Virtual instruments are instruments with a visual interface built on a computer by adding relevant hardware and software. Virtual instruments completely break the traditional situation where instruments can only be defined by the manufacturer and cannot be changed by the user. This allows any user to easily and flexibly operate various "knobs" on the virtual instrument's soft panel on the computer screen using a mouse or buttons to conduct tests. Users can switch between different virtual instruments according to different test requirements, or modify the software to change, add, or remove functions and scale from the virtual instrument system. These superior characteristics of "developability" and "scalability" give virtual instruments strong vitality and competitiveness. Virtual instrument technology consists of three main parts: 1. Efficient software. Software is the most important part of virtual instrument technology. Using the right software tools and by designing or calling specific program modules, engineers can efficiently create their own applications and user-friendly human-computer interaction interfaces. NI's industry-standard graphical programming software—LabVIEW—not only easily connects to various hardware and software, but also provides powerful subsequent data processing capabilities, allowing users to set data processing, transformation, and storage methods, and display the results to the user. In addition, NI offers more interactive measurement tools and higher-level system management software tools to meet customers' needs for high-performance applications. II. Modular I/O Hardware. Facing today's increasingly complex test and measurement applications, NI provides comprehensive hardware and software solutions. Whether users use PCI, PXI, PCMCIA, USB, or 1394 buses, NI offers corresponding modular hardware products. The product range covers everything from data acquisition, signal processing, sound and vibration measurement, vision, motion, instrument control, distributed I/O to industrial communication such as CAN interfaces. NI's high-performance hardware products, combined with flexible development software, allow engineers responsible for test and design to create fully customized measurement systems to meet a variety of unique application requirements. Currently, NI has achieved a hardware product release rate of one every two business days, greatly expanding users' choices. III. Hardware and Software Platforms for Integration. NI's PXI hardware platform, first proposed specifically for test tasks, has become the standard platform for today's test, measurement, and automation applications. Its open architecture, flexibility, and the cost advantages of PC technology have brought about a revolutionary change in the measurement and automation industry. The PXI Systems Alliance, initiated by NI, has now attracted 68 manufacturers, and the number of products under the alliance has surged to nearly a thousand. 1. Hardware Technology 1.1 Card-based Instruments Traditional instruments mainly consist of a control panel and internal processing circuitry; however, card-based instruments do not have their own instrument panel. They must rely on the powerful graphics environment of a computer to create a graphical virtual panel to complete instrument control, data analysis, and display. Taking a data acquisition card as an example, it typically has functions such as A/D conversion, D/A conversion, digital I/O, and counters/timers. Some also have digital filtering and digital signal processing functions. Modern multi-functional data acquisition cards often employ "Virtual Hardware" (VH) technology. This concept originates from programmable devices, allowing users to easily modify hardware functions or performance parameters through programming, thereby enhancing applicability and flexibility through hardware flexibility. Currently available VH cards offer variable sampling rates and accuracy. Because card-based instruments are tightly integrated with computers, they can fully utilize existing computer resources, offering lower costs, greater flexibility, and stronger performance compared to traditional instruments, making them a highly promising type of instrument. 1.2 Bus Technology 1.2.1 Instrument Bus The GPIB bus (IEEE-488 bus) is a digital parallel bus primarily used to connect test instruments and computers. This bus can connect up to 15 devices (including the host computer acting as the master controller). If the high-speed HS488 handshake protocol is used, the transmission rate can reach up to 8MBps. The VXI bus (IEEE 1155 bus) is an extension of the VME bus, a high-speed computer bus, in the instrumentation field. It was developed in 1987 by five test and instrument companies (Hewlett-Packard, Wavetek, Tektronix, Colorado Data Systems, and Racal-Dana Instruments). The VXI bus is widely used due to its open standard, compact structure, high data throughput (up to 40 Mbps), precise timing and synchronization, reusable modules, and support from numerous instrument manufacturers. However, its relatively high price has limited its widespread application, primarily focusing on defense and military sectors such as aerospace. The PXI bus is based on CompactPCI and is an extension of the open PCI bus (proposed by NI in 1997). The PXI bus conforms to industry standards and fully leverages the mechanical, electrical, and software advantages of the PCI bus. PXI is similar in architecture to VXI, but it offers lower device costs, faster operating speeds, and a more compact size. Currently, PCI-based hardware and software can be used in PXI systems, resulting in excellent compatibility. PXI also boasts high scalability, offering eight expansion slots, compared to only three to four in desktop PCI systems. PXI systems can be expanded to 256 expansion slots using PCI-PCI bridges. The PXI bus has achieved a transmission rate of 132 Mbps (maximum 500 Mbps), the highest currently available transmission rate. Therefore, PXI-based instrument hardware will see increasingly widespread application. 1.2.2 Computer Bus The ISA bus is an 8-bit or 16-bit asynchronous data bus with an operating frequency of 8MHz. Its maximum data transfer rate is 24MBps for 8-bit and 48MBps for 16-bit. This bus is effective for low-speed data sampling and processing, but for multi-tasking operating systems and high-speed data acquisition systems based on high-performance PCs, the ISA bus, due to its bandwidth and bit width limitations, cannot meet the system's operational requirements. Newer motherboards and higher-version operating systems no longer support the ISA bus. The PCI bus is a synchronous, CPU-independent 32-bit or 64-bit local bus with a clock frequency of 33MHz and a data transfer rate as high as 132–264MBps. The PCI bus technology's unlimited read/write burst mode can send large amounts of data in an instant. Peripheral devices on the PCI bus can work concurrently with the CPU, thereby improving overall performance. The PCI bus also has an automatic configuration function, enabling all PCI-compatible devices to achieve true "plug and play." Due to these advantages, the PCI bus has been widely used and has become the de facto standard in the PC industry. USB Universal Serial Bus (USB) The USB bus (also known as the IEEE-1394 bus or Fireware bus) is one of the two buses widely used in PCs and has been integrated into the computer motherboard. The USB bus can connect 127 devices in a daisy chain and requires a pair of signal lines and a power line. The data transfer rate of the USB 2.0 standard can reach 480Mbps. This bus is lightweight, inexpensive, and easy to connect, and is now widely used in broadband digital cameras, scanners, printers, and storage devices. The IEEE-1394 bus is a high-performance serial bus designed by Apple Inc. in 1989. Currently, its transmission rate is 100, 200, and 400Mbps, and it will reach 3.2Gbps in the future. This bus requires two pairs of signal lines and a pair of power lines and can connect 63 devices in any way. It is designed specifically for digital cameras, hard drives, etc., which require serial transmission of large amounts of data [1]. Both USB and IEEE-1394 buses have "plug and play" capability and are more suitable for connecting multiple peripherals compared to parallel buses. 1.2.3 Industrial Fieldbus In order to share test system resources, more and more users are turning to networks. Industrial fieldbus is a network communication standard that enables products from different manufacturers to communicate using a common protocol through a communication bus. Currently, there are many fieldbus standards, such as ISA-SP50, ProfiBus, CAN, FieldBus, and DeviceNet, which are highly competitive. The development of a general fieldbus will take some time. 1.3 Virtual Instrument System Construction Scheme The outstanding achievement of virtual instruments is that they can not only be built into flexible virtual instruments using PCs, but more importantly, they can be used to build automatic test systems of different scales through various interface buses. According to the hardware configuration, virtual instrument systems can be constructed in the following ways: (1) GPIB instruments form a GPIB system with a computer through a GPIB interface card. (2) VXI instruments form a VXI system with a computer. (3) PXI instruments form a PXI system. (4) PC-DAQ test system is constructed with DAQ and signal conditioning parts as hardware. (5) Parallel bus instruments form a parallel bus system. (6) Serial bus instruments form a serial bus system. (7) Fieldbus devices form a fieldbus system. Generally speaking, GPIB, VXI, and PXI are suitable for large-scale high-precision integrated test systems; PC-DAQ, parallel port, and serial port (such as USB) systems are suitable for popular, low-cost systems; fieldbus systems are mainly used for large-scale network testing. Sometimes, different scales of automatic test systems can be built according to different needs, or the above-mentioned schemes can be combined to form a hybrid test system. 2. Software Technology Software is the key to virtual instruments. The following introduces the development platform, instrument driver, and I/O interface software of virtual instrument application software. 2.1 Software Development Platform The main development environments for virtual instruments include Visual C++, Visual Basic, HP's VEE, and NI's LabVIEW and Lab Windows/CVI. Although VC, VB, and Lab Windows/CVI are visual development tools, they require high programming skills from developers and have a long development cycle. HP-VEE is a graphical virtual instrument programming environment with many users. Its disadvantage is that the applications it generates are interpreted and run slowly. LabVIEW is currently the only dataflow-based compiled graphical programming environment internationally. It simplifies complex, tedious, and time-consuming language programming into a simple graphical programming method that uses simple or icon-based prompts to select functions (graphics) and connects various graphics with lines. This allows engineers unfamiliar with programming to quickly "draw" their programs and instrument panels according to test requirements and tasks, greatly improving work efficiency and reducing the workload of researchers and engineers. Therefore, LabVIEW is an excellent virtual instrument software development platform. 2.2 Instrument Drivers Instrument drivers are one of the most important components of a test system, used to implement the communication and control functions of the instrument hardware. Traditionally, instrument drivers are provided by the instrument hardware manufacturer with the hardware. Due to differences in hardware between different manufacturers, it is necessary to modify the test code when replacing the instrument hardware. To enable free interchangeability of instrument hardware without modifying the test program, i.e., to solve the instrument interoperability problem, the VXI plug & play alliance developed the instrument driver standard VISA. VISA is written in G language (graphical language) or ANSI C language and can be used in various virtual instrument development environments and operating systems. In 1999, NI proposed the Interchangeable Virtual Instrument Standard IVI (IVI). Instruments), making program development completely independent of hardware. IVI is built on the VXI plug & play driver standard, which solves the interoperability problem of instruments. IVI drivers control instruments through a general class driver. A class driver is a set of functions and attributes of an instrument, which enables control of instruments in a class of instruments (oscilloscope, digital voltmeter, function generator, etc.). The application calls the class driver, which then communicates with the physical instrument through a dedicated driver. The dedicated instrument driver (and the corresponding physical instrument) can be changed, but the application code remains unchanged [2]. Using IVI technology can reduce software maintenance costs, reduce system downtime, improve the reusability of test code, and make instrument programming simpler. 2.3 I/O Interface Software I/O interface software is the foundation of virtual instrument system software and is used to handle the low-level communication protocol between the computer and the instrument hardware. Today's excellent virtual instrument test software is built on a general kernel of a standardized I/O interface software component, providing users with a consistent, cross-computer platform application programming interface (API), enabling users' test systems to select different computer platforms and instrument hardware [3]. 3. Development Trends With the continuous improvement of computer technology, instrument technology, and network communication technology, virtual instruments will develop in the following three directions: (1) External Virtual Instruments PC-DAQ type virtual instruments are currently popular virtual instrument systems. However, since PCI bus-based virtual instruments require opening the chassis when inserting DAQ, it is quite troublesome. Moreover, the PCI slots on the host are limited. In addition, the test signals directly enter the computer, and various field test signals pose a great threat to the computer's security. At the same time, the strong electromagnetic interference inside the computer will also have a great impact on the test signals. Therefore, external virtual instrument systems with USB interfaces will become the mainstream of low-cost virtual instrument testing systems in the future. (2) PXI type high-precision integrated virtual instrument testing system The high scalability and good compatibility of the PXI system, as well as its higher cost performance than the VXI system, will make it the mainstream virtual instrument platform for large-scale high-precision integrated testing systems in the future. (3) Networked Virtual Instruments Although Internet technology initially did not consider how to connect embedded intelligent instruments, companies like NI have developed products that allow observation of these embedded instruments via web browsers, enabling people to operate the instruments via the Internet. Based on the characteristics of virtual instruments, we can easily form computer networks around them. Using network technology to connect test devices with different functions and geographical locations allows expensive hardware and software to be shared on the network, reducing redundant investment. Currently, standards for MCN (Measurement and Control Networks) are actively being developed and have made some progress. Therefore, networked virtual instruments have broad application prospects. 4. Four Major Advantages of Virtual Instrument Technology Virtual instrument technology utilizes high-performance modular hardware combined with efficient and flexible software to complete various testing, measurement, and automation applications. Only by simultaneously possessing three major components—efficient software, modular I/O hardware, and a hardware and software platform for integration—can the four major advantages of virtual instrument technology—high performance, strong scalability, short development time, and excellent integration—be fully realized. 4.1 High Performance: Virtual instrumentation technology is developed based on PC technology, thus fully inheriting the advantages of the latest commercial technologies dominated by off-the-shelf PC technology, including high-performance processors and file I/O, allowing users to perform complex analyses in real time while data is imported to disk at high speed. Furthermore, the ever-evolving Internet and increasingly faster computer networks enable virtual instrumentation technology to demonstrate even greater advantages. 4.2 Strong Scalability: Thanks to the flexibility of NI software, users can improve their entire system with minimal hardware investment and little or no software upgrades simply by updating their computer or measurement hardware. When utilizing the latest technologies, users can also integrate them into existing measurement equipment, ultimately accelerating time-to-market at a lower cost. 4.3 Shorter Development Time: At both the driver and application levels, NI's efficient software architecture integrates with the latest technologies in computing, instrumentation, and communications. NI designed this software architecture to facilitate user operation while providing flexibility and powerful functionality, allowing users to easily configure, create, publish, maintain, and modify high-performance, low-cost measurement and control solutions. 4.4 Integrated virtual instrument technology is essentially an integrated hardware and software concept. As products become increasingly complex, engineers often need to integrate multiple measurement devices to meet complete testing requirements, and connecting and integrating these different devices is always time-consuming. NI's virtual instrument software platform provides standard interfaces for all I/O devices, helping users easily integrate multiple measurement devices into a single system, reducing task complexity.