Various brands of PLCs have a self-diagnosis function, but the trick of PLC repair is to make full use of this function to analyze and then pinpoint the problem. Sorted out the 8 common types of errors that PLC repairmen need to know when the PLC presents abnormal alarms.
1. The CPU is abnormal
When the CPU is abnormally alarmed, check all the devices connected to the internal bus of the CPU unit. The specific method is to replace the potentially problematic units sequentially, find out the problematic 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 still cannot be solved after ab initio programming, this situation may be caused by the disturbance of noise to change the program, otherwise the memory should be replaced.
3. The input/output unit is abnormal, and the expansion unit is abnormal
When this kind of alarm occurs, the connection status of the input/output unit and the extension unit adapter and the cable connection should be checked first, and the unit should be replaced after determining the unit where the problem occurred.
4. Non-implementation of procedures
In general, it can be viewed according to the process of input-program execution-output
(1) Input viewing is identified by the input LED indicator or by the input monitor composed of a writer. When the input LED is not lit, you can start to conclude that it is a problem with the external input system, and then check it with the multimeter. If the output voltage is abnormal, it can be concluded 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 a monitor on the writer. When the contact status of the ladder diagram does not match the result, it is a program error (such as the use of two layers of internal relays, etc.), or a problem in the calculation part.
(3) Output view: Identification can be identified by output LED indicator. If the calculation result is correct but the output LED indication is incorrect, it can be considered to be a problem with the CPU unit or 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.
Due to the different PLC models, the I/O and LED connection methods are different (some are connected to the I/O unit interface, and some are connected to the I/O unit). Therefore, the scale of the problem based on LED discrimination is also different.
5. Some procedures are not implemented
The inspection 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 echo problem, in this case, it is necessary to check whether the input time is large enough, and the check can be carried out according to the connection of the input time (the maximum response time of the input unit + the time of operation and scan multiplied by 2).
6. If the power supply is powered off for a short time, the program content will also disappear
(1) First check if there is a problem with the battery.
(2) After repeatedly turning on and off the power supply of the PLC itself. In order for the microprocessor to start up 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. When there is a problem with this 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 replacing the battery, 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 is always synchronized with the machine system, and the noise impact of the machine system can be viewed. Since the disconnection of the power supply is often a problem that occurs at the same time as the operation of the machine system, most of it is caused by the strong noise generated by the motor or winding.
7. PROM does not work
Check to see if the PROM connection is good, and then decide if you need to replace the chip.
8. After the power is restarted or reset, the action stops
This problem can be attributed to noise disturbance or poor contact within the PLC. The cause of noise is generally caused by the reduction of small capacitor capacity or poor function of components on the circuit board, and the cause of poor contact can be viewed by gently knocking on the PLC body, and the connection status of the cable and the adapter can also be checked.
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