Analysis of Circuit Board Fault Diagnosis System Based on VXI Instrument

A certain electromechanical control system uses a centralized distributed computer, which is powerful and easy to operate, but the configuration is large and complex. The hundreds of printed circuit boards it contains are prone to failure and require high maintenance and testing technology. In order to shorten the time to troubleshoot circuit board failures and improve the quality of maintenance, we have developed a circuit board fault diagnosis system based on VXI bus instruments to detect and diagnose faults on the above circuit boards and locate the faults at the component level.

1. Hardware of the fault diagnosis system

The VXI (VME bus extension for instrumentation) bus is one of the most advanced measurement and control system chassis backplane buses. It has the advantages of open standards, compact structure, high data throughput, reusable modules, and support from many instrument manufacturers. The advent of the VXI bus standard and its products has set off a new revolution in the field of automatic testing at home and abroad, and has had a significant impact on the development of international automated testing technology. It has become the development direction of today's measurement and control technology. The circuit board fault diagnosis system is based on the system controller, VXI bus host chassis and its test instrument modules, supplemented by some IEEE488 program-controlled instruments, and the circuit board to be tested is connected to the system through adapters and adapters. The block diagram of the system hardware composition is shown in Figure 1.

(1) System controller. Choose an external host computer, which is equipped with a GPIB communication interface card and connected to the VXI bus host box through a GPIB-VXI translator. Embedded computers have the advantages of high speed, small size and easy use, but their cost is too high and difficult to upgrade, so external computers are still selected.

(2) VXI bus host box and its test instrument modules. Considering the advantages of VXI bus, the developed circuit board fault diagnosis system selected the C-size 13-slot HPE8403A high-performance VXI bus host box, which has 12 slots available for test instrument modules. Although only 4 test instrument modules were purchased, the spare slots provide the possibility for system expansion and functional improvement.

In the VXI bus host box, the HPE1406A command module is used as a zero-slot module. It is connected to the external controller through the GPIB.VXI translator, has translator and interface functions, provides public system resources and performs resource management; the HPE1418A (8-channel D/A conversion) module is used to provide the required analog signals for the circuit board to be tested; the HPE1458A (96-channel digital V0) module is connected to the data bus, address bus and control bus of the circuit board to be tested to complete the control and read and write operations of the circuit board; the HPE1411B (5-bit and a half digital multimeter) is used as the signal parameter test module.

(3) IEEE488 program-controlled instruments. In order to provide a stable voltage source for the circuit board under test and facilitate program control, the desktop instruments HPE3631A and HPE3632A are selected as the program-controlled excitation sources of the system.

(4) Adapters and universal adapters. This part is mainly to establish a basic unified connection and communication method between the excitation source and the measuring instrument equipment and the test object. Since there are many types of circuit boards to be tested, it is impossible to meet the test requirements with only a single adapter. Therefore, an adapter is designed based on the connection of each universal adapter with as many circuit boards to be tested as possible. The adapter is designed to avoid repeated plugging and unplugging and connection of wires and cables when replacing the universal adapter. The lead wires of all excitation sources and VXI measuring instrument modules are centrally connected to the adapter, and then connected to the universal adapter through a 96-pin connector. Since the circuit boards to be tested are of standard design and most of them have 96-pin anode connectors, the universal adapter can be connected to a variety of circuit boards to be tested, and finally the fault diagnosis system and the circuit boards to be tested are connected into a complete whole.

In addition, in order to provide the required clock signal for the circuit board to be tested, the system also includes a DF1631 power function signal generator, which is connected to the adapter through a cable. The first three parts are connected together through the GPIB bus to form the main framework of the test and diagnosis system. GPIB (general purpose interface bus) is an 8-bit parallel interface invented by HP in the 1970s. It enables rack-mounted and desktop instruments to communicate with each other and with the host computer. Because of its simplicity and flexibility, almost all instrument manufacturers use this interface. Until today, it is still the preferred interface for automatic test systems composed of independent instruments. Although the data transmission rate of the GPIB bus is low, it can meet the technical requirements of the developed test and diagnosis system. Considering the performance-price ratio, the circuit board fault diagnosis system is mainly based on desktop instruments, forming an integrated test system mixed with GPIB and VXI buses. The VXI card-type measuring instruments and GPIB program-controlled instruments used in the system meet international standards, have a high degree of standardization, generalization, and modularization, strong compatibility, and are easy to expand and update the system, laying a solid foundation for the improvement and development of the system in the future.

2. Software design of fault diagnosis system

HPVEE was selected as the software design platform for the fault diagnosis system, and the basic operating environment is Windows 98. Windows 98 supports 32-bit program development. Since it is a multi-task operating system environment, it is easy to switch between tasks and exchange information conveniently, providing users with a convenient and good operating interface. It is an ideal operating system. The system software platform uses the graphical programming language HPVEE, which is currently the most productive programming language used in test and measurement. VEE (visual engineering mviroment) is a modular programming language for instrument control and is also one of the main software development environments for VXI bus test systems. It can handle daily tasks such as instrument control, measurement processing and test reports, and simplify the tasks encountered in the test development process: system integration, debugging, structured programming design and document processing. At the same time, programs written in other languages ​​such as C/C++, VC, VB, FORTRAN, etc. can be easily combined with HPVEE programs, which is particularly important for today's test development. In short, VEE can greatly shorten the development time of test software.

Based on the universal requirements of system software, the software design adopts a test-oriented intelligent automatic test system decision platform, and its overall structure block diagram is shown in Figure 2. The following is a brief description of the main components of the software:

(1) System and circuit board information display. After entering the system software main control module, the system information or circuit board information display subroutine can be called through the human-machine interface to display the composition and status of the entire system and the relevant information of the circuit board to be tested to the operator. If the circuit board type required to be entered is not included in the system diagnosis range, it will be automatically prompted.

(2) System equipment self-check. This subroutine performs a self-check on the system equipment. If the self-check passes, the test and diagnosis program can be entered. Otherwise, the program will be terminated and further execution will be returned to the software main control module and an error will be reported.

(3) Circuit board function test program. This subroutine performs fault detection on the circuit board to be tested. When performing fault detection, the required programmable power supply and measuring instrument equipment are first initialized and started to work according to the information provided by the test diagnosis database, and then the circuit board to be tested is initialized (if necessary), and then tested according to the requirements of the test diagnosis database. The measurement results are compared with the relevant data in the test diagnosis data to determine whether the fault detection is completed. If no fault is detected, it is reported and returned to the software main control module; if a circuit fault is detected, it is reported and the circuit board diagnosis program is entered.

(4) Circuit board diagnostic procedure. Similarly, based on the test diagnostic database, the fault isolation and fault location are completed in the same way as the functional test procedure.

(5) Circuit board test diagnosis database. This database corresponds to the circuit board to be tested one by one. Before testing and diagnosing a certain type of circuit board, a database corresponding to it must be established. In other words, the number of databases in the system software can diagnose the number of types of circuit boards. The database source program is written by Delphi software, and its contents mainly include diagnostic steps, analog stimulus, digital stimulus, signal type, test node, test position, feature range, correct situation, error situation and prompt information. When testing, the system test diagnosis program reads a certain data in the database, which contains the diagnostic steps, analog stimulus, prompt information and other contents mentioned above, and then applies stimulus to the circuit board to be tested according to the data content and reads the response, and compares the result with the expected value to determine the next step. It can be seen that the correct establishment of the database is very important, and it is the core of the entire software part.

(6) Test diagnosis result report: Display and print the test diagnosis results of the circuit board under test.

The preparation of the test diagnosis database mentioned above requires an in-depth understanding of the fault diagnosis mechanism of the circuit to be tested. Many types of circuit boards to be tested are mixed digital and analog, and fault diagnosis of mixed digital and analog circuits is a difficult point. Discrete event system (discrete event systems, hereinafter referred to as DES) theory can provide a unified approach that is effective for both digital and analog circuit testing. DES theory is used for fault diagnosis of mixed digital and analog circuits, and its main functions are as follows:

① Check the testability of the circuit. DES theory uses the sum and product operations of sets in discrete mathematics to define the testability of the circuit, that is, to verify whether the expected fault isolation can be achieved through observable events. The specific steps are to first artificially determine the state set T. Each element in T is a set composed of several states, and each state corresponds to the fault or normal state of a component of the circuit; then find several observable events, and establish a partition (a set) corresponding to the event according to the possible and impossible states of the circuit when each event occurs. The partition consists of two subsets, one of which contains all the possible states of the circuit after the event occurs, and the other contains all the impossible states of the circuit after the event occurs; then perform the set product operation on the partitions corresponding to the observable events one by one, and if the final result Tout (still a partition) is "more refined" than the expected T, then the circuit is testable. If the circuit is not testable, observable events must be added, otherwise the diagnostic results will not achieve the expected effect. The inherent meaning of circuit testability is: only when any two states that cannot be distinguished by the observed events are in the same element in T, the circuit can be tested. Obviously, the measurability of a circuit depends on T and all observable events.

② Calculate the minimum test set minOES(T). In order to complete the fault diagnosis of the circuit at a faster speed and lower cost, it is necessary to find the minimum test set. If the circuit to be diagnosed is testable, it is possible to consider removing some of the observable events without affecting the fault isolation. The set of remaining events is EElinoES(T). It can be proved that minOES(T) is not unique, but for each minOES(T), removing any element from it will make the circuit to be diagnosed untestable. As for which minOES(T) is the most ideal, it is mainly considered from the test cost. The smaller the cost, the more ideal the minOES(T).

③ Find the fault isolation rate. After selecting appropriate observable events, the faults corresponding to all states in the above Tout can be detected, and the faults corresponding to the elements in Tout that contain only one state can be located. Therefore, the fault isolation rate P.=m/n is easy to calculate, where m is the number of states in Tout that can achieve fault location, and n is the number of all states contained in Tout.

Overall, the system software design is oriented to the test process, is an open software platform, has a high degree of generalization, is easy to develop and operate, but the new fault diagnosis method for mixed analog and digital circuits needs to be verified in practice.

3. Conclusion

The circuit board fault diagnosis system based on VXI bus has incomparable advantages over any traditional instruments and means, with good inheritance and strong adaptability, and will have broad application prospects.