1. CPU abnormality: When the CPU is abnormally alarmed, check the device connected to the internal line of the CPU unit. The specific method is to replace the units that may cause failures in turn, find out the faulty units, and deal with them accordingly.
2. Memory abnormality: When the memory is abnormally alarmed, if it is a problem with the program memory, the fault will be reproduced after reprogramming. In this case, it may be that the interference of noise causes a change in the program, otherwise the memory should be replaced.
3. Abnormal input/output unit and abnormal expansion unit: When this kind of alarm occurs, the insertion status and cable connection status of the connector of the input/output unit and the expansion unit should be checked first, and the unit should be replaced after determining the fault.
4. Do not execute the program: the input check is identified by the input LED indicator, or the input monitor composed of a writer is checked. When the input LED is not lit, it can be preliminarily determined that the external input system is faulty, and then check with the multimeter.
If the output voltage is abnormal, it can be determined that the input unit is faulty. When the LED is on but the internal monitor is not displayed, it is considered to be a fault of the input unit, CPU unit, or expansion unit.
The program performs a check through a monitor check on the writer. When the contact status of the ladder diagram does not match the result, it is a program error (e.g., double use of internal relays, etc.), or a malfunction in the calculation part.
Output checks can be identified using the output LED indicator. If the calculation result is correct but the output LED indicates an error, it can be considered as a fault of the CPU unit or 1/0 interface unit. When the output LED is bright but there is no output, it can be determined that the output unit is faulty, or the external load system is faulty.
5. Some programs are not executed: the check method is the same as the previous item, but if the input time of the counter, step controller, etc. is too short, there will be no response failure, then the input time should be checked whether the input time is large enough, and the verification can be carried out according to the relationship between the input time < the * response time of the input unit + the operation scanning time multiplied by 2.
6. If the power supply is powered off for a short time, the program content will also disappear: in order to make the microprocessor start correctly, the PLC is equipped with an initial reset point circuit and a saving program circuit when the power supply is disconnected. When such a circuit fails, the program cannot be saved. Check the on/off of the available power supply.
7. PROM cannot operate: first check whether the PROM is inserted well, and then determine whether the chip needs to be replaced.
8. After the power supply is re-put into or reset, the action stops: this fault can be regarded as caused by noise interference or poor internal contact of the PLC. The cause of noise is generally caused by the reduction of the capacitance of the small capacitor in the circuit board or the poor performance of the components, and the cause of the poor contact can be checked by gently tapping the PLC body. Also check the plugged in status of the cables and connectors.
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