Troubleshooting of input and output signals
Check the PLC output status: When the actuator (such as motor, cylinder) does not respond, first check the output status of the PLC. You can monitor the status of the output point online through the PLC programming software to see if there is a signal output. If the PLC does not output a signal, check whether the program logic is correct and whether the output conditions are met.
Check the output circuit power supply: In the case where there is an output signal but the actuator does not work, check the power supply of the output circuit. For example, the PLC controls the relay coil, and the relay controls the AC contactor to drive the motor. Check whether the auxiliary contacts of the relay are normal and the coil power supply of the AC contactor is normal, including whether the voltage meets the requirements and whether the line is unobstructed.
Check the actuator itself: If the power supply and PLC output are normal, check whether the actuator is faulty. For example, for motors, check whether the motor windings are open or short-circuited, and whether the mechanical part is stuck; for cylinders, check whether the gas source is normal and whether the piston of the cylinder is stuck.
Check the sensor power supply: If the PLC does not receive the expected input signal, first check the sensor power supply. For example, for a proximity sensor, its operating power supply may be DC 24V. Use a multimeter to measure the voltage between the sensor power terminals to see if it is within the normal range. If the power supply voltage is abnormal, check whether the power supply line is open, short-circuited, or the power module is faulty.
Check the signal line connection: Confirm whether the sensor signal line is correctly connected to the input terminal of the PLC. You can check along the signal line to see if the line number is correct and whether the wire is loose or falling off. Sometimes, due to vibration and other reasons, the wire on the terminal may become loose, and re-tightening the loose wire may solve the problem.
Check the sensor status: If the power supply and connection are normal, check whether the sensor itself is working properly. For example, for a photoelectric sensor, you can observe whether the sensor indicator light flashes normally by blocking and releasing the light beam. If the sensor is damaged, you need to replace it with a new sensor.
Communication troubleshooting
Check the communication line: If the communication between the PLC and the host computer or other equipment is interrupted, first check the communication line. Check whether the communication cable is firmly connected and whether there is any damage. For Ethernet communication, check whether the crystal head of the network cable is properly plugged in and whether there is poor contact caused by damaged core. For serial communications such as RS-485, check whether the A and B lines of the communication line are connected in reverse.
Check the communication parameter settings: confirm whether the communication parameters of the PLC and the communication device are consistent. Including baud rate, data bit, stop bit, check mode, etc. For example, different baud rate settings of the PLC and the host computer will cause communication failure, and the communication parameters of the two need to be set to the same value.
Check the communication module status: check whether the communication module of the PLC is working properly. The communication module may have a fault light on. At this time, you need to refer to the manual of the communication module to determine the cause of the fault. For example, overheating of the communication module, damage to the internal circuit, etc. may cause communication failure, and the communication module may need to be replaced to solve the problem.
Power supply troubleshooting
Check the main power input: If the PLC control cabinet loses power as a whole, first check the main power input. Check whether the fuse is blown and whether the circuit breaker is tripped. If the fuse is blown, check whether the circuit is short-circuited, replace the fuse and try again. If the circuit breaker is tripped, check whether there is overload, etc., reset the circuit breaker and observe the operation of the equipment.
Check the power module output: If some devices lose power, check the output of the power module. For example, if the digital output module of the PLC has no power, check whether the voltage of the power output terminal that supplies power to the module is normal. If the output voltage of the power module is abnormal, it may be an internal fault of the power module, and the power module needs to be repaired or replaced.
