PCB Blog - On-line tester module composition for double-sided PCB board

1. Circuit online testing technology
1) Online testing principle: The basic principle of online testing is that the tester provides input excitation for the chip under test on the printed circuit boards, and at the same time automatically collects and records the output response and state value of the chip under test under the control of the computer. All recorded state values are compared with the standard state truth table to judge the fault condition of the tested object.
2) Post-drive test technology: Post-drive test technology is mainly used for online testing of digital circuits. Its essence is to sink or pull out a large transient current at the input stage of the device under test (the output stage of the front-stage driver chip), forcing its potential to become higher or lower as required, so as to apply test excitation to the device under test online. Purpose. To ensure functional testing of the device on the board, it is necessary to force the logic level of the device to be driven, and each pin driver must be able to sink or source sufficient current. According to the post-drive safety standard recommended by the international protection standard document (00-53/1), the drive current of the tester is designed to be 240mA, and the test time is within 200ms. Through the experiment, the device under test can basically be well isolated, and the safety of the device under test is also ensured.

2. Composition of the tester
1) Hardware module: The tester consists of a portable computer, a single-chip test platform and test analysis and processing software. Among them, the single-chip test platform completes the acquisition of the measured object data under the control of the computer. Some functions and descriptions are as follows:
The microcontroller circuit mainly completes data acquisition, control, command processing, and data exchange with the computer. In the design of the tester, MCS-51 series 8031 single-chip microcomputer is used, 2764 is used as the expansion ROM, and 6264 is used as the expansion RAM. The decoding chip circuit is 74LS138. For serial communication with computer, MCl488 and MC1489 are used to convert between RS-232C level and TTL level. The single-chip microcomputer system clock frequency selects 6MHz crystal oscillator, the communication baud rate selects 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 single-chip microcomputer bus to improve its driving capability, and selects 74LS244 and 74LS245 line drivers.

The drive control circuit mainly completes the control of the TTL and CMOS test thresholds in the test process, and selects 4-fold SPST (single pole single throw) DG211 analog switch. The switch control is completed by the decoding circuit and the 74LS373 latch. To ensure that the DG211 is in normally open (OFF) state when powered on, a pull-up resistor (10kΩ) is added 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, a 4-way parallel connection is used. To prevent damage to the device, an LC network is added for high current buffering, and a diode protection circuit is designed. The data acquisition circuit reads the output response of the chip under test, and uses the dual voltage comparator LM393 to control the output signal. It has low power consumption, high comparison accuracy, and is compatible with TTL logic. The LM393 output is connected with the 74LS373 data latch, which is controlled by the microcontroller to read in the comparison data.

The voltage-driven D/A circuit completes the output of the step voltage in the VI test process. Adopt 8-bit parallel D/A converter MC1408. The chip power supply voltage is +5V and -12V. The reference voltage is provided by the constant current voltage regulator TL431. The output selects bipolar output, which is completed by the two-stage amplifier LM348. The current conversion acquisition A/D circuit implements the acquisition of the current data of the test point. In the circuit, the load resistance and the differential amplifier circuit LM343 are used to follow the voltage of the test point, and the current value of the test point is converted into the voltage that the A/D conversion circuit can handle. Select AD7574 eight-bit successive comparison type high-speed A/D conversion circuit. The conversion time is 15μS, and it is powered by a single +5V supply. The reference voltage selects VREF=-8V. The input voltage range is 0~+|VREF|. A/D conversion can be started by generating a negative pulse at the RD end of the program control chip.

2) Software module: The tester is controlled by the portable main control computer through the serial port, the single chip test platform completes the excitation control, data acquisition and other work, and all data analysis processing and command control are completed by the portable main computer. The whole set of test software consists of main control software, data communication software, offline test software, online function test software, online status test software, VI characteristic test software, node voltage test software, electronic manual, test development software, system self-test software, etc. The main modules are composed.

3. Main functions of the tester
The tester adopts the circuit online test technology, which can be used to test and analyze the common faults of various small and medium-scale integrated circuit chips online or offline, and test the V/I characteristics of analog and digital devices. The basic principle of functional testing of digital chips is to detect and record the input/output state of the chip, and compare the recorded state with the standard state truth table to judge whether the function of the tested chip is correct. Digital chip state test Each digital device on the circuit board has three state characteristics after power-on: the logic state of each pin (power, ground, high resistance, signal, etc.), the connection relationship between pins, input The logical relationship between the outputs. When a device fails, its state characteristics generally change. The tester can extract the state characteristics of each IC device on a good circuit board, store it in the computer database, and then compare it with similar faulty circuit boards, so as to accurately find the fault location. VI Characteristic Test Analysis This test function is based on the analog characteristic analysis technology and can be used for the testing of analog, digital, special-purpose devices, programmable devices, and large-scale and ultra-large-scale devices. The tester automatically extracts the characteristic curve of the measured point through the test probe or test clip, displays it on the computer screen, and stores it in the computer. In special fault diagnosis, compare the VI curve measured with the standard curve stored in advance, and then find the fault. Nodal voltage test Since the test object of the tester includes not only digital circuit devices, but also a large number of analog circuit devices, in order to further improve the application scope of the tester, the node voltage test technology is adopted in the tester. By applying the working voltage to the measured object, the computer reads the voltage response value of the test node, and establishes a standard test information database for the operator to analyze and judge the fault location. In addition to the above main functions, other functional testers also have auxiliary test functions such as electronic manuals, test development, and system self-checking on PCB board.