Thread Content
Changhui Instruments http://yunrun.com.cn/ As most of us know, PLCs of various brands come equipped with diagnostic functions. When a PLC reports an error, we should make use of these diagnostic tools in the software to identify the problem and its cause. For example, when it is detected that the ERROR indicator light on the PLC is flashing or staying on constantly, one can open the software’s diagnostic menu to find out what the error is; it shows the cause of the error as well as solutions, which is very convenient. In addition, under normal circumstances, when an error occurs, the value of 8009, or one of 8060–8068, is written into the special data register D8004 (taking a Mitsubishi PLC as an example). Assuming that the value written to D8004 is 8064, the error code can be determined by checking the content of D8064. The error details corresponding to the error code can be found in the PLC manual’s error code table, which also helps to determine the cause of the error. Of course, generally speaking, when a PLC malfunctions, we first need to check whether the power supply voltage is normal, whether the screws and connectors of the I/O terminals are loose, and if there are any other abnormalities. Then, based on the status of the various LED indicators on the PLC, it is possible to check for any abnormalities either within the PLC itself or in its external connections, and troubleshooting is carried out in this manner. Below, using the Mitsubishi FX series PLC as an example, the empirical method is illustrated to determine the cause of errors in the PLC based on the status of the LED indicators. 1. Power indicator (LED light on): A. When power is supplied to the Mitsubishi PLC, the POWER LED on the PLC will light up. If the PLC is powered on but the [POWER] LED does not light up, check the power connections. Additionally, if the same power supply is used to drive sensors and other devices, it is necessary to check for any occurrences of load short circuits or overcurrent. B. If this indicator light does not turn on after the PLC is powered on, please unplug the wiring connected to the “24+” terminal. If the indicator light then lights up normally, it indicates that the DC load is too high; in such a case, do not use the DC power from the “24+” terminal, and instead prepare a separate DC 24V switching power supply. C. If it is not due to the reasons mentioned above, it may be that conductive foreign objects have entered the PLC or some other abnormal condition has occurred, causing the fuse in the basic unit to blow; this can be resolved by replacing the fuse. 2. Error indication ([EPROR] LED flashing): The [EPROR] LED will flash when there is a syntax error in the program (such as forgetting to set the constants for timers or counters), when parameter settings are incorrect, or when abnormal noises or foreign substances with electrical conductivity get into the system and cause changes in the content of the program memory. In such a situation, it is necessary to check whether there are errors in the program, as well as to look for any conductive foreign objects or high-intensity noise sources. 3. Error indication ([EPROR] LED lights up) A. If conductive foreign objects get into the Mitsubishi PLC or if it is affected by external noise, causing the CPU to lose control or the operation time to exceed 200ms, a WDT error occurs; the [EPROR] LED lights up, the PLC enters STOP mode, and all outputs are turned OFF. A power-off reset can be performed at this time; if the PLC returns to normal operation, check for any sources of abnormal noise or the presence of conductive foreign objects. Also, please check whether the PLC’s grounding meets the requirements. B. If the [EPROR] LED remains on, check whether the program’s operation cycle is too long (the maximum scanning time can be viewed by monitoring D8012). After conducting various checks, the problem of the [EPROR] LED not lighting up persists; it is likely due to a hardware fault within the PLC. Please contact Mitsubishi’s after-sales service. 4. Battery indicator ([BATT LED] light is on). When this [BATT LED] light turns red, it indicates that the lifespan of the lithium battery inside the PLC is nearly exhausted (about one month left). At this point, it is necessary to replace the lithium battery as soon as possible to prevent the programs stored in the PLC (when RAM is used) from being lost. If the LED light remains on after replacing it with a new lithium battery, it is likely that the CPU board of this PLC is damaged; please contact Mitsubishi’s after-sales service. 5. Input indicator light: Regardless of whether the LED light of the input unit is on or off, make sure to check whether the input signal switch is indeed in the ON or OFF position. If input components such as photo sensors are used, changes in sensitivity may occur due to dust or dirt adhering to the light-emitting/light-receiving parts, which could prevent it from reaching the ON state. Furthermore, it is possible that the ON and OFF inputs will not be received within a time shorter than the operation cycle of the Mitsubishi FX2N series PLC. Additionally, check whether a different voltage has been applied to the input terminal; if so, it will damage the input circuit. 6. Output indicator light: Whether the LED of the output unit is on or off, if the load cannot be turned ON or OFF, it is mainly due to overload, load short circuits, or inductive currents from capacitive loads, which cause the relay’s output contacts to stick together or result in poor contact due to inadequate contact surfaces. The above are the fault diagnosis methods for Mitsubishi PLCs. Have you ever encountered such situations without knowing how to determine the issue? Now you know, everyone can use the methods mentioned above to conduct checks~ Feel free to communicate in the comments section! Author: Cai Huiying