Application of PXI in Virtual Instruments

1. Introduction

PXI (PCI Extensions for Instrumentation) is a new modular instrument platform that provides high-performance measurements at an affordable price. With PXI modular instruments, you can fully enjoy the advantages of low cost, ease of use, flexibility and high performance brought by open industrial standardized PC technology. The core technology of PXI is CompactPCI industrial computer architecture, Microsoft Windows software and VXI timing and triggering functions.  

2. Development of electronic measuring instruments  
Since its development, electronic measuring instruments can be roughly divided into four generations: analog instruments, digital instruments, intelligent instruments and virtual instruments.

The first generation of analog instruments, such as pointer multimeters and transistor voltmeters, can still be seen in some laboratories. The

second generation of digital instruments, such as digital voltmeters and digital frequency meters, are currently quite popular. These instruments convert analog signal measurements into digital signal measurements and output the final results in digital form. They are suitable for fast response and high-accuracy measurements. The

third generation of intelligent instruments, such instruments have built-in microprocessors, which can perform automatic testing and have certain data processing capabilities. They can replace some mental labor and are usually called intelligent instruments. All its functional blocks exist in the form of hardware (or fixed software). Compared with virtual instruments, they lack flexibility in both development and application.

The fourth generation of virtual instruments is the product of the combination of modern computer technology, communication technology and measurement technology. It is a huge change in the concept of traditional instruments and an important direction for the development of the instrument industry in the future.  

3. What is a virtual instrument?

The concept of virtual instruments (VI) was proposed by the National Instruments Corporation (NI) in 1986. Virtual instruments are measurement and control systems composed of computer hardware resources, modular instrument hardware and software for data analysis, process communication and graphical user interface; they are modular instrument systems controlled by computers.  

3.1. Advantages of virtual instruments

Compared with traditional instruments, virtual instruments have the following advantages:

(1) Integrating powerful computer hardware resources, virtual instruments break through the limitations of traditional instruments in data processing, display, storage, etc., and greatly enhance the functions of traditional instruments. High-performance processors, high-resolution displays, large-capacity hard disks, etc. have become standard configurations of virtual instruments.

(2) It makes use of the rich software resources of computers to realize the softwareization of some instrument hardware, saving material resources and increasing system flexibility; through software technology and corresponding numerical algorithms, various analysis and processing of test data can be carried out in real time and directly; through graphical user interface (GUI) technology, it truly achieves user-friendly interface and human-computer interaction.

(3) Based on computer bus and modular instrument bus, the instrument hardware has realized modularization and serialization, greatly reducing the system size, and can easily build modular instruments (Instrument on a Card).

(4) Based on computer network technology and interface technology, the VI system has convenient and flexible connectivity and widely supports various industrial bus standards such as CAN, FieldBus, PROFIBUS, etc. Therefore, VI technology can be used to easily build an automatic test system (ATS) to realize the networking of measurement and control processes.

(5) Based on the open standard architecture of computers. The hardware and software of virtual instruments are open, modular, reusable and interchangeable. Therefore, users can choose products from different manufacturers according to their needs, making the development of instrument systems more flexible and efficient, and shortening the system construction time.

3.2. Virtual Instrument Hardware System

The hardware system of virtual instruments is generally divided into computer hardware platform and measurement and control function hardware.

Computer hardware platform can be various types of computers, such as ordinary desktop computers, portable computers, workstations, embedded computers, etc. Computers manage the hardware and software resources of virtual instruments and are the hardware foundation of virtual instruments. The development of computer technology in display, storage capacity, processing performance, network, bus standards, etc. has led to the rapid development of virtual instrument systems.

According to the different measurement and control function hardware, VI can be divided into four standard architectures: GPIB, VXI, PXI and DAQ.

(1) GPIB (General Purpose Interface Bus) is a general purpose interface bus, which is a standard communication protocol between computers and instruments. The hardware specifications and software protocols of GPIB have been incorporated into the international industrial standards-IEEE 488.1 and IEEE 488.2. It is the earliest instrument bus. At present, most instruments are equipped with GPIB interfaces that comply with IEEE 488. A typical GPIB test system includes a computer, a GPIB interface card and several GPIB instruments. Each GPIB instrument has a separate address and is controlled by a computer. Instruments in the system can be added, reduced or replaced by making corresponding changes to the computer's control software. This concept has been applied to the internal design of instruments. In terms of price, GPIB instruments cover instruments from relatively cheap to extremely expensive. However, the data transmission speed of GPIB is generally lower than 500kb/s, which is not suitable for applications with high system speed requirements. (For a standard interface bus within a distance of 20m, if the standard load equivalent to each 2m is equivalent to using a 48mA open collector transmitter, the maximum operating rate is 250kb/s. If a three-state gate transmitter is used, the general rate is 500kb/s and the maximum rate can reach 1000kb/s.)

(2) VXI (VMEbus eXtension for Instrumentation) is the expansion of the VME bus in the field of instruments. It is an open instrument bus standard jointly developed by major instrument manufacturers in 1987 based on the VME bus, Eurocard standard (mechanical structure standard) and IEEE 488. The VXI system can contain up to 256 devices, mainly composed of a main chassis, a "0-slot" controller, modular instruments with multiple functions, driver software, and system application software. The functional modules in the system can be replaced at will, and a new system can be formed by plug-and-play. At present, there are two VXI bus organizations in the world. ① VXI Alliance, responsible for formulating the hardware (instrument level) standard specifications of VXI, including chassis backplane bus, power distribution, cooling system, zero-slot modules, electrical characteristics, mechanical characteristics, electromagnetic compatibility, system resource management and communication procedures of instrument modules; ② VXI Bus Plug & Play (VPP) System Alliance, the purpose is to provide an open system structure by formulating a series of VXI software (system level) standards, and truly realize the "plug-and-play" of VXI bus products. These two sets of standards constitute the VXI standard system, realizing the modularization, serialization, and generalization of VXI, as well as the interchangeability and interoperability of VXI instruments. The price of VXI is relatively high, which is suitable for cutting-edge testing fields.

(3) PXI (PCI eXtension for Instrumentation) is an extension of PCI in the field of instrumentation. It is a new open, modular instrument bus specification released by NI in 1997. Its core is the CompactPCI structure and Microsoft Windows software. PXI is formed by adding mature technical specifications and requirements to the PCI core technology. PXI adds a trigger bus and reference clock for multi-board synchronization, a star trigger bus for precise timing, and a local bus for high-speed communication between adjacent modules to meet the requirements of test and measurement users. PXI is compatible with the CompactPCI mechanical specification and adds requirements such as active cooling and environmental testing (temperature, humidity, vibration and shock testing). In this way, the interoperability of multi-vendor products and the ease of system integration can be guaranteed.

(4) DAQ (Data AcQuisition) Data acquisition refers to a built-in function card based on a computer standard bus (such as ISA, PCI, PC/104, etc.). It makes more effective use of computer resources and greatly increases the flexibility and scalability of the test system. DAQ can be used to quickly and easily build computer-based instruments, achieving "one machine with multiple types" and "one machine with multiple uses". In terms of performance, with the rapid development of A/D conversion technology, instrument amplification technology, anti-aliasing filtering technology and signal conditioning technology, the sampling rate of DAQ has reached 1Gb/s, the accuracy is as high as 24 bits, the number of channels is as high as 64, and it can arbitrarily combine digital I/O, analog I/O, counter/timer and other channels. Instrument manufacturers have produced a large number of DAQ function modules for users to choose from, such as oscilloscopes, digital multimeters, serial data analyzers, dynamic signal analyzers, arbitrary waveform generators, etc. By connecting several DAQ function modules to a PC computer and matching the corresponding software, a PC instrument with several functions can be constructed. [page]

3.3. Virtual Instrument Software System

The core idea of ​​virtual instrument technology is to use the hardware/software resources of the computer to softwareize (virtualize) the technology that originally required hardware implementation, so as to minimize the system cost and enhance the system's functions and flexibility. Based on the important role of software in the VI system, NI put forward the slogan "The software is the instrument". The VPP system alliance proposed a series of VPP software standards such as system framework, driver, VISA, soft panel, component knowledge base, etc., which promoted the process of software standardization.

The software framework of virtual instruments includes three parts from low level to top level: VISA library, instrument driver, and application software.

VISA (Virtual Instrumentation software Architecture) is the virtual instrument software architecture, which is essentially the general term for standard I/O function library and its related specifications. This I/O function library is generally called VISA library. It resides in the computer system to perform special functions of the instrument bus and is the software layer connection between the computer and the instrument to realize program control of the instrument. It is a set of callable operation functions for instrument driver developers. The

instrument driver is a set of software programs that complete the control and communication of a specific instrument. It is a bridge for application programs to realize instrument control. Each instrument module has its own instrument driver, which is provided to users by the instrument manufacturer in the form of source code.

The application software is built on the instrument driver and directly faces the operating user. It completes the automatic test task by providing an intuitive and friendly measurement and control operation interface and rich data analysis and processing functions.

The writing of virtual instrument application software can be roughly divided into two ways:

① Writing with general programming software. Mainly including Microsoft's Visual Basic and Visual C++, Borland's Delphi, Sybase's PowerBuilder;

② Developing with professional graphical programming software. Such as HP's VEE, NI's LabVIEW and Lab windows/CVI.

The application software also includes general digital processing software. General digital processing software includes various functional functions for digital signal processing, such as power spectrum estimation, FFT, FHT, inverse FFT, inverse FHT and refinement analysis of frequency domain analysis; correlation analysis, convolution operation, deconvolution operation, root mean square estimation, differential integral operation and sorting of time domain analysis. And digital filtering, etc. These functional functions provide a basis for users to further expand the functions of virtual instruments.

4. Introduction to LabVIEW

In the early 1980s, computer interfaces became more and more sophisticated and user-friendly. NI engineers realized that a powerful software interface was needed to allow users to obtain simpler and more effective testing and control through their computers. Apple's Macintosh provided the best environment for this upcoming graphical software language: G language. Soon, NI developed a software package for computer-based measurement and automation: LabVIEW.

LabVIEW is a revolutionary graphical development language based on G language, which is used for data acquisition and control, data analysis and data expression. Its goal is to simplify program development and allow engineers and scientists to fully utilize the functions of PCs to complete their work quickly and easily. More than ten years of continuous enrichment have made LabVIEW a rich and powerful practical tool package. LabWindows/CVI was launched simultaneously with LabVIEW, which is characterized by the use of ANSI C programming language to establish an interactive development environment with practical instruments. Both are equipped with library functions for GPIB, VXI, serial ports and plug-in DAQ boards, as well as instrument drivers from hundreds of manufacturers around the world. A variety of accessories have also been developed around these core software.

The birth of LabVIEW marks the beginning of NI's entry into a period of specialization in VI (virtual instruments).  

5. Comparison of PCI, PXI, and VXI

The virtual instrument test systems based on PCI bus, PXI bus, and VXI bus have their own characteristics and application scopes due to the different buses.

Compared with the test system composed of traditional instruments, the virtual instrument test system based on PCI bus has absolute advantages in performance, flexibility, ease of use, and low price. Its instrument hardware is card-type and has the same size as the computer card. The test system can be formed by directly inserting the hardware card into the PCI slot in the computer, making full use of the computer's resources to realize intelligent measurement and control such as data acquisition and processing, fault analysis and diagnosis, and process control. Compared with virtual instrument test systems based on other buses, its low price makes it widely used in industry, military industry, education, and scientific research. The disadvantage is that the virtual instrument test system based on PCI bus lacks trigger line standardization and the computer environment in which it is located. This environment cannot meet the requirements of complex and precise test tasks with high power, high-quality cooling, and careful consideration of RFI/EMI shielding. The connection of the card may also be difficult to produce due to the limitations of the computer model used. The number of slots is very limited and it is difficult to accommodate a large number of channels.

Virtual instrument test systems based on PXI bus are fully compatible with PCI bus products. In many fields, they can replace virtual instrument test systems based on PCI bus, and have better performance than the former, but the price is slightly higher. If users want to switch from existing virtual instrument test systems based on PCI bus to virtual instrument test systems based on PXI bus, they only need to invest in hardware, and the original software can run on PXI system without any modification. At the same time, since PXI bus has strict regulations on the working environment of components inside the chassis and PXI system has more expansion slots than desktop design, PXI system can work normally in harsh working environment, so it can adapt to various larger and more complex test fields. Since PXI bus is a combination of instrument characteristics of VXI bus based on PCI bus, PXI system is between PCI system and VXI system in price and performance.

Although VXI bus has a short history since its birth in 1987, VXI bus products have been developed from scratch and from small to large, and have formed large-scale production. Especially since the 1990s, the development of VXI bus products has shown an exponential upward trend. The VXI bus template itself does not have a power supply, panel, buttons, knobs, or display. The setting of electrical parameters and the display of measurement results must be achieved through the software panel. It is a good virtual instrument system platform. By combining the VXI bus technology with computer network technology and utilizing existing Internet resources, remote communication and test networks can be established on the basis of interactive web pages. The system structure of the VXI bus provides a more ideal environment for the development of virtual instruments. The virtual instrument test system based on the VXI bus will become the mainstream of program-controlled test systems in the 21st century. At present, due to its high price, it is mainly used in the field of cutting-edge testing. According to data, 72% of VXI system users come from the communications industry and the military industry.

Different test tasks have different requirements for the test system. A virtual instrument test system cannot cover the measurement requirements of the entire society. There should be an objective understanding of the development of virtual instrument test systems. Virtual instrument test systems based on the PCI bus are usually suitable for low-frequency and low-speed process measurement and control systems, teaching experiments, and routine laboratory tests. Virtual instrument test systems based on the PXI bus can be used in general automatic test system occasions and automatic test systems with limited system total price due to the improvement of electromagnetic compatibility performance and cooling performance and modular structure. The VXI-based virtual instrument test system has good performance and can be used for automatic measurement systems, especially high-speed and large-volume automatic test systems, wide-band automatic test systems and military automated measurements, representing the development direction of test technology in the 21st century.  

6. Conclusion

PXI combines the high performance of the compact PCI standard and the high reliability of the VXI instrument system. At the same time, it maintains a more attractive price advantage than VXI, and is the best cost-effective choice to meet your high-standard test requirements. The open PXI specification utilizes a number of existing industrial standard technologies to provide the best measurement and automation platform. The most important electrical specification is extended from the very successful PCI bus. The electrical extensions for instruments include built-in triggers and local buses, which are extended from the high-performance VXI instrument structure.

Therefore, PXI is the best choice for the measurement platform I have built.