VTI Instruments platforms, software, applications and product families based knowledge base and frequently asked questions (FAQ)
HARDWARE SYNCHRONIZATION
IEEE-1588 provides incredible advances in over the-wire synchronization; however, there will always be instances where additional accuracy is required. The most accurate and deterministic synchronization mechanism between multiple devices involves the implementation of a hardware trigger interface. As a result of this requirement, the LXI standard defines a high performance trigger interface referred to as TriggerBus.
The LXI Trigger Bus is required in LXI Class A devices, and provides the link between all devices in the test system for both triggering and clock signal distribution Deterministic trigger generation and propagation between multiple devices is accomplished with an eight-channel, multipoint low voltage differential signal (LVDS) interface. This architecture permits individual lines to be configured as a source and/or receiver and supports external, time based or software generated triggering as well as clock distribution. Common topologies are supported including star, daisy-chain, and hybrid configurations providing the flexibility to distribute the trigger lines as dictated by the application requirements.
Additional flexibility is realized with the addition of a star hub; this device permits very tight trigger tolerances to be maintained throughout a large distribution network. Many data acquisition applications require a large number of parallel-channel instruments to be synchronized to a common clock or trigger pulse. Furthermore, the clock signals are often linked to a system clock such as IRIG-B or GPS for overall time correlation. The Trigger Bus interface simplifies the process of distributing these signals, and when proper cabling and interconnects are used, ensures low channel-to-channel skew.
LAN SYNCHRONIZATION
LAN synchronization, incorporating the IEEE-1588 Precision Time Protocol (PTP), highlights another fundamental advantage of LXI Class B devices that is ideal for distributed measurements. This completely over-the-wire approach provides an ideal mechanism to synchronize multiple instruments separated by hundreds or thousands of meters.
PTP defines a precision clock synchronization protocol for networked measurement and control systems. The protocol is designed to enable the synchronization of systems that include clocks of different precision, resolution and stability. Sub microsecond accuracy can be achieved with minimal network and local clock computing resources, and with little administrative attention from the user. There are several ways in which PTP can be implemented ranging from user level software control, to kernel-level driver modifications, to hardware implementations utilizing dedicated FPGA devices. The highest level of precision is obtained when hardware implementations assist in the time stamping of incoming and outgoing network packets or frames; delay fluctuations can be in the nanosecond range with this approach. PTP provides multiple device synchronization while eliminating the need for external cabling between devices. Utilization of this approach is less accurate than hardware triggering; however, Giga-bit Ethernet can provide synchronization times in the hundreds of nanosecond range which may be suitable for slower data acquisition rates common with thermocouple measurements.
SYSTEM CALIBRATION
It is critical that test engineers are able to confidently rely on their measurement devices, and the integrity of the data they produce. Generally a major part of this confidence is achieved through instrument calibration and traceable verification standards. Specifically, a traceable source is used by the instrument undergoing calibration to both adjust and verify the quality of measurement. Typically, this has been viewed as a painful but necessary process involving system disassembly and downtime.
In most cases, test engineers are required to disassemble test stations and send each
individual instrument to their respective vendor’s factory for calibration. Some costly workarounds to such a problem include ordering spare instruments for each test station, hiring an outside calibration service, or construction of an in-house calibration laboratory. Reducing these costs and alleviating the downtime associated with the calibration process ultimately benefits all test and measurement applications. Leading instrumentation manufacturers have invested significant engineering resources to vastly simplifying the calibration process, and have also added features that guarantee
measurement accuracy. By taking advantage of the distributed measurement advantages of the LXI, and designing instruments with on-board precision voltage references, calibration becomes more convenient and more reliable than ever.
Specifically, the LXI standard allows vendors to embed an easy-to-use calibration process directly into the instrument’s firmware, allowing the end user to execute a complete calibration in minutes, at the click of a button. Furthermore, a precision on-board voltage source can extend the calibration capabilities by also offering a “self calibration” routine which the end user can initiate at any time. This on-board procedure guarantees customers the most precise measurements, regardless of changes in surrounding environmental conditions. The fully integrated web interface that the LXI specification requires, streamlines the calibration process, making it truly turnkey. The embedded user interface, combined with a built-in internal voltage reference, makes calibration in any location more convenient and cost effective. All that is necessary for many instruments to perform a complete NIST traceable calibration is a host computer and precision voltmeter. Simply connect the voltmeter to the instrument utilizing banana jacks, access the web interface using a standard internet browser, and click the button that commands the instrument to perform the automatic factory calibration. The instrument’s firmware can be configured to recognize and communicate with a number of different voltmeters, including the most commonly used instruments, to measure the on-board precision voltage source. Storing this value, the instrument can route the source back through the input signal paths and reliably perform internal adjustments. When compared to other approaches, this takes very little user interaction,and no calibration software development investment. Furthermore, the simplified equipment setup enables the process to be executed almost anywhere. Instead of sending the instrument to the metrology lab, test engineers can send the metrology lab to the instrument. In addition to a turn-key, cost effective, full calibration procedure, additional features can guarantee the highest level of measurement accuracy before each and every data acquisition sequence. Specifically, the device can be designed with a self-calibration procedure that is executed directly from software or a web browser interface. Before any measurement is initiated, users can initiate a self-calibration sequence which routes the precision source back through the input signal.
This process makes minor gain and offset adjustments by routing the source through the actual signal path. Whenever the device undergoes any changes in its surrounding thermal environment, which is typical with many data acquisition applications, this process can be executed to ensure the highest degree of measurement quality.
DISTRIBUTED DATA ACQUISITION
Distributed measurements provide the user with numerous advantages over more traditional centralized approaches, and have become increasingly popular especially in data acquisition applications. Advances in electronic component designs and packaging, combined with the LXI interface standard, have provided the basis for a powerful new generation of instrumentation.
The strategic placement of data acquisition instrumentation around or near the test article can result in significant advantages. The benefits of this approach include:
-Quicker Setup Due to Instrument Proximity
-Simplified Calibration
-Excitation Source Closer to Bridges
-Cabling Noise Reduced
-Less Debug Time Required
-Less Installation Time Required
-Reduced Costs Associated With Cabling
-Simplified Maintenance
-Improved Transportability
Clearly these benefits encompass the entire operational life of a project including installation, maintenance, support, and calibration. Cost savings begin at the time of installation by greatly reducing the cost of cabling and associated installation, debug and testing. Simplified calibration and excitation by placing instrumentation near the associated transducers further improves system performance. Furthermore, increased accuracies are realized by reducing the effects of noise on transducer cables and difficulties calibrating such devices
LXI – The Future of Calibration
Test and measurement applications involve the test and analysis of a wide array of products such as satellites, rocket engines, automotive engines, medical devices, and in most cases measurement integrity is of the utmost importance. Countless dollars and man hours go into the design of these products, and accurate testing is essential. Poor measurement quality can be devastating resulting in schedule setbacks, substantial monetary losses, and endangerment of human safety. It is critical that test engineers are able to confidently rely on their measurement devices, and the integrity of the data they produce.
Measurement integrity or confidence is achieved through instrument calibration and traceable verification standards. A traceable source is used by the instrument undergoing calibration to both adjust and verify the quality of measurement. This has been viewed as a painful but necessary process involving station disassembly and downtime. Test engineers are required to disassemble test stations, and send each individual product to their respective vendor’s factory for calibration. Some costly workarounds to this problem include ordering spare instruments for each test station, hiring an outside calibration service, or construction of an in-house calibration laboratory. Reducing these costs and alleviating the downtime associated with the calibration process ultimately benefits all test and measurement applications.
VTI has invested significant engineering resources into simplifying the calibration process for customers, and adding features that guarantee measurement accuracy. This is achieved by taking advantage of the benefits of the next generation measurement platform With respect to improving calibration the notable break through is that LXI class
A compliant products are capable of peer to peer device communication independent of a host controller. Taking advantage of this and designing instruments with on-board precision voltage references provides the means for making the calibration process more convenient and more reliable than ever. Now vendors can embed an easy-to-use calibration process directly into the product's firmware, allowing the end user to execute a complete calibration in minutes, at the click of a button. The precision voltage source extends the calibration capabilities by also offering a “self-calibration” routine which the users can initiate at any time. This on-board procedure guarantees users the most precise measurements, regardless of changes in surrounding environmental conditions.
INTERFACE PROTOCOLS
The LXI Standard is based on TCP/IP, a set of protocols, notably UDP and TCP, which were defined for the ARPAnet and now form the basics of Internet communication. Briefly, the protocols specify how hosts are addressed, how messages find their way between hosts (i.e., routing), and the general formats for those messages. The IP protocols form the Network Layer, Layer 3 of the ISO OSI Model, and TCP and UDP form Transport Layer 4. TCP is connection-oriented, setting up a persistent connection between two hosts, and it is reliable.
Reliable in this case means that an application that sends a message via TCP will know if that message is received at the other end. TCP uses a number of techniques to guarantee delivery: requiring messages to be acknowledged, automatic retransmission of missed messages, and timeouts to detect missed messages and hosts that are unavailable. With all of these features it should not be surprising that TCP has a larger message overhead when compared to other interfaces; this is commonly referred to as first-byte latency.
OPEN ARCHITECTURE
The advantages of adopting an open architecture test platform spans both hardware and software, providing a wide range of choices not available to those locked into proprietary designs. An open hardware approach guarantees that a well-defined set of signal and interface characteristics have been adopted, and that multiple vendors will provide support and products. All of this results in reduced cost, extended test system life cycles, and commercial-off-the-shelf (COTS) product availability.
The VXIbus is a prime example of the features that can be realized through open architecture designs; this platform continues to evolve providing viable high-density modular solutions utilizing other open-platform interfaces such as LXI.The LXI Standard was designed with hardware independence in mind and this is largely accomplished by leveraging well established industry standards.
The LAN interface for LXI devices is based upon IEEE 802.3 standards and it is intended to support current and future networks with 10/100baseT or faster connections. Utilizing this standard greatly reduces entry obstacles for instrumentation manufacturers primarily due to implementations that have been driven by the PC market. The very nature of this interface also makes it ideal for many distributed applications; standard CAT-5 copper connections can reach 100 meters point-to-point and fiber optic implementations can span several kilometers without additional switches or routers. Open software support also plays an important role in reducing development costs and ensuring life cycle management.
Software independence is built upon well-defined standards such as the familiar plug&play and IVI driver sets. This is further extended into the application development environment providing the freedom of choice to select software environments best suited to meet specific needs. Even with all of the benefits of the LXI platform listed above, there are inherent differences between this TCP/IP Ethernet-based interface and other serial and parallel interfaces currently in use. Understanding these differences will simplify the integration task greatly.
HISTORICAL TEST PERSPECTIVE
The introduction of the personal computer (PC), accompanied by GPIB instrumentation, resulted in a production test revolution. Smaller, inexpensive test stations were configured and maintained at a fraction of the cost of larger proprietary test behemoths. These new stations started to move towards open standards using the SCPI command language and were programmed with Basic or Quick Basic. Both the PC and programming languages continued to evolve with processing speeds eclipsing many megaflops, and languages expanding to include C/C++, Visual Basic, and graphical environments such as HP VEE and LabVIEW.
The increased program execution speeds were welcomed by all, but resulted in unforeseen troubles to the test engineer. Suddenly, stable proven test programs were failing when ported over to new computers with expanded RAM and faster processors. It soon became apparent that timing was everything; typical functional test timing sequences that had executed flawlessly in the past now failed. The increased program execution speeds did not compensate for delays and settling times that were inherent in the older systems. As a result it became necessary to modify test sequences or insert delay loops to slow program execution.
These problems can be attributed to a number of different issues such as program execution exceeding settling times, measurements beginning before instrument aperture times have been met, or streaming commands beyond the buffering capabilities of the device. Architectural changes, even as simple as improving PC speed, can have an observable impact on test execution. Even the most innocuous enhancements, such as adopting the most widely used open-architecture communications platform in the world, can have unforeseen consequences within the test community .
CROSS-PLATFORM FUNCTIONALITY
Test engineers are commonly tasked with integrating hardware utilizing different physical interfaces such as RS232, GPIB, MXI-2, and IEEE-1394. This combination of interfaces is inevitable because the best solution may not be available in a designated package, resulting in added complexity and integration time. Even though LXI-based instruments reside on a common inexpensive bus, and provide many attractive features, the need for cross platform integration will continue throughout the industry adoption cycle.
The Ethernet interface is platform and operating system independent, and is integrated into nearly every computer available on the market ensuring long-term stability. Even with this stability, it is still critical to characterize real-world use-case scenarios in order to determine if LXI can address all applications and, if not, what the alternatives are. Mixing these various interface types requires an understanding of how each one operates, and how this inherent operation will affect the overall test sequence and performance for different applications. Several scenarios are easy to imagine: The evolution of communication interfaces continues to drive the need to understand these types of issues, a task that test engineers are not unfamiliar with.
A NEW INTERFACE EMERGES
Faced with the growing need for a replacement for an aging IEEE-488 (commonly known as GPIB), the instrumentation community established the LXI Standard. It provides the flexibility and performance common on backplane-based implementations, such as VXIbus, to the next generation of small to medium sized systems. LXI-based instrumentation also provides an ideal distributed measurement architecture.
The LXI Consortium was co-founded in the fall of 2004 by VXI Technology, Inc. to address this need. Membership has since grown to nearly 50 leading test and measurement companies from around the globe, making LXI the fastest growing programmable instrumentation bus standard in the history of the industry. connectivity, ease of cable routing, and inexpensive networking hardware clearly make Ethernet an attractive alternative to current parallel bus and other serial-based interfaces. Many of the attributes that have made Ethernet so popular to the computer industry are also attractive to the instrumentation community; however, instrumentation manufacturers and users alike demand key functionality not typically associated with this vanilla flavor Ethernet.
Determinism, synchronization, triggering, device discovery, and predictable software driver interoperability are all essential functional requirements that extend beyond standard Ethernet. Different application areas will also drive other functional requirements, and these requirements can vary tremendously with applications ranging from bench top to functional test to distributed data acquisition. Concrete functional requirements are not the only aspect that must be considered, and this has been clearly demonstrated to many engineers as test system methodologies have evolved.
LXI: A SHIFT IN THE FUNCTIONAL TEST PARADIGM
Abstract- LAN-based instrumentation, leveraging the advantages that LXI (LAN eXtensions for Instrumentation) has to offer, has resulted in a paradigm shift in functional test methodology. The early days of test program design involved utilizing test instrumentation with parallel interfaces such as IEEE-488. These interfaces were slow and had limited bandwidth, but the response to individual message queries was relatively quick and ideal for multiple command response sessions used for setup, control, and data gathering purposes.
The very nature of LAN-based instrumentation requires a re-evaluation of this approach and with it the classis test paradigm. The TCP/IP protocol stack, triggering, synchronization, security, and inter-device interaction are fundamental areas of LAN-based test that provide tremendous opportunity, but also involve understanding any subtle differences that may exist. Other parallel advances in instrumentation such as powerful inexpensive digital signal processors, FPGA devices, and large onboard memory also has changed the way particular test sequences are implemented.
The need to rely on the host computer for processor intensive operations is no longer needed, and therefore reduces the burden on both the communications interface and the host. This paper will address how the adoption of LAN-based instrumentation changes the way test engineers approach test program set (TPS) design. It will also provide recommendations to leverage the strengths that LAN-based test has to offer.
THE FAMILIARITY OF THE WEB PAGE
HTML and JAVA driven web pages have been enormously popular for Internet users world-wide as web page designers focused on elaborate, yet user-friendly interfaces which can be used for all aspects of a company’s business from initial introduction, through the sales ordering cycle. Test instrumentation vendors have been able to apply this technology to their LAN-based products, providing a soft-front panel (GUI) that allows a user to control a device through common browser applications such as Internet Explorer or Firefox.
The LXI specification requires that all LXI class instruments include an embedded web page for direct communication to instruments through a browser. This is a powerful tool and as applied to the RFIU Core, allows users to view the system contents; component attributes and also provides a password-protected mechanism for manual control of the device. embedded JAVA applet that gives the user the ability to view system layout, component attributes and allows for direct control of functionality This is a very convenient mechanism for first-line maintenance as there is no software that is required on the PC aside from the internet browser of choice. A field technician can access and control the box simply with a laptop and a standard 10/100T interface.
OPEN ARCHITECTURE SOFTWARE
While it is important for hardware engineers to have the freedom to choose the component type and vendor that is part of their design, it is equally important to provide similar freedom to software engineers who are responsible for providing the tools necessary for integrating the RFIU in an automated environment.
The LXI specification requires all LXI compliant devices be delivered with an Interchangeable Virtual Instrument (IVI) programmer’s interface (API). The RFIU Core incorporates the IVISwitch definition as its API and defines instrument specific functions for programming non-switch devices through IVI instrument specific calls. An additional ‘IVI-like’ interface provides the ability to operate under non-Windows operating systems. IVI inherently provides path-level programming that can significantly simplify development and output code that closely resembles the system architecture.
FLEXIBLE HARDWARE PLATFORM
The functional block diagram depicts the two primary components of the LXIbased RFIU Core, the digital interface/communications board and the driver/digital I/O board. It is a flexible and scalable architecture designed to accommodate larger channel count requirements by simply adding additional driver boards. Microwave component manufacturers provide a multitude of component types, such as relays, attenuators and filters and various options for controlling and reading back device status. Latching and non-latching relays present an additional variable in that non-latching relays require a constant flow of current for contact closure, where latching relays require a short pulse of current, with the minimum pulse width dependent on the type of relay.
The driver board is designed to accept sources in the range of 5 –48 V to account for the variety of options offered by component manufacturers. On-board logic is able to read from a file stored in non-vol that defines latching or non-latching functionality and the FPGA is then responsible for providing the proper sequencing for applying power to the relays without burdening the application code. There is an additional 32 bit parallel bus that is available for directly controlling TTL logic devices.
The digital controller board is the main communications interface to the host PC. It embeds the main features required by the LXI specification including LAN discovery and reset mechanism, a robust graphical user interface (web page), IEEE-1588 synchronization capability and LVDS/LXI triggering. It ensures that a custom design connected to the driver board complies with LXI requirements, facilitating interoperability with other LXI devices on the system network. This greatly reduces integration time by providing a well-documented communications interface, and delivers a framework in LXI that is designed to mitigate obsolescence.
INTEGRATING THE LXI STANDARD
A critical component of many microwave test systems is the RF interface unit (RFIU) which sits between source and measurement devices and routes, splits, modulates, attenuates and otherwise distributes system I/O across the various system components. The layout of a microwave interface unit is generally unique to a given test system.
A modular building block approach, such as that which has been implemented within the VXIbus framework, can be used to provide a flexible design in combination with external fixtures and cabling. Many RF and microwave systems, however, require an interface solution that is driven by specifications that demand tighter control of phase matching and loss management that can only be satisfied by a fully integrated custom design. However, the methods for interfacing to the box and the components, as well as the software tools required to control its operation, can vary considerably which puts the design at risk when trying to meet aggressive implementation schedules. Too often, compromises are made at development time with regards to software development and communications interfaces at the expense of documentation and reusability. This has a ripple effect on obsolescence mitigation and maintainability over time. By developing a scalable microwave interface infrastructure based on the LXI platform, custom solutions can more closely resemble a standard open-platform product, greatly minimizing development time, and promoting a common architecture that can be applied to virtually any requirement.
LAN SYNCHRONIZATION
LAN synchronization, incorporating the IEEE-1588 Precision Time Protocol (PTP), highlights another fundamental advantage of LXI Class B devices. PTP defines a precision clock synchronization protocol for networked measurement and control systems. The protocol is designed to enable the synchronization of systems that include clocks of different precision, resolution and stability. Sub microsecond accuracy can be achieved with minimal network and local clock computing resources, and with little administrative attention from the user.
There are several ways in which PTP can be implemented ranging from user level software control, to kernel-level driver modifications, to hardware implementations utilizing dedicated FPGA devices. The highest level of precision is obtained when hardware implementations assist in the time stamping of incoming and outgoing network packets or frames; delay fluctuations can be in the nanosecond range with this approach.
PTP provides multiple device synchronization while eliminating the need for external cabling between devices. Utilization of this approach is less accurate than hardware triggering; however, Giga-bit Ethernet can provide synchronization times in the hundreds of nanoseconds range. Synchronization of hybrid test systems, including standalone instruments as well as VXI-based subsystems, can easily be accomplished with a test approach that utilizes time based execution. The background PTP functionality ensures that each device is synchronized to within a high degree of certainty of one another.
Test sequences can then be initiated at specific time slots based upon the relative PTP time. An LXI-VXI slot-zero control bridge can respond to a PTP based event in a number of different ways ranging from initiating a specific control source/measure function on a discrete instrument to generating an entire sub-system test sequence involving multiple source, measure, and switch devices.
HARDWARE TRIGGERING
The most accurate synchronization mechanism between multiple devices, regardless of the platform, involves the implementation of a hardware trigger interface. Most functional test applications follow a relatively straight forward approach that involves defining a signal path, applying a stimulus to the unit under test (UUT) and then measuring the results.
The key to generating accurate results is often linked to the timing associated with the test sequence, and this is where the trigger interface comes into play. As a result of this requirement a high-performance trigger interface, the LXI TriggerBus has been implemented in LXI Class A devices and provides the link between all devices in the test system for both triggering and clock signal distribution.
Deterministic trigger generation and propagation between multiple devices is accomplished with an eight-channel, multipoint, low-voltage, differential signal (LVDS) interface. This architecture permits individual lines to be configured as a source and/or receiver and supports external, time-based or software-generated triggering as well as clock distribution. Common topologies are supported including star, daisy-chain, and hybrid configurations, providing the flexibility to distribute the trigger lines as dictated by the application requirements.
Additional flexibility is realized with the addition of a star hub; this device permits very tight trigger tolerances to be maintained throughout a large distribution network zero control bridge, providing a mechanism to link a VXI chassis and all other LXI hardware. The LXI-VXI slot-zero control bridge will provide a direct extension of the eight VXI trigger lines to any external device, providing the ability to individually control specific instruments and switch devices with the VXI chassis. This type of flexibility will provide the user the ability to integrate stand-alone instruments, such as spectrum analyzers and power sources, into a homogeneous test environment leveraging the strengths of each subsystem.
Required functionality, such as a high-density, high performance switch subsystem, uniquely inherent in VXI-based devices, can function transparently with LXI-based synthetic instruments without additional integration activities
SOFTWARE INTERFACE
Another key attribute of any bridge device is the ability to seamlessly transition between interfaces A typical test system will have significant man hours invested in software and test program development; therefore, the ability to minimize the impact on currently fielded software is essential.
Updating a traditional VXI-based system should be as simple as removing the existing slot 0 interface, updating the instrument drivers, and installing the new LXI-VXI slot-zero control bridge. The transition to a LXI-VXI bridge will typically involve no user code modifications. This is primarily because of the functionality provided by the VISA I/O libraries, which are designed with utilities for configuring, programming, and troubleshooting instrumentation systems. There are essentially two widely accepted VISA implementations in the market, Agilent VISA and National Instruments (NI) VISA, and the upgrade process varies slightly depending on the architecture that the manufacturer has adopted.
The process defined in assumes that the LXI-VXI slot-zero control bridge implements the Agilent VISA interface, and illustrates the steps required for upgrading the bridge in both a native Agilent and native NI environment One step of particular interest involves adding the interface using Agilent’s Connection Expert. This step essentially maps the LXI compliant resource string to the VXI resource string that would be in use in the existing system. Once this has been completed all existing code will execute without any additional modifications.
BRIDGE DEVICE FUNCTIONALITY
One key purpose of a bridge device is to provide the communications link between the platform of interest and the LXI network. Essential characteristics of this device include compliance to all LXI Class C requirements which define the network and LAN functionality; device discovery,IP address allocation, and behavior in the event of network conflicts are just some of the requirements. However, how will a platform that incorporates a host interface to control a variety of independent, highly synchronized devices, such as VXI or PXI instruments and switches, function within this environment?
An LXI-VXI slot-zero control bridge, for example, must perform all of the functions expected of any VXI controller, in addition to providing the LXI interface functionality for the external network.This includes providing a communications path for the host computer, facilitating instrument and switch card discovery (within the chassis),memory allocation, trigger distribution and generation, and error reporting. Individual instruments within the chassis will continue to be identified utilizing the familiar VISA resource name scheme:
TCPIP[board]::host address[::LAN device name][::INSTR]
TCPIP::10.1.2.119::INSTR
Each of the instruments or switch cards within the chassis will function independent of the LXI interface, and retain all of the exceptional timing and synchronization characteristics that have driven wide industry acceptance of platforms such as VXI. Internal backplane functionality is unaffected by the LXI bridge, and performance characteristics such as data transfer rates, device triggering, high-speed local bus, and power supply capabilities are retained. Furthermore, other devices (LXI, LXI-VXI Controllers, or non-LXI) can easily interface with bridge devices, thanks to the functionality described in the Hardware Trigger Section.
Application Specific Products
The EX7000 family of microwave switching products begins with an Ethernet controlled mainframe based on the LXI platform. Up to 12 miniature microwave building blocks can be added per 1U of rack space. There is also a removable tray that can house components such as splitters, combiners and attenuators, for custom configurations. The EX7000 series is a scalable architecture and available in multiple rack-U height configurations, with up to 72 relays in a 6U mainframe.
The EX7000 incorporates a flexible and modular approach to RF/microwave sub-assembly design, by allowing the user to populate single or multiple mainframes with the exact number and types of components that the application demands. If expansion is required, additional slices can easily be added. This approach not only provides maximum amount of flexibility during the initial system configuration phase, but also permits the ability to modify the system at a later date without being faced with additional non-recurring engineering and design costs.
As with the majority of our LXI devices, the EX7000 series is designed as a class A compliant device. As such, it has the necessary features to support device synchronization and timing within an LXI network via IEEE-1588 and the LXI Trigger Bus. This is critical for maintaining tight deterministic control with instrumentation that is necessary for the most challenging RF applications. A full-featured graphical user interface is supplied that provides immediate control of each relay. Additionally, an IVI driver is supplied to facilitate integration into an automated test environment.
Instrument Grade Ethernet LXI
The LXI Specification extends the capabilities of typical Ethernet by addressing key functional areas that are necessary to ensure instrument interoperability, performance, and usability.
The primary sections include the following:
- Physical
- LAN Device Synchronization and LAN-based Triggering
- Module-to-Module Data Communications
- Hardware TriggerBus
- Programmatic Interface
- LAN Configuration
- WEB Interface
The base level of LXI compliance features device discovery, web browser control, and a common application programming interface (API). Web browser functionality provides out-of-the-box operation without the installation of any software or drivers, while device discovery identifies all LXI compliant devices on the network, simplifying startup and configuration.
IEEE 1588 further extends LXI instrument functionality by adding LAN-based synchronization. IEEE-1588 defines a precision clock synchronization protocol for networked measurement instrumentation exclusively over the LAN connection
Instruments can be synchronized, and activities initiated, using LAN commands with accuracies in the sub-microsecond range. This approach is ideal for distributed measurement applications. The addition of the TriggerBus Hardware Trigger Interface provides the highest level of synchronization and platform interoperability. This interface is ideal when applications require hardware-level triggering for deterministic command, response and handshaking; system-level clock and trigger distribution is also possible. The TriggerBus interface also provides a convenient mechanism for interfacing to other open-standard platform architectures, such as the VXIbus, thus leveraging current test hardware investments.
LXI test and measurement modules are optimized for use in design validation and manufacturing test systems with LAN connectivity enabling modules to be accessed from anywhere in the world. Unlike a modular cardcage, LXI modules are self-contained with their own processor, LAN connections, power supply and trigger inputs. Signal I/O connections are typically located on the front panel, with LAN and power connections located on the rear. LXI modules eliminate the need for displays, buttons and dials traditionally found on rack-and-stack instrumentation. They use standard web browsers for troubleshooting and implement IVI-COM drivers for communications, thus simplifying system integration
LXI Why
The LXI consortium was co-founded by VXI Technology (VTI) and Agilent Technologies in 2004 to offer the industry an open-architecture instrumentation bus that would leverage all of the benefits of Ethernet for test and measurement. The intent was to provide ‘one’ forward looking architecture for small-to-medium density and distributed applications. By leveraging the two most successful instrumentation platforms (VXI and GPIB), LXI has been adopted and carried forward by approximately 50 of the leading manufacturers of test and measurement instrumentation.
LXI’s compact, flexible packaging, high-speed I/O, and reliable operation meet the needs of R&D and manufacturing engineers delivering electronics for the aerospace/defense, automotive, industrial, medical, and consumer electronics markets.
The VXIbus is an ideal platform for our customers that require high-channel density in their applications. LXI now allows us to develop leading edge products with the same longevity advantages that VXI has given the industry, for the applications that VXI was not designed to address
what is LXI
LXI (LAN eXtensions for Instrumentation) is a powerful test instrumentation platform supported by the world’s leading instrumentation companies. LXI combines the synchronization and triggering features inherent in VXIbus and IEEE-1588 devices, with the benefits of Ethernet and GPIB.
The standard defines a platform for small to medium channel density and distributed modular instruments using low-cost, open-standard LAN (Ethernet) as the system backbone. LXI was developed to offer the size and integration advantages of modular instruments without the cost constraints of card-cage architectures. The standard continues to evolve, leveraging current and future LAN functionality, far exceeding legacy T&M connectivity capabilities.
Six key attributes set LXI apart from other architectures:
- Speed, simplicity, worldwide reach, low implementation cost, and backward compatibility of LAN.
- Quick, easy configuration through the intuitive web interface built into compliant instruments.
- Simplified programming and greater software reuse through IVI drivers.
- The ability to create hybrid systems that include LXI, GPIB, and VXI.
- Enhanced system performance and event handling via hardware- and LAN-based triggering modes.
- Synchronization of local and remote instruments through the IEEE-1588 precision time protocol.
Hardware Trigger
The most accurate and deterministic synchronization mechanism between multiple devices involves the implementation of a hardware trigger interface. As a result of this requirement, the LXI standard defines a high-performance trigger interface referred to as TriggerBus. TriggerBus can provide the link between all devices in the test system for both triggering and clock signal distribution.
Deterministic trigger generation and propagation between multiple devices is accomplished with an eight-channel, multipoint low voltage differential signal (LVDS) interface. This architecture permits individual lines to be configured as a source and/or receiver and supports external, time based or software generated triggering as well as clock distribution. Common topologies are supported including star, daisy-chain, and hybrid configurations providing the flexibility to distribute the trigger lines as dictated by the application requirements. Additional flexibility is realized with the addition of a star hub; this device permits very tight trigger tolerances to be maintained throughout a large distribution network.
The TriggerBus can also be automatically extended to other platforms, such as VXI, with a LXI-VXI slot-zero control bridge, providing a mechanism to link a VXI chassis with other LXI hardware. The LXI-VXI slot-zero control bridge will provide a direct extension of the eight VXI trigger lines to any external device, providing the ability to individually control specific instruments and switch devices within the VXI chassis. This type of flexibility will provide the user the ability to integrate other instruments into a homogeneous open test environment, leveraging the strengths of each subsystem.
LAN Synchronization
LAN synchronization, incorporating IEEE-1588 Precision Time Protocol (PTP), provides the ability to synchronize multiple devices utilizing only the LAN Ethernet connection; another fundamental advantage of LXI based devices. PTP defines a precision clock synchronization protocol for networked measurement and control systems which is designed to enable the synchronization of systems that include clocks of different precision, resolution and stability. Sub-microsecond accuracy can be achieved with minimal network and local clock computing resources, and with little administrative attention from the user.
There are several ways in which PTP can be implemented ranging from user level software control, to kernel-level driver modifications, to hardware implementations utilizing dedicated FPGA devices. The highest level of precision is obtained when hardware implementations assist in the time stamping of incoming and outgoing network packets or frames; delay fluctuations can be in the nanosecond range with this approach. PTP provides multiple device synchronization while eliminating the need for external cabling between devices.
Utilization of this approach is less accurate than hardware triggering; however, Giga-bit Ethernet can provide synchronization times in the hundreds of nanosecond range which may be suitable for slower data acquisition rates common with thermocouple measurements.
Distributed Measurement Approach
Open hardware and software are fundamental requirements for any truly sustainable platform, but system designs utilizing the LXI platform also provide an inherent benefit that is ideal for distributed data acquisition applications. Many current data acquisition implementations involve placing the instrumentation in a control room that may be located hundreds of feet from the article that is being tested. This distance presents numerous challenges to the test engineer including cable cost, maintenance, calibration, noise, and debugging. Fortunately the effects from most of these issues are greatly reduced by placing the instrumentation as near to the test article as possible.
Open hardware and software are fundamental requirements for any truly sustainable platform, but system designs utilizing the LXI platform also provide an inherent benefit that is ideal for distributed data acquisition applications. Many current data acquisition implementations involve placing the instrumentation in a control room that may be located hundreds of feet from the article that is being tested. This distance presents numerous challenges to the test engineer including cable cost, maintenance, calibration, noise, and debugging. Fortunately the effects from most of these issues are greatly reduced by placing the instrumentation as near to the test article as possible.
Distributed LXI-based instrumentation, such as VXI Technologyʼs EX1629, 48-channel high-performance remote strain measurement unit can greatly simplify the task. Each EX1629 can be placed near the test structure and connected to the test local area network (LAN) utilizing standard Ethernet cable and networking accessories. A single Ethernet cable can then be routed to the control room for data collection and control. While this approach simplifies installation and addresses several key areas of concern, a distributed approach must still address synchronization and trigger control, issues common to most data acquisition applications
LXI Bridge Objective
The main objective behind any bridge or adapter device is to retain all native platform functionality while providing a transparent path to the protocol or interface of choice, LXI in this instance. Not only can the design engineer count on the native performance of the VXIbus, but also leverage the features of LXI such as long communications path lengths, high data transfer rates, hardware and software independence, and easy cable routing.
LXI instruments and interfaces have been architected so that hybrid systems can be easily configured. LAN-based test systems will use standard LAN connectivity with off-the-shelf switches and hubs permitting simple setup without expensive interface cards and proprietary cable configurations. Instrumentation can now be placed near test articles, or where most convenient, without any undue restrictions using standard copper or fiber optic cable.
Test & Measurement Local Area Network Implementations
Selecting the right network topology is also a concern, and can have a significant impact on the overall performance and timing of Ethernet-based instrumentation. Ethernet networks function by sending packets of data between different nodes on the network. If both nodes transmit data at the same time, a collision will occur, thus affecting system throughput Consequently, more collisions will occur as the number of nodes increase, and data throughput will be reduced. Therefore, the performance of an LXI™ based instrumentation network will be optimized if it is isolated from general purpose corporate network paths.
Dedicated networks are inexpensive to install and provide the necessary isolation between corporate wide network traffic and the test system Additionally, this network can be easily interfaced with the rest of the corporation, or the World Wide Web, with little effort.
Isolated instrumentation networks also eliminate many of the logistical issues that may arise when trying to conform to corporate network requirements. Security concerns can also be addressed by simply not allowing physical access to outside network connections.
LXI Overview
Several approaches can be utilized for synchronization utilizing Ethernet interfaces, and the most common approaches include an auxiliary trigger subsystem (Trigger Bus), Network Time Protocol (NTP), or IEEE-1588. An auxiliary trigger subsystem can provide extremely precise timing and control signals to multiple instruments with minimum phase skew. Additionally, this approach provides the means to synchronize with ease LXI™ instruments and other standard platform implementations, such as VXIbus
Another approach that is unique to Ethernet-based interfaces, and inherent in LXI™, is the Network Time Protocol (NTP), which is the current de-facto standard for network time synchronization. This approach is designed to synchronize the clocks of networked devices; however, it only offers synchronization in the millisecond range at best. There are many factors that can affect the actual precision of NTP, including network traffic, bus, switches, and routers. This approach will provide a reasonable level of synchronization, and may be acceptable for low speed measurements such as thermocouple devices.
The most accurate Ethernet-based timing is available through IEEE-1588, a standard that defines a precision clock synchronization protocol for networked measurement and control systems. The protocol is designed to enable the synchronization of systems that include clocks of different precision, resolution and stability. Sub-microsecond accuracy can be achieved with minimal network and local clock computing resources, and with little administrative attention from the user. This standard will provide LXI™ a level of synchronization that will easily address many typical multiple instrument installations without the need for additional cabling or clock distribution systems.
Ethernet: The Logical Choice
Ethernet is by far the most widely accepted communications interface in use today; nearly every computer is manufactured with an integrated Ethernet interface and networking hardware is becoming increasingly inexpensive. Many of the attributes that have made Ethernet so popular to the computer industry are also attractive to the instrumentation
community.
Technical advantages, such as TCP/IP error checking and fault detection, as well as long inter-device connectivity clearly exceed the limitations of parallel bus and other serial based interfaces. The TCP/IP stack provides error detection and correction that will typically not interfere with throughput rates, especially when a dedicated test system network issued. Furthermore, Ethernet connections can span 100 meters point-to-point, encompass a radius of 200 meters with the use of a hub, switch, or router, or extend to thousands of kilometers if fiber interfaces are used. There are, however, instrumentation-specific requirements that must be addressed before Ethernet can be accepted as a next generation platform for modular instruments.
Some of these include:
• Cooling
• Triggering
• Interrupt Handling
• Mechanical Interfaces
• Multiple Device Synchronization
• Software Interfaces
• Network Routing, Switching
• EMI/RFI
Some of these performance areas, such as mechanical interfacing and cooling, while important, do not represent significant technical challenges. The most challenging aspects of LXI™ implementation involve instrument synchronization, test network architecture, and software inoperability. The following sections will discuss different implementation approaches to address these issues from within the LXI™ framework
Introduction
The functional test and data acquisition community has been instrumental in the development of many industry standards ranging from the definition of communication interfaces to instrumentation backplanes. These functional requirements are often tightly coupled to, and have evolved from, the application space that they serve. Applications may range from small systems to medium sized combinational systems to high-channel count, highly integrated implementations.
The general purpose interface bus (GPIB) is an example of an instrumentation interface that has addressed small systems, and some aspects of medium sized combinational systems, while VXI-based systems have dominated the high-channel count application market. However, GPIB is now being challenged by the need for ever increasing bandwidth and higher data transfer rates. Many other standards have been advertised as the next generation replacement for GPIB, such as the universal serial bus (USB), FireWire, CAN bus, PCI, and PXI, but none of these have met with wide industry acceptance.
As a result of this need, Agilent Technologies and VXI Technology, Inc. have combined engineering resources to develop the next generation instrumentation interface with the release of LAN eXtensions for Instrumentation (LXI™). LXI™ is based upon industry standard Ethernet technology and will provide the flexibility and performance commonplace on larger VXI bus systems, to small and medium size systems
Arbitration
This process is invoked when there is a dispute regarding the validity of a complaint regarding the conformance of an LXI Device. It is presumed at the outset of this process that a written complaint as described above is available, as well as a written document from the provider of the LXI Device stating why it disputes the complaint.
To resolve the complaint, a Conformance Review Committee, chartered as a subcommittee of the LXI Technical Committee, will review and comment on claims. The Technical Committee chairman is responsible for creating the Conformance Review Committee and ensuring that all members of the Technical Committee have an opportunity to volunteer for the Conformance Review Committee. The Technical Committee chairman will initiate this process as soon as is convenient after being notified of the dispute. The membership will be made up of volunteer members from the Technical Committee; they shall elect an impartial chair from their membership. The committee may include both the LXI Device manufacturer and/or the person or company that initiated the complaint regarding the LXI Device in question.
The Conformance Review Committee will review the complaint. They will discuss the problem either in person or via phone meeting with the LXI Device manufacturer. The Conformance Review Committee will then formulate an authoritative opinion regarding the facts of the matter. The committee shall create a document either stating that the LXI Device appears to be conformant or stating the specific problems with the LXI Device, including references to the appropriate LXI specifications as to why the LXI Device in question does not comply. This will be sent to both the LXI Device manufacturer and the person or company that initiated the complaint.
If the flaw in the LXI Device is found to be based on a lack of clarity in the specification then the Conformance Review Committee will forward the matter to the Technical Committee and the Technical Committee shall initiate a request to update the specification using defined operating procedures for submitting specification changes.If the LXI Device is found to be conformant, the matter is finished.
If the LXI Device is found to not be conformant, and if the LXI Device manufacturer agrees in
writing to remedy the situation, the LXI driver manufacturer will be given three months from the time they are informed of the problem to remedy the situation (either update the LXI Device or remove claims of conformance).
If the LXI Device manufacturer is not satisfied with the written conclusions of the Conformance Review Committee, the LXI Device manufacturer may summarize the situation in writing to the LXI Board of Directors and request they take action on it. The Board of Directors shall review the findings of the Conformance Review Committee. If it does not agree, a new Conformance Review Committee will be formed to repeat the work of the previous committee. If the Board of Directors is in agreement with the Conformance Review Committee that the LXI Device is falsely claiming conformance to LXI, or falsely using the LXI Consortium logo, the company providing the LXI Device will be given one month to remedy the problem.
Hybrid Systems
The LXI Consortium anticipates that some systems will use both LXI conformant devices and devices conformant with other standards. These systems are described in the Hybrid System section.
A system that contains both LXI Devices and other non LXI devices accessed through non LXI interfaces is referred to as an Aggregate System. If a system contains only LXI Devices and non-LXI devices accessed through an interface that make them conformant and indistinguishable from native LXI Devices, they are part of a Conformant Hybrid System.
The interface types that provide LXI conformant interfaces are identified as Bridges, Adaptors or Adaptor Toolkits.
Bridges provide an LXI conformant interface for the Bridge and means to control the devices connected to it. Although the Bridge is LXI conformant, the devices connected to it are not exposed through an LXI interface, and it is part of Aggregate System. The Bridge provides a different mechanism for controlling the devices being adapted.
Adaptors present a complete LXI interface the devices being adapted. The combination of the Adaptor and Adaptee is completely LXI conformant and provides the full LXI experience – it is indistinguishable from a native LXI device. It can be part of a Conformant Hybrid System.
An Adaptor Toolkit is hardware and software that provides an adaptor function capable of
presenting an LXI interface for adaptee(s), but it is provided as an incomplete solution. The user must invest additional effort to make the interface LXI conformant.
The interfaces that make a non-LXI device appear to be LXI conformant can carry the LXI logo after conformance testing. The adapted products are not permitted to carry the LXI logo since they are not conformant without the additional adaptor interface.
It is believed that it is not possible for a single physical adaptor interface to expose multiple devices as LXI conformant, but further work is anticipated to be carried out in this area.
LXI Trigger Interface Overview
LXI provides three trigger mechanisms, one based on triggering over the LAN, the second based on IEEE 1588 Precision Time Protocol running over the LAN interface, the and the third based on a wired trigger interface (LXI Trigger Bus).
The LXI trigger facilities use the principal of a uniform approach – within the performance limitations of each trigger mechanism trigger functions can be performed by any of the methods and can be connected together. A trigger event on the wired trigger, for example, can initiate a LAN or IEEE 1588 trigger event.
The LAN trigger provides a way of programmatically triggering events through driver commands either from the controller to the LXI device or by message exchange between LXI devices. It is available on all Functional Classes of LXI devices that require trigger operation. This trigger mode is the simplest to implement but has the lowest performance because of the potential latency in the LAN communication.
IEEE 1588 Precision Time Protocol is available on Functional Class A and B devices and provides a way of synchronizing clocks across many LXI devices, giving the system a coherent understanding of time that can be used to set up triggered events based on the system time. IEEE 1588 can either be implemented entirely in software or can be supported by dedicated hardware that provides more accurate timing synchronization. Timing accuracy and uncertainty is dependent on the module and the IEEE 1588 implementation, but can be expected to be in the range of 10’s of microseconds to 10’s of nanoseconds.
Triggers or events (measurements) can be initiated at specified times, or can be generated immediately on receipt of the instruction, though in this case there will be a higher degree of time error because of latency in the control system.
The LXI Trigger Bus available on Functional Class A devices connects LXI Devices by a daisy chain or star configuration transmission line system to provide a more deterministic trigger interface that can be event driven (by a device for example) or timed by IEEE 1588 (generating a trigger into the wired trigger system). The interface is based on an 8 channel Multipoint LVDS (M-LVDS) signaling system that allows LXI devices to be configured as sources and/or receivers of trigger signals. The interface can also be configured to a wired OR configuration, permitting LXI devices to respond to trigger events requiring the detection of an event by any of multiple devices initiates or where the last device to be ready initiates events.
The LXI Trigger Bus has an input and an output connector to allow easy daisy chaining of LXI devices. The last device in a daisy chain must have its output connector terminated in a specified load to ensure that transmission lines are correctly terminated.
The LXI Trigger Bus provides a more deterministic inter-module trigger than IEEE 1588 and more closely emulates the trigger facilities provided on instruments, such as oscilloscopes, connected directly by physical connectors. Trigger Adaptors can be used to translate triggers from the LXI Trigger Bus to other trigger levels, or to convey a trigger command from another trigger system to a LXI Wired Trigger event.
The LXI Trigger Bus can include a Star Hub that supports a number of LXI Device daisy chains from a single buffered hub. LXI devices maybe connected to the Star Hub as a simple star network, or they can be set up as hybrid system of star and daisy chain connections. In addition to extending the capacity of the trigger system, the Star Hub can be used to translate a trigger from one channel to another which permits, for example, a device to send a trigger to other devices (including itself) with equal delay times if they are connected to the Star Hub with equal length cables. Star Hubs may be internally terminated, forcing them to be placed at the end of a chain, or they can use two connectors for each port and be placed near the centre of a chain.
The LXI Trigger Bus also permits the exchange of clock signals across one or more of the 8 channels available
LXI LAN and Web Overview
The LAN interface for LXI devices is intended to use 100baseT or better connections based on the IEEE 802.3 standards. Modules should be designed for fast boot times under all conditions (including circumstances where the network is disconnected). Network speed and duplex settings are automatically detected to simplify system integration.
The LAN interface is defined to minimize the amount of user intervention required for configuring the TCP/IP parameters and the standard ensures that instruments can be quickly ported from one system to another without risk of system hang-ups and a minimal amount of user intervention.
Access to the instrument functions is via a web browser that provides essential information (such as instrument type, serial number, Functional Class) and about key settings on the device Welcome page. The web interface is required to provide an additional IP Configuration page and a Synchronization page (if the device is Functional Class A or B). The Synchronization page includes information about the IEEE 1588 parameters. If the LXI Device allows the user to change any of the instrument settings these have to be password protected. Examples of sample web pages are available to show how the web pages might be presented by an LXI Device.
LXI Devices can be assigned aliases to make it easier for users to identify, particularly for circumstances where more than one of the same type of module may be in the system.
The LXI Standard allows modules to have automatic lookup for the latest firmware or software through a homepage created by the module manufacturer.
The LAN connection is made to the LXI Device with physical cables and can include a wireless access system in the network to cater for applications where a physical connection is difficult to arrange or technically not desirable. Provision is made for the use optical connection systems in the future.
