With the development of electronic technology, digital oscilloscopes have greatly expanded their capabilities thanks to digital technology and software. Shortcomings of early products, such as low sampling rates, large dead times, and low screen refresh rates, have been significantly improved. Complex signals, such as modulation signals, communication eye diagrams, and video signals, which were previously difficult to observe, are now easier to observe. Digital oscilloscopes can perform calculations and analysis on data, making them particularly suitable for capturing all the details and anomalies generated in complex dynamic signals. Therefore, they have been widely used in scientific research and industrial production. To ensure that oscilloscopes operate in a qualified state, regular, rapid, and comprehensive calibration, guaranteeing traceability of measurement values, is an urgent task for test engineers. Manual calibration is inefficient and prone to errors, requiring test engineers to consult a large amount of documentation for each type of oscilloscope. Automated test systems, on the other hand, offer accurate and rapid parameter measurement, intuitive display of test results, and automatic storage of test data—features that traditional manual testing cannot achieve. Using automated test systems for programmed calibration of oscilloscopes will be the trend in instrument calibration. GPIB, VXI, and PXI are standard buses for automatic test systems. GPIB has won user recognition due to its stable performance, ease of operation, and low price. GPIB was chosen as the bus for this test system. 1 Hardware Design The hardware of the GPIB-based automatic calibration system for digital oscilloscopes consists of a GPIB controller, a FLUKE5500A, the digital oscilloscope under test, a PC, and peripheral devices such as a printer. The system composition is shown in Figure 1. 1.1 GPIB Interface Controller 1.1.1 GPIB Bus GPIB is a practical instrument interface system developed by Hewlett-Packard in the late 1960s and early 1970s. Due to its convenient control of test instruments and high transmission speed (1 Mbps), GPIB was adopted as the IEEE 488 standard in 1975 and revised to IEEE 488.1-1987 in 1987. The GPIB bus is a digitized 24-pin parallel bus. Eight lines are ground and shield, and the remaining 16 lines are TTL level signal transmission lines, including eight data lines, five interface management lines, and three data transmission control lines. GPIB uses 8-bit parallel, byte-serial, asynchronous communication, with all bytes transmitted sequentially through the bus. GPIB system devices have three attributes: controller, speaker, and listener. Actual devices may possess one, two, or all three. As a controller, it can address and connect to devices with speaker attributes on the bus to become speakers and devices with listener attributes to become listeners, including designating itself. Speakers can send data to other devices through the bus. Listeners can receive data sent by speakers from the bus. Generally, in a GPIB system, the computer is the controller and possesses all three attributes: speaker, listener, and controller. To avoid bus conflicts, IEEE 488 specifies that only one speaker can exist at a time, but several listeners can exist simultaneously. Because the operating speeds of various devices in a GPIB system can vary significantly, three handshake lines are configured in the GPIB bus to ensure bidirectional, asynchronous, and reliable transmission of multi-line messages: a data valid line, an unready-to-receive line, and a data not received line. 1.1.2 BC-1402-2 Interface Controller The GPIB controller used in this system is the BC-1401-2 USB-GPIB interface controller developed by Beka Technology Co., Ltd. It has a USB interface, converting the USB bus to a GPIB bus for operating GPIB instruments. Its features include: full compliance with IEEE 488.1 and IEEE 488.2 international standards; support for PCI, USB, and Ethernet industry standards; a data transfer rate of 900kbps, suitable for high-speed data transfer between PCs and instruments; a set of I/O GPIB operation function libraries, whose functions are the same as those of the ES1400 series interface controllers of the ISA bus; a set of VPP-compliant Virtual Instrument Software Architecture (VISA) function libraries, enabling applications developed using VISA functions to operate without modification when switching between different manufacturers and models of GPIB interface controllers; and the ability to program test programs in multiple languages ​​such as C/C++, VC++, VB, LabView, LabWindows/CVI, HP-VEE, and Delphi, offering convenience and flexibility. 1.2 FLUKE5500A The FLUKE5500A is a high-performance, multi-functional calibrator from Fluke Corporation, USA. It can calibrate various instruments, including handheld and benchtop multimeters, oscilloscopes, power meters, electronic thermometers, data acquisition units, power harmonic analyzers, and process calibrators. The FLUKE5500A provides three standard interfaces: GPIB (IEEE-488), RS-232, and 5725A. In terms of safety, it meets IEC 1010-1 (1992-1), ANSI/ISA-S82.01-1994, and CAN/CSA-C22.2NO.1010.1-92 standards. The FLUKE5500A can output voltage up to 1100V and current up to 11A. It can provide various waveforms and harmonics for DC voltage and current, AC voltage and current, and can simultaneously output two voltages or one voltage and one current, simulating power, resistance, capacitance, thermocouples, and RTDs. Its oscilloscope calibration kit also provides stable sine waves, fast edges, time stamps, and amplitude signals. 1.3 Main Control PC The PC acts as the system's "master controller," issuing commands to the GPIB interface controller to control the FLUKE5500A and the oscilloscope under test. This mainly includes: instrument initialization, reset, and parameter settings; commanding the FLUKE5500A to generate standard signals, which are simultaneously displayed on the oscilloscope under test; reading/saving instrument data and transferring it to the PC, etc. 2 Software Design 2.1 Software Platform Selection Software is the core of this automatic digital oscilloscope calibration system. The stable and coordinated operation of the hardware and software is fundamental to the system's ability to quickly and reliably calibrate digital oscilloscopes. This system uses the stable Windows 2003 Server operating system, SQL Server 2005 (development version) database, and Visual.NET 2005 as the development platform, with C/C++ as the programming language. For driver development, NI's LabWindows/CVI 7.0 is used for some program drivers. Simultaneously, MAX (Measurement & Automation) is used as the IVI driver configuration program. 2.2 Key Technologies VISA and IVI VISA is an I/O interface software standard developed by the VXI plug & play alliance. The purpose of developing VISA is to ensure that instruments from different manufacturers and with different interface standards can be compatible, communicate, and exchange data. Its significant features are: VISA is implemented using advanced object-oriented programming concepts; it is a hyper-integration of function lines for all current instrument interface types, and is very concise, with only about 90 functions; as a standard function, VISA is independent of the instrument's I/O interface type, facilitating program portability. For driver and application developers, the VISA library functions are a set of functions that can be easily called to control various devices such as GPIB, VXI, PXI, etc. IVI (Interchangeable Virtual Instrument) is an instrument driver programming interface launched by the IVI Foundation to further improve the executable performance of instrument drivers, achieve true instrument interchangeability, and realize complete hardware independence for applications. The IVI system consists of five parts: IVI class drivers, specific drivers, the IVI engine, IVI configuration utilities, and IVI configuration information files. Class drivers encapsulate unified functions at the upper layer and interact with the operator, while specific drivers handle communication with the specific instrument. The test program calls the class driver, which in turn calls the specific driver to achieve hardware independence. The IVI engine handles state caching, instrument attribute tracking, and mapping from class drivers to specific drivers. The IVI configuration utility uses software MAX to create and configure IVI logical names. In the test program, logical names are passed to a class driver initialization function, mapping operations to specific instruments and their drivers. The IVI configuration information file records all logical names and the mapping information from class drivers to specific instrument drivers. 2.3 Test Software Architecture 2.3.1 Test Software Modules The test software consists of three parts: a test data management module, a test parameter management module, and a test program module. The test data management module manages the instrument's calibration date, calibration personnel, calibrated items for the specific instrument, and calibration data. The test parameter management module manages the calibration items and standard values ​​for each calibration item in the database. The testing program module calls the corresponding testing instrument to perform the test based on the test parameters selected by the user on the soft panel, compares the test data with the standards in the database, and determines whether it is qualified. 2.3.2 Structured Process of Testing Software After the system self-test upon startup, the calibration operator selects/inputs the instrument model to be calibrated on the software interface. The program retrieves the corresponding calibration items, the standard values ​​of the tested items, and the connection diagram between the tested instrument and the FLUKE5500A and GPIB controller from the database based on the instrument model. The calibrator connects the instrument according to the connection diagram (FLASH animation). After confirming the correct connection, the operator checks for the presence of the IVI driver. After installing the driver, the MAX configuration tool is run. After completing the configuration, the corresponding test program can be run, the test results are saved to the database, and the corresponding pass/fail report is printed. 2.4 Developing IVI Drivers For IVI instruments, the manufacturer provides IVI drivers. Only a small amount of code is needed to calibrate the instrument. The main program is simple and easy to manage. The IVI Foundation's goal is to support 95% of instruments. The calibration of digital instruments based on IVI technology will be the inevitable path for instrument calibration. However, not all instruments support IVI. For non-IVI instruments, the IVI driver development wizard in LabWindows/CVI is used to encapsulate all low-level commands in the instrument's programmable command tree into a series of high-level functions with graphical panels, completing the development of the IVI driver and making it an IVI instrument. Its characteristic is that the initial development of the IVI driver involves a large workload, but the later development and maintenance of the test program is less. 2.5 Database Management Database management mainly includes six modules: user management, instrument model management, calibration item management, calibration report management, calibration item indicator management, and data query. 3 Application Examples The test system built using this method calibrated IVI instruments such as the HP54815, and developed IVI drivers for non-IVI instruments such as the XJ4321. It calibrated five aspects: vertical sensitivity, transient response, steady-state response, scan time factor error, and scan time factor linearity error. The calibration results were saved and calibration certificates were printed. Practice has proven that the calibration process becomes faster and simpler; the results of automatic and manual calibration are consistent. This article introduces a digital oscilloscope calibration system that uses GPIB as the bus and integrates IVI and database technologies to achieve automatic calibration of digital oscilloscopes. It features convenient operation, strong scalability, and good working stability, and provides a reference for building an integrated automatic calibration system for digital instruments such as power meters, spectrum analyzers, arbitrary waveform/function generators, and digital multimeters.