(3) The crystal is deteriorated to change its parameters. It can only be detected with an oscilloscope and a frequency meter. Although the crystal oscillator can oscillate, its clock frequency deviates from its nominal value. At this time, although there are oscillation pulses, the system circuit cannot work due to the wrong number of pulses. At this time, only the frequency meter can accurately measure the deviation.
(4) In the actual clock circuit, the crystal oscillator is connected with a ceramic capacitor of several picofarads to several tens of picofarads from both ends of the crystal oscillator. The fault of the clock circuit caused by leakage and deterioration of the capacitor is also common. It is best to measure the quality of the crystal with an oscilloscope and a frequency meter. It is difficult to judge whether the multimeter is good or bad.
For example, a machining center controlled by FANUC 6M, after working for a while, suddenly CRT black screen, the machine does not move. Turn off the power, then send the power, the machine can work for a while. Check that the power supply is working fine. The fault may be on the system board. After repairing the main board A16B - 1000 - 0220/ 04A, it was found that one of the two crystal oscillators had a poor internal contact with the 16.840MHz crystal oscillator. No similar faults have occurred since the replacement.
3, reset circuit
The reset circuit is also a circuit existing on the system board, which is a circuit unique to a large-scale digital integrated circuit. The microprocessor, interface circuit, etc. all have reset terminals.
The reset pulse generated by the reset circuit clears the program counter, causing the CPU to call the initialization file from the memory and initialize each control chip port. If the reset circuit is defective, the system will malfunction such as turbulence or crash.
Generally, when observing the reset pulse with an oscilloscope, the power should be turned on and off repeatedly, and the reset pulse should be observed every time the switch is turned on. The reset pulse should be an ideal rectangular square wave. If there is no reset pulse, check the resistors, capacitors, transistors, etc. in the reset circuit. The reset terminal of the integrated circuit should be a regular low or high level. Otherwise, it should be a reset circuit failure or an integrated circuit damage.
For example, a large machining center using the PLASMA CNC system, the system cannot be started, and the CRT has no alarm display. After checking that the ±5V, ±12V, and ±24V power supply voltages are normal, the clock circuit is normal. Suspected to be a problem with the system board. When checking the reset circuit, it was found that there is no reset pulse on the CPU reset side. Further inspection found that a 3. 3k / 0. 5W resistor open circuit on the reset end, the system starts normal after replacement.
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