Signal conditioning platform design is a crucial step in building a large-scale missile test system based on the VXI bus, and it is also the primary task in its hardware implementation. Currently, the increasing number of large missile models, their larger scale, and greater complexity present new challenges to the design of signal conditioning platforms for test equipment. Designing dedicated signal conditioning platforms for different models of large missiles would be extremely labor-intensive, involve significant duplication of development, and be economically unfeasible, hindering the formation of equipment generalization, standardization, and serialization. The emergence of in-system programmability (ISP) technology represents the next generation of PLD development. It provides the possibility of field system reconfiguration and user-defined field systems, enabling remote field upgrades and maintenance. It is highly suitable for implementing the programmable control unit of the signal transfer module. Therefore, this paper designs a general-purpose signal conditioning platform for a large-scale missile test system based on ISP, meeting the testing requirements of different missile models. 1. ISP Technology Programmable Logic Devices (PLDs) originated in the 1970s. They are a new type of logic device that allows users to program specific functions. Since their inception, they have evolved from low-density PLDs such as PROM, PLA, PAL, and GAL to high-density PLDs such as CPLD and FPLD. Currently, the integration level of these devices is increasing, their functions are constantly being enhanced, and the number of logic gates has increased from 5,000 to 2 million, with some even reaching tens of millions of gates. The ISP technology, which emerged in 1991, provided new development space for PLDs and represents the direction of the next generation of PLD technology development. It has the following main characteristics: (1) It shortens the system design and trial production cycle and reduces trial production costs; (2) It reduces the size of the chip and simplifies the production process; (3) It facilitates system maintenance and upgrades; (4) It improves the testability of the system and increases the reliability of the system. The development of ISP devices does not require a programmer; the logic function code can be directly downloaded into the device via a cable. VHDL, as a mainstream development platform, has been standardized by IEEE as IEEE 1076.3. It uses specific syntax to describe the logical functions of devices, providing a convenient development tool for field system reconfiguration and functional customization, simplifying system design. 2. Signal Conditioning Platform The signal conditioning platform is the intermediate link connecting the backend large missile equipment under test (DUT) and the frontend VXI module resources, as shown in Figure 1. Its main functions include the following two points: First, regarding the DUT, it realizes the distribution and transfer of the DUT signal on the conditioning bus, as well as amplification, isolation, and filtering transformations within the conditioning module, providing clean and stable DUT signals to the VXI test resources. Second, regarding the VXI module resources, it is responsible for transmitting power signals and excitation signals to the DUT, connecting and switching test signals with the DUT, and maintaining maximum compatibility with VXI electrical specifications. 3. Implementation of the Signal Conditioning Platform 3.1 Hardware Framework The signal conditioning platform adopts a structure of "adapter + signal conditioning bus + signal conditioning module," as shown in Figure 2. The adapter aggregates the measured signals, test signals, and excitation signals, and transmits them to the signal conditioning bus. Platform integration is achieved by plugging in plug-and-play signal conditioning modules onto a standardized, electrically independent signal conditioning bus. To improve the versatility and standardization of the signal conditioning platform, the conditioning circuit is divided into control modules, general-purpose modules, special-purpose modules, and expansion modules. It adopts a standard card structure, using a 96-pin DIN connector to form a pluggable structure with the signal conditioning bus, and is fixed in an embedded chassis. The connection relationship between the modules is shown in Figure 3. The control module receives instructions from the industrial computer's digital I/O card and implements programmable management of the entire conditioning platform. The general-purpose module mainly performs dynamic allocation and preprocessing of test signals. Its internal circuitry includes module control circuitry, signal allocation circuitry, analog signal processing circuitry, and I/O signal processing circuitry, as shown in Figure 4. The special-purpose module implements certain special functions, such as channel self-testing and special signal conditioning when testing certain objects under test. The expansion module is used for system function expansion. 3.2 Signal Conditioning Module Design 3.2.1 Selection of ISP Devices General-purpose signal conditioning platforms do not have high requirements for the scale of ISP devices, but they must be reliable, flexible in development, have strong reconfiguration capabilities, and good versatility. Lattice's ispLSI series, Altera's 7000S and 9000 series, and Xilinx's XC9500 series are commonly used ISP devices. Lattice was the pioneer in ISP design and has relatively mature technology in this area. Its ISP chips belong to the small-to-medium scale CLPD category, including six series, offering a wide variety of products, low prices, and flexible development, and can be directly applied to systems. Its development platforms, ispDesign EXPERT and PACDesigner, are powerful and easy to use. In 1999, Lattice also pioneered the in-system programmable analog device (ispPAC), bringing a revolutionary breakthrough to the interface design of analog circuits. Therefore, the control circuit and digital signal conditioning circuit of the signal conditioning platform in this paper are implemented using Lattice's ispLSI032, and the analog signal conditioning circuit is implemented using ispPAC20. 3.2.2 Design Flow The design of the signal conditioning module in this system consists of two parts: the functional design of the ISP device and the design of its peripheral circuits. The functional design of the ISP device is primary, as it determines the design of its peripheral circuits. The functional design of the ISP device is also divided into two parts: the functional design of the digital part—ispLSI1032—based on the ispDesign EXPERT platform, and the functional design of the analog part—ispPAC20—based on the PAC-Designer platform. The design flow of the ISP device is shown in Figure 5. "Behavioral Analysis" determines the functions and performance requirements of the device, defining the input and output signals; "Structural Design" determines the implementation details of the system functions, providing a flowchart of the system design and a functional description using VHDL, and, if necessary, a timing diagram; "Logic Description" implements the structural design using ispDesign EXPERT and PAC-Designer software, performs compilation and simulation, verifies the design results, and generates a downloadable file—JEDEC; "Hardware Implementation" designs and completes the specific circuit based on the previous work, including circuit board design, device soldering, downloading the JEDEC file to the ISP device, and system function integration testing. 4. Working Process of Signal Conditioning Platform Application When integrating the platform with a missile testing system, only a dedicated signal conditioning module needs to be designed according to the testing requirements of that missile. The tested signal, the excitation signal, and the signal under test are distributed to the signal conditioning bus via their adapters. Then, convenient integration with the missile testing system is achieved through programmed control of signal switching. During testing, control commands from the main control computer's digital I/O card are sent to the control module via the conditioning bus. After decoding, the control module controls the corresponding conditioning module to switch the target signal. The main control computer selects the signal via the VXI bus multiplexer module, thus completing the test. The excitation process is the reverse. 5. Several Key Issues in the Platform's Universality Design Process To ensure good universality, the following requirements must be met: the signal conditioning bus must be standardized and electrically independent; the plug-and-play signal conditioning circuit must be programmable, modular, and have automated channel management; the entire platform must be scalable. 5.1 Standardized and Electrically Independent Design of the Signal Conditioning Bus The signal conditioning bus is the channel connecting the device under test (DUT), VXI resources, and signal conditioning modules. Its standardized and electrically independent design is a fundamental requirement for a universal and standardized signal conditioning platform. The signal conditioning bus was rigorously defined based on requirements analysis of various types of large missiles under test and functional definition of VXI test resources. It mainly consists of a conditioning control bus and a data transmission bus. The conditioning control bus transmits control commands from the industrial computer's digital I/O card to the control module, which then decodes them to control the entire platform's operation. The data transmission bus forms the connection channel between the tested signal, the conditioning module, and VXI resources (see Figure 3). Under strict bus definition, the selection and layout of connectors, signal connection methods, and connection status definitions must fully consider electromagnetic compatibility. Therefore, this platform adopts a "baseboard + backboard" structure (baseboard adapter, backboard conditioning) in the embedded chassis, isolating the adapter and conditioning bus and maximizing signal transfer space. Furthermore, ground layers are specifically added to the circuit board design of the baseboard and backboard to separate analog ground, digital ground, power ground, and test signals, ensuring that excitation and test signals operate in a clean and unobstructed transmission channel, minimizing interference. 5.2 Programmable and Modular Design of the Conditioning Circuit Modularization and programmability are the development direction of test systems. Designing a programmable and analog signal conditioning platform is an objective requirement for achieving platform versatility. The conditioning circuit design adopted a modular design concept of "control module + power module + special module + expansion module". These modules form a pluggable structure with the conditioning bus (see Figure 2). The control unit and digital conversion circuit of each module are implemented by ispSL1032, and the analog conversion circuit is implemented by ispPAC20. The external interface is connected to the conditioning control bus and can be programmed for control. The general module realizes most of the conditioning functions of the test system, while the special module corresponds to different test objects and test tasks. When building large missile test systems of different models, system integration can be achieved by simply increasing or decreasing the number of general modules and designing different special modules. 5.3 Automated Design of Channel Management for Plug and Play Conditioning Modules The management of signal conditioning channels is a key step in improving the intelligence of the general signal conditioning platform. This includes the establishment (electrical interconnection of ports) and cancellation of channels, and the control and monitoring of channel status. This is generally achieved by directly controlling the status of the channel management unit—ISP device. Therefore, the measurement and control software and the conditioning circuit control unit are inseparable. To achieve automatic management of signal conditioning circuits, the control of the measurement and control conditioning circuit must be based on the signal transmission characteristics of each channel in the conditioning circuit. Here, the management of the conditioning circuit channels is divided into three independent files: interface configuration, control functions, and control models. The interface configuration file stores the interface mapping information between the object under test and the test resources, as well as the channel conditioning parameters. This can be achieved through a software development platform (such as LabWindows/CVI). The general signal conditioning platform has already been applied in the Army's general maintenance and testing system for missile equipment. Experiments have shown that it can meet the performance requirements of different test objects and different test tasks, exhibiting high versatility, standardization, and expandability. (Edited by: He Shiping)