With the continuous development of industrial automation, PLC is widely used in industrial control professions. No matter how strong the PLC product is, it often fails to work normally. All brands of PLC have the function of self-diagnosis, but the skill of PLC repair is that when the PLC is abnormal, it should be fully used to self-diagnose and analyze the cause of the problem, and then accurately find the problem.
1. The CPU is abnormal
When the CPU is abnormally alarmed, you should check all the devices connected to the internal bus of the CPU unit. The specific method is to replace the units that may have problems sequentially, find out the problem units, and deal with them accordingly.
Second, the memory is abnormal
When the memory is abnormally alarmed, if it is a problem with the program memory, it is still not possible to solve the problem after reprogramming, which may be caused by noise disturbance and the program is changed, otherwise the memory should be replaced.
3. The input/output unit is abnormal, and the expansion unit is abnormal
When such an alarm is generated, you should first check the connection of the connector and cable connection between the input/output unit and the expansion unit, and then replace the unit after confirming the unit caused by the problem.
4. Non-performance of procedures
In general, you can check it according to the steps of input-program performance-output
(1) Input viewing is identified by the input LED indicator or by an input monitor consisting of a writer. When the input LED is not lit, it can be preliminarily determined that it is a problem with the external input system, and then cooperate with the multimeter to check. If the output voltage is abnormal, it can be determined that the input unit is a problem. When the LED is on but the internal monitor is not visible, it can be considered a problem with the input unit, CPU unit, or expansion unit.
(2) The program is viewed through the monitor on the writer. When the contact situation of the ladder diagram is inconsistent with the result, it is a program error (such as the use of two layers of internal relays, etc.), or the operation part is a problem.
(3) Output view: available output LED indicator identification. When the calculation effect is correct and the output LED indicates the error, it can be considered as a fault of the CPU unit and I/O interface unit. When the output LED is bright but there is no output, it can be determined that there is a problem with the output unit or an external load system.
Because of the different PLC models, the connection methods of I/O and LED are different (some are connected to the interface of the I/O unit, and some are connected to the I/O unit). Therefore, the scale of the problem judged by the LED is also different.
5. Some procedures are not performed
The viewing 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 a problem of no photo, in this case, you should check whether the input time is large enough, and the verification can be carried out according to the connection of the input time (the maximum response time of the input unit + the calculation scan time multiplied by 2).
6. If the power supply is powered off for a short time, the program content will also disappear
(1) In this case, in addition to checking the battery, check as follows.
(2) Check by repeatedly turning on and off the PLC's own power supply. In order for the microprocessor to operate correctly, the PLC is equipped with a circuit for the initial reset point and a circuit for saving the program when the power supply is disconnected. If there is a problem with such a circuit, the program cannot be saved. Therefore, you can check the power supply on and off.
(3) If the battery abnormal alarm still appears after the battery is replaced, it can be determined that it is caused by the abnormal increase of the leakage current of the memory or external circuit.
(4) The on/off of the power supply always occurs synchronously with the machine system, and the noise impact of the machine system can be viewed. This is because the disconnection of the power supply is often associated with the operation of the machine system, and most of it is caused by the strong noise generated by the motor or winding.
7. PROM cannot be operated
First, check whether the PROM articulation is excellent, 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 problem can be thought to be caused by noise disturbance or poor contact within the PLC. The cause of noise is generally caused by the reduction of the capacitor capacity of the small capacitor or the poor function of the component on the circuit board, and the cause of the poor contact can be viewed by gently tapping the PLC body, and the insertion of the cable and adapter can also be checked.
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