To judge the quality of a button, limit, line and other input circuits, you can press the button (or other input contacts) when the PLC is powered on (in non operation state, in order to prevent misoperation of the equipment). At this time, the corresponding PLC input terminal and the common terminal are short circuited, and the PLC input indicator corresponding to the button is on, indicating that the button and the line are normal. If the light is not on, it may be twisted, the line may be in poor contact, or the line may be broken.
If further judgment, if the button is good, then use a probe of the multimeter, one end is connected to the common end of the PLC input end, and the other end is connected to the corresponding PLC input point (the above operation should be careful, never touch the 220 V or 110 V input terminal). At this time, the indicator light is on, indicating that there is a fault in the circuit. The indicator light is not on, indicating that the PLC input point has been damaged (this situation is rare, generally caused by strong current intrusion).
Maintenance of output circuit
For the PLC output point (here we only talk about the relay output type), if the indicator light corresponding to the action object is not on, and it is determined that the PLC is in the running state, then the PLC input and output logic function of the action object is not satisfied, that is to say, the input circuit is out of order. Check the input circuit according to the above.
If the corresponding indicator light is on, but the corresponding executive components such as solenoid valve and contactor do not work, check the control power supply and fuse of the solenoid valve first, and use the electric pen to measure the common terminal of the corresponding PLC output point. If the pen doesn't light up, it may correspond to power failure such as fuse blow. When the pen is on, it shows that the power supply is good and the corresponding solenoid valve, contactor and circuit are faulty.
If the fault of solenoid valve, contactor and circuit is still abnormal, one end of the multimeter is connected to the corresponding output common terminal, and the other end is connected to the corresponding PLC output point. At this time, the solenoid valve still does not act, indicating that the output circuit is faulty. If the solenoid valve acts at this time, the problem lies in the PLC output point.
Because the electric pen sometimes makes a false report, it can be analyzed by another method. The voltage between the PLC output point and the common terminal can be measured by the voltage level of the multimeter. If the voltage is zero or close to zero, it means that the PLC output point is normal and the fault point is outside. If the voltage is high, it means that the contact resistance of this contact is too large and it has been damaged.
In addition, when the indicator light is not on, but the corresponding solenoid valve and contactor act, this output point may be burned due to overload or short circuit. At this time, the external wiring of the output point should be removed, and then the resistance of the output point and the common end should be measured by the resistance gear of the multimeter. If the resistance is small, it means that the contact is broken. If the resistance is infinite, it means that the contact is good, and the corresponding output indicator light is broken.
