Mitsubishi PLC maintenance:
Maintenance methods: Open the machine and find that the power is burned out. After the rectifier bridge filter electrolytic capacitor has been blown open, the fuse is burnt black, and the filter capacitor is short-circuited when viewed with a multimeter. Open the fuse, check other components one by one and found no burnout. Replace the fuse and filter electrolytic capacitor after power, test the power of each group has been normal, installed the entire machine, power indicator light after the power, the input and the public end of the short circuit, the input light, the corresponding output light is the same. Use the system test platform to verify that everything is normal.
PLC diagnosis confirmed:
All brands of PLCs have self-diagnosis functions. When the PLC is abnormal, it should make full use of its self-defining diagnosis to analyze the cause of the disease. When the PLC is abnormal, first check the supply voltage, PLC and I/O terminal screws and connectors are loose, and whether there are other abnormalities. Then, according to the indicator status of various LEDs set on the basic unit of the PLC, to see whether the PLC itself and the outside are abnormal. In case of a seizure error, one of the values ​​of 8009, 8060 to 8068 is written in the special data register D8004. Assuming that the content of the write D8004 is 8064, the error code can be known after viewing the contents of D8064. Refer to the error code table of the PLC application manual for the actual mistakes that correspond to the error code.
The Mitsubishi FX series PLC is used as an example to illustrate the method to diagnose the causes of PLC faults based on the status of the LED indicators.
1. Power indicator ([POWER] LED indication)
When power is supplied to the Mitsubishi PLC basic unit, the [POWER] LED on the surface of the unit is lit. If the power is turned on but the [POWER] LED is off, please acknowledge the power wiring. In addition, if the same power supply has a drive sensor, etc., please acknowledge whether there is a load short circuit or overcurrent. If it is not for the above reason, the PLC may be mixed with conductive foreign matter or other abnormal conditions, so that the fuse in the basic unit is blown. At this moment, it can be solved by replacing the fuse.
2. Error indication ([EPROR]LED shines)
When the program syntax is wrong (such as forgetting to set timer or counter constants, etc.), or there is abnormal noise, conductive foreign matter mixed into the other causes the program memory content changes, [EPROR] LED will shine, Mitsubishi PLC is in STOP condition, All output turns OFF. In this situation, check for any errors in the program and check for sources of conductive foreign matter and high-intensity noise.
3. Error indication ([EPROR] LED is on)
Because Mitsubishi PLC internal conductive foreign matter mixed or affected by external abnormal noise, resulting in CPU out of control or computing cycle exceeds 200ms, WDT mistakes, [EPROR] LED lights, PLC is in STOP, all outputs are turned OFF. At this moment, power off reset can be performed. If the PLC recovers, please check whether there is any abnormal noise source and conductive foreign matter. In addition, please check whether the grounding of the PLC meets the requirements. If the [EPROR] LED is turned on and the brightness changes, check the procedure. If the [EPROR] LED still insists on the status of the light, please admit that the program operation cycle is too long (supervised D8012 shows the maximum scan time). If the [EPROR] LED's status is still not cleared after full inspection, consider that there is something wrong with the PLC. Please contact your skills service provider.
4. Enter instructions
Regardless of whether the LED of the input unit is on or off, check whether the input signal switch is actually ON or OFF. If the rated current capacity of the input switch is too large or because of oil intrusion, etc., contact failure occurs easily. When the input switch is connected in parallel with the LED light resistance, the Mitsubishi FX series PLC can still be input even if the input switch is OFF but the parallel circuit is still ON. If an input device such as an optical sensor is used, the sensitivity may change due to dirt and other parts of the light emitting/receiving area, and the ON state may not be completely entered. The ON and OFF inputs cannot be received within a time shorter than the Mitsubishi FX2N Series PLC operation cycle. If a different voltage is applied to the input terminals, the input circuit will be damaged.
5. Output instructions
Regardless of the output unit's LED light is still off, if the load can not be turned on or off, mainly because of overload, load short circuit or capacitive load inrush current, causing the relay output contact adhesion, or the contact point contact surface is not good cause contact bad.
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