Have you experienced the eight pits in the PLC debugging process?

“Various brands of PLCs have a self-diagnosis function, but the trick to PLC repair is to make full use of this function to analyze, and then accurately find the problem. 8 types of common errors that PLC repairers need to know when PLC displays abnormal alarms.

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Various brands of PLCs have a self-diagnosis function, but the trick to PLC repair is to make full use of this function to analyze, and then accurately find the problem. 8 types of common errors that PLC repairers need to know when PLC displays abnormal alarms.

1. Abnormal CPU

When the CPU alarms abnormally, 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 in sequence, find out the problem units, and deal with them accordingly.

2. Abnormal memory

When the memory is abnormally alarmed, if it is the problem of the program memory, it cannot be solved after reprogramming. This situation may be caused by the disturbance of noise and cause the program to be changed. Otherwise, the memory should be replaced.

3. The input/output unit is abnormal and the expansion unit is abnormal

When such an alarm occurs, you should first check the connection status of the input/output unit and the expansion unit connector, and the cable connection status, and then replace the unit after determining the unit that has the problem.

Fourth, do not execute the program

Under normal circumstances, it can be viewed according to the process of input – program execution – output

(1) Input check is to use the input LED indicator to identify, or check with the input monitor composed of the writer. When the input LED does not light up, it can be determined that it is the problem of the external input system, and then check with the multimeter. If the output voltage is abnormal, it can be concluded that the problem is the input unit. When the LED is on but the internal monitor does not appear, it can be considered a problem with the Input Unit, CPU Unit, or Expansion Unit.

(2) The program is checked through the monitor on the writer. When the contact status of the ladder diagram is inconsistent with the results, it is a program error (such as the use of two layers of internal relays, etc.), or there is a problem in the operation part.

(3) Output viewing can be identified by output LED indicators. When the operation result is correct but the output LED indicates an error, it can be considered as a problem with the CPU unit and I/O interface unit. When the output LED is on but there is no output, it can be judged that there is a problem with the output unit or the external load system.

Due to different PLC models, 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 according to the LED judgment is also different.