, and control systems. These systems serve applications such as manufacturing test, military and aerospace, machine monitoring, automotive, and industrial test. With PCI based communication, PXI benefits from low latency and high throughput at 132 Mbytes/s. In addition, PXI provides additional timing and triggering with a 10 MHz reference clock, an eight line trigger bus, and STAR trigger lines which provide dedicated trigger lines with inter-module skew within 1 nanosecond. PXI is heavily used as a platform for modular instrumentation providing an attractive alternative to traditional stand-alone instrumentation through compact, high performance measurement hardware devices with integrated timing and synchronization resources.

PCI Express
As PC applications become more bandwidth intensive, the PCI bus is reaching its physical limits in many situations. As a result, the PCI-SIG, the standards body that defines PCI, has introduced PCI Express, with the main goals of providing a scalable, low-cost interface that serves many differing markets and providing compatibility at the software level with existing PCI card drivers and software. Compatibility with the PCI addressing model is maintained to ensure that all existing applications and drivers operate unchanged. An evolution of PCI, PCI Express provides a basic communication lane of 250 Mbytes/s in each direction in a x1 implementation up to 4 Gbytes/s in a x16 implementation. In addition, by connecting each PCI Express slot to a switch fabric, PCI Express provides independent bandwidth to each slot as opposed to the shared bandwidth in PCI. Designed with compatibility needs in mind, PCI Express uses a well-designed layered architecture to ensure compatibility with future generations as well as software compatibility with PCI. As with PCI, PCI Express is typically not used directly for instrument control but as a peripheral bus to connect GPIB devices to PCs for instrument control. Because of its tremendous speed, PCI Express can be used as a carrier bus for modular instruments.

PXI Express
As its use of PCI in the communication backplane helped drive the rapid adoption of PXI, PXI has the ability to meet even more application needs by integrating PCI Express into the PXI standard. By taking advantage of PCI Express technology in the backplane, PXI Express increases the available PXI bandwidth from 132 MB/s to 6 GB/s for a more than 45X improvement in bandwidth while still maintaining software and hardware compatibility with PXI modules. With this enhanced performance, PXI can reach into many new application areas, many of which were previously only served by expensive and proprietary hardware. With the software compatibility of PCI Express, the standard software framework provided by PXI will carry into PXI Express. To provide hardware compatibility, the new CompactPCI Express specification defines a new hybrid slot that gives engineers the ability to install modules with either a PCI or PCI Express architecture in a slot. With this technology, engineers and vendors can preserve their existing investments in PXI systems and products through both hardware and software compatibility.

Importance of Software Framework

Hybrid systems enable the combined use of both modular instrumentation buses and commercial bus interfaces for stand-alone instruments so that users can benefit from the high speed and flexibility of modular instrumentation and use existing or specialized stand-alone instruments. The software framework employed is vital to ensuring the successful integration of these various platforms into one system and the ease of flexibility to adjust to changing instrumentation buses. With the growth of commercial buses, the software layers of the hybrid architecture are becoming even more important. Because commercial buses by definition change very quickly, the software layer becomes more important by providing a layer of abstraction to adapt and keep up with changing commercial buses.

An integral part of the software framework is the Measurement and Control Services layer which includes flexible device drivers that bridge hardware and software and simplify configuration and integration of hardware with the test code. To seamlessly integrate hardware into software, engineers need drivers that offer fast performance, programming flexibility, and a consistent and scalable application programming interface (API). The Virtual Instrumentation Software Architecture (VISA) standard provides a common API to communicate with the driver software independent of the instrumentation bus used. VISA provides a standard set of function calls to communicate with instruments based on PXI, VXI, GPIB, LAN/LXI, etc. By providing controller interchangeability and bus interchangeability, VISA allows developers to interchange controllers used. In addition, instrument drivers are a critical piece of the application since they abstract instrument functionality in an appropriate way for use in the application development environment. A standard for instrument drivers, Interchangeable Virtual Instrument (IVI) drivers are useful for instrument replacement since they allow engineers to interchange instruments in a system without modifying test software for specified classes of instruments such as oscilloscopes or switches. An IVI instrument driver that conforms to one of these classes may be substituted with another instrument of the same class, regardless of manufacturer or bus connection. For instance, by using IVI drivers, a developer could use the same code to communicate with a PXI, VXI, GPIB, or LAN/LXI instrument.

Summary

Hybrid test systems enable users to combine modular instrumentation buses with peripheral buses for stand-alone instrumentation. In order to take advantage of new instrumentation buses like PXI and PXI Express or USB and LAN/LXI instruments, developers can use hybrid test systems to integrate multiple platforms into one system. With the ability to mix platforms, developers can utilize the high speed, flexibility, and customizable software available with modular instrumentation buses in conjunction with stand-alone instrumentation. The key to successfully integrating instruments based on buses like PXI, USB, or LAN/LXI to a system is taking advantage of the software architecture and abstraction available through tools like VISA and IVI. With this software, even as commercial buses change, users have a layer of abstraction that minimizes the effect of changing buses.
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