A general circuit board in-circuit tester
The universal circuit board online tester adopts the communication technology between the computer and the single-chip microcomputer. The tester communicates with the computer through the serial port. The portable performance is good, and it is suitable for the testing and diagnosis of various kinds of double-sided printed circuit boards.
1. The circuit on-line test technology
1.1 Principle of Online Testing
The basic principle of online testing is that the tester provides input stimulus for the tested chip on the printed circuit board. At the same time, the tester automatically collects and records the output response and state value of the tested chip under the control of the computer, and records all state values of the recorded The standard state truth table is compared to determine the fault condition of the measured object.
1.2 Post-drive testing technology
Post-drive test technology is mainly used for on-line testing of digital circuits. Its essence is to inject or pull transient large current at the input stage of the device under test (output stage of the pre-driver chip), forcing its potential to become higher or lower as required to achieve on-line application of test excitation to the device under test. purpose.
To ensure functional testing of the device on the circuit board, the logic level of the device must be forced, and each leg driver must be able to sink or output sufficient current. According to the post-drive safety standard recommended by the International Protection Standards Document (00-53/1), the tester's maximum drive current is designed to be 240mA and the test time is within 200ms. Through experiments, the device under test can be well isolated, and the safety of the device under test is also ensured.
2. The tester composition
2.1 Hardware Module
The tester consists of a portable computer, a microcontroller test platform, and test analysis and processing software. One-chip computer test platform under the control of the computer to complete the measured object data collection, the basic principle diagram shown in Figure 1. The function and description of each part are as follows:
The single-chip circuit mainly completes the data acquisition, control, command processing, and data exchange with the computer. In the design of the tester, the MCS-51 series 8031 single-chip microcomputer was used, 2764 was selected as the expansion ROM, and the 6264 was used as the expansion RAM. The decoding chip circuit is 74LS138. For serial communication with a computer, MCl488 and MC1489 are used to convert RS-232C levels and TTL levels. The system clock frequency of the one-chip computer chooses 6MHz Crystal Oscillator, the baud rate of the communication chooses 2400, the one-chip computer adopts working mode 3 to carry on the serial communication. Timer T1 is set to mode 2. Set SMOD=1, time constant F3H.
The bus driver expands the MCU bus to increase its drive capability. The 74LS244 and 74LS245 line drivers are used.
The drive control circuit mainly completes the control of the TTL and CMOS test thresholds during the test process. It selects 4 SPST (single-pole single-throw) DG211 analog switches, and the switch control is completed by a decoding circuit and a 74LS373 latch. To ensure that the DG211 is in the normally open (OFF) state when it is turned on, add a pull-up resistor (10kΩ) to the control line. The test drive circuit applies a test input signal to the chip under test and uses a micro-relay to control the input signal. The test signal is generated by the data buffer 74ACT244. In order to ensure that the input current meets the design requirements, 4-way parallel connection is adopted. To prevent damage to the device, increase the LC network for large current buffers and design the diode protection circuit.
The data acquisition circuit reads the output response of the tested chip and uses the dual voltage comparator LM393 to control the output signal. It has low power consumption, high accuracy, and is compatible with TTL logic. The LM393 output is connected to the 74LS373 data latch and the microcontroller reads the comparison data.
The voltage-driven D/A circuit completes the step voltage output during the VI test. 8-bit parallel D/A converter MC1408 is used. The chip power supply voltage is +5V and -12V. The reference voltage is provided by the constant current regulator TL431. The output selects the bipolar output and is completed by the two-stage amplifier LM348.
The current conversion acquisition A/D circuit implements the collection of current data at the test point. The load resistor and differential amplifier circuit LM343 are used in the circuit to follow the voltage of the test point, and the current value of the test point is converted into the voltage that can be processed by the A/D conversion circuit. The AD7574 eight-time successive comparison high-speed A/D conversion circuit was selected. Conversion time is 15μS, single +5V power supply. The reference voltage is VREF=-8V. The input voltage range is 0~+|VREF|. A negative pulse is generated on the RD side of the program control chip to start the A/D conversion.
2.2 Software Module
The tester is controlled by a portable host computer through a serial port. The microcontroller test platform completes the incentive control, data acquisition, etc. All data analysis processing and command control are performed by a portable host computer. The whole set of testing software consists of main control software, data communication software, offline testing software, online function testing software, online status testing software, VI characteristic testing software, nodal voltage testing software, electronic manual, test development software, system self-checking software, etc. The main module consists of.
3. The main function of the tester
The tester adopts the circuit on-line test technology, which can be used to test and analyze the common faults of various small and medium-scale integrated circuit chips on-line or off-line and test the V/I characteristics of analog and digital devices.
The basic principle of the functional testing of the digital chip is to detect and record the input/output status of the chip, compare its recorded status with the standard status truth table, and determine whether the function of the tested chip is correct. State of digital chip testing Each digital device on a circuit board has three state characteristics after power-up: logic state of each pin (power, ground, high impedance, signal, etc.), connection between pins, input The logical relationship between the outputs. When the device fails, its status characteristics generally have to change.
The tester can extract the state characteristics of each IC device on the good circuit board, store it in the computer's database, and compare it with the same kind of faulty circuit board to accurately find the fault location.
VI Features Test Analysis This test function is based on analog feature analysis technology and can be used for analog, digital, dedicated devices, programmable devices, and large-scale, ultra-large-scale device testing. The tester automatically extracts the characteristic curve of the measured point by testing the probe or test clip, displays it on the computer screen, and finally stores it in the computer. In the special fault diagnosis, the measured VI curve is compared with the previously stored standard curve, and the fault is found. Node voltage test Because the tester test object not only includes digital circuit devices, but also includes a large number of analog circuit devices, in order to further improve the scope of application of the tester, the node voltage test technology is used in the tester. By applying a working voltage to the measured object, the computer reads the voltage response value of the test node and establishes a standard test information library for the operator to analyze and determine the fault location.
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