1 Introduction This article describes the common circuit board tester using online communication between Computer and Technology, Test and computer through serial communication, portable performance, suitable for all kinds of double-sided printed circuit board test and diagnosis.
2 circuit Test
2.1 online test theory
the basic principles of online testing is to test the instrument for the printed circuit board under test chips provide the stimulating, while automatically collecting computer records under the control of the measured chip output response and status values, through the computer records of all its values and standards of the state of the state a truth table, so as to judge the measured object fault conditions. After the driving test
2.2 technology
drive the testing technology is mainly used for online testing of digital circuits. The essence is in the input stage of the device under test (pre-driver chip output level) poured or pulled out a large transient current required to force changes its potential high or low, to impose tests on the device under test online incentive purpose.
order to ensure that the devices on the circuit board functional test, we must force the logic level driven devices, the foot or the driver must be able to absorb enough of the current output. In accordance with international protection standards for file (00-53/1) after the driver recommended by the safety standards, test instrument designed for maximum drive current is 240mA, test time in less than 200ms. Through the experiment, the basic can better isolate the tested devices, while also ensuring the safety of the device under test.
3 Test form
3.1 hardware module
tester from the portable computer, single chip test platform and test analysis software composition. SCM test platform in which to complete the computer-controlled data collection measured object, the basic block diagram shown in Figure 1. The function of each, and described as follows:
microcontroller circuit mainly for data acquisition, control, order processing, and computer data exchange. The design of the tester using MCS-51 series of 8031, use 2764 as an extension ROM, 6264, as extended RAM. Decoder chip circuit 74LS138. Serial communication with the computer, using MCl488 and MC1489 to RS-232C level and TTL level of mutual transformation. MCU system clock frequency of use 6MHz crystal, communication baud rate selection 2400, the SCM work three serial communication. Timer T1 is set to mode 2. Set SMOD = 1, time constant F3H.
bus driver on the MCU bus expansion, to improve their driving ability, use 74LS244, 74LS245 line driver.
drive control circuit mainly to complete the testing process TTL, CMOS test threshold control, use 4 heavy SPST (SPST) DG211 analog switches, switch control from the decoding circuit and finished into a 74LS373 latch . To ensure the DG211 is normally open at boot time (OFF) state, the line of control to increase pull-up resistor (10kΩ). Test drive circuit applied to the measured test chip input signals, using micro-relay for input signal control, test signal generated by the data buffer 74ACT244. To ensure the input current to meet the design requirements, using 4-way in parallel. To prevent damage to the device, increase the LC network of high current buffer, and the design of diode protection circuit.
data acquisition circuit to read the measured chip output response, LM393 dual voltage comparator to output signal. Its low power consumption, relatively high precision and can be compatible with TTL logic. LM393 output data latch 74LS373 linked, read by the MCU control comparative data.
voltage-driven D / A circuit to complete the testing process in step VI of the output voltage. 8-Bit Parallel D / A converter MC1408. Chip power supply voltage of +5 V,-12V two. Reference voltage provided by the constant current regulator TL431 source. Bipolar output select output, completed by the two amplifier LM348.
current transformation in acquisition A / D circuit implementation of the current test point data collection. Circuit with the load resistor and differential amplifier LM343 voltage follower of the test points, test points will convert the current value of A / D converter can handle the voltage capacity. Comparison of eight successive selection AD7574 high-speed A / D converter. Conversion time 15μS, single +5 V power supply. Selected reference voltage VREF =- 8V. Input voltage range of 0 ~ + | VREF |. RD-side program control chip produced a negative pulse can start A / D conversion.