The output is abnormal after the touch screen is disconnected from the PL

Fault Name:

The output is abnormal after the touch screen is disconnected from the PLC.

Fault Symptom:

After disconnecting the touch screen from the PLC, the inverter stops running and runs normally after being turned on.


DC 0V is not connected to the output common terminal.


PLC output COM terminal can be solved by normal wiring


Such problems are generally caused by unqualified electrical personnel from equipment manufacturers! Their understanding of the wiring problem is not comprehensive.

Such problems are generally excluded from my personal point of view from the following points:

The first thing I think of is to check the program, because the program is not written by myself, I don't understand the PLC output.

Associated with the touch screen, so view the program:

Check if the touch screen has a script that judges the communication error. After viewing the touch screen program, no script is found using the touch screen. Troubleshoot touch screen programs.

Check the PLC program. After careful inspection, there is no problem that determines the abnormal shutdown of the communication.
Combining two basics can basically eliminate program problems.

Check the inverter parameter settings to check if the inverter is started and stopped by communication. By looking at the parameters, the inverter is started and stopped by an external terminal, and no communication is used. So you could exclude any inverter parameter settings fault.

Finally, there are only circuit problems left. Because the customer insists that their line is ok, it has always been the case. Think it is a problem with the program. Because the customer wiring is connected by the electrician himself, only the IO meter, there is no detailed secondary wiring diagram and line number, so it is difficult to find the line, you must use the multimeter to measure the line. After continually trying to find the problem, I suddenly found that as long as the DB9 metal case of the communication line and the touch screen DB9 metal case came together, the inverter runs immediately. This kind of problem is really strange. This phenomenon can be initially determined to be related to GND. After carefully checking the line, it is found that the PLC output common terminal COM is not actually connected to the GND of the switching power supply, but is suspended.

In this case, the loop is generated by the communication line shielding, so disconnecting the communication line will cause the PLC output to be disconnected.

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