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The Mitsubishi FX2N PLC indicator light PR0G is flashing, the input and output signal indicators are off, and the parameters set on the touch screen have disappeared. Research shows that PROG indicates a program error. After retrieving the program on the computer, everything appears fine. Replacing it with another PLC of the same model still results in the same fault. Repairs were paused as it was late.
When I resumed repairs the next day, I thought, since the PLC program is fine, could it be a circuit fault?
The PLC output load has a silicon-controlled rectifier adapter board. If there is a short circuit in the line, it could damage the silicon-controlled rectifier, and the signal light would show abnormal behavior. Upon checking the input, I found a short circuit in the power line of a proximity switch, which is powered by the PLC.
The cause of the fault was identified: the PLC output 24V power supply short-circuited, triggering the PLC’s internal fault protection. After replacing the switch, the system powered on normally.
In another case, the main pump’s touch screen was set to N0, but the main pump motor showed no response. The PLC input and output signals were normal, but there was no 4-20mA signal from the variable frequency drive’s analog input. It was determined that the FX2N-2DA output module was faulty.
After purchasing a new one, the signal was restored, but the variable frequency drive still wouldn’t start. The drive displayed normal parameters and no fault alarms. Upon disassembling the drive, I found that the drive board was burnt out. After replacing the drive and setting the parameters, the main pump started normally.
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