Composition of two-sided printed circuit board online test module
1 Circuit On line Testing Technology
1) On line test principle: The basIC principle of online test is that the tester provides input excitation PCB for the test chip, and automatically collects and records the output response and state value of the tested chip under the control of the computer Compare all recorded state values with the standard state truth table to judge the fault state of the test object
2.) Post driving test technology: post driving test is mainly used to test digital circuits online. Its essence is to absorb or pull out large transient current at the input stage of the device under test (the output stage of the front driver chip), forcing its potential to become higher or lower as required, so as to apply test excitation to the equipment under online test Intent To ensure the functional test of the equipment on the circuit board, the logic level of the equipment must be forced to drive, and each pin driver must be able to absorb or provide sufficient current A. According to the post driving safety standard recommended by the international protection standard document (00-53./1), the driving current of the tester is designed as 240mA, and the test time is within 200ms Through the experiment, the tested equipment can be isolated well and the safety of the tested equipment is guaranteed
Printed pcb board

2. Composition of tester
1) Hardware module: The tester is composed of portable computer, single-chip microcomputer test platform and test analysis and processing software Among them, the SCM test platform completes the data collection of the tested object under the control of the computer Some functions and descriptions are as follows:
The micro control ler circuit mainly completes 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 is used, 2764 is used as the extended read-only memory, and 6264 is used as the extended memory decoding chip. The circuit is 74LS138. It is used for serial communication with computers. MCl488 and Mac1489 are used for conversion between RS-232C level and TTL level The clock frequency of the single chip microcomputer system is 6 MHz crystal oscillator, the communication serial transmission rate is 2400, and the single chip microcomputer adopts the working mode 3. Serial communication is carried out. The timer time 1 is set as the mode 2. Set SMOD=1, and the time constant f3 hours. Bus bar driver extends the single chip bus to improve its driving ability, and selects 74L 244 and 74lb 245 line drivers
The drive control circuit mainly controls the test threshold of TTL and metal oxide semiconductor during the test process, and selects a 4 times SPST (single pole single throw) DG211 analog switch. The switch is controlLED by the decoding circuit and the 74 lb 373 latch To ensure that DG211 is normally open (OFF) when it is powered on, a pull-up resistor (10k Ω) is added to the control line ?）。 The test drive circuit applies the test input signal to the chip under test, and uses the micro relay to control the input signal The test signal is generated by the data buffer 74 actual 244 In order to ensure that the input current meets the design requirements, a 4-circuit parallel connection is used to prevent equipment damage. A new LCD network is added for large current buffer, and a diode protection circuit is designed The data acquisition circuit reads the output response of the tested chip and uses a 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 to the 74 lb 373 data latch and controlled by a microcontroller to read comparison data
Voltage driven day/During the virtual instrument test, one circuit completes the output of step voltage 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 on page 431 Bipolar output is selected as the output, which is completed by two-stage amplifier LM348 Company The current conversion acquisition A/D circuit realizes the current data acquisition at the test point In the circuit, the load resistor and differential amplifier circuit LM343 are used to track the voltage of the test point, and can be processed by converting the current value of the test point to the A/D conversion circuit The 8-bit continuous comparison high-speed A/D conversion circuit in 7574 AD is selected The switching time is 15mS, powered by a+5V power supply. The reference voltage is VREF=- 8V. The input voltage range is 0~++| VREF |. A/D conversion can be started by generating a negative pulse at the first end of the program control chip
2) Software module: The tester is controlled by the portable master computer through the serial port. The SCM test platform completes excitation control, data acquisition and other work. All data analysis and processing and command control are completed by the programmable master 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 inspection software, etc The main modules are composed of
3. Main functions of the tester
The tester adopts circuit online testing technology, which can test and analyze the common faults of various SMAll and medium-sized integrated circuit chips online or offline, and test the characteristics of voltage/current analogy and digital devices The basic principle of digital chip function test is to detect and record the input/output state of the chip, and compare the recorded state with the standard state truth table to determine whether the function of the tested chip is correct Each digital device on the digital chip status test circuit board has three status characteristics after power on: the logic status of each pin (power, ground, high resistance, signal, etc.), the connection relationship between pins, and the logic relationship between input and output When an equipment fails, its state characteristics usually change The tester can select the state characteristics of each integrated circuit device on a good circuit board, store them in a computer database, and then compare them with SIMilar faulty pcb boards to accurately find the fault location 6、 Characteristic test analysis This test function is based on analog characteristic analysis technology, and can be used for analog test, digital special equipment, programmable devices, and large-scale and super large-scale equipment The tester automatically selects the characteristic curve of the measuring point through the test probe or test clamp, displays it on the computer screen, and stores it in the computer In the special fault diagnosis, compare the measured curve with the standard curve stored in advance, and then find out the fault Node voltage test, because 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 tester uses node voltage test technology By applying working voltage to the tested 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 function testers also have auxiliary test functions, such as electronic manual, test development, and system self inspection of printed circuit boards