A Shanghai-made P40 rotary offset press in the factory, once after power-on, there was no indication in the PLC programmable logic controller (PLC) part, and all the external display of the whole machine had no operation failure.

This machine adopts Japanese MITSUBISHI FX2N-4DA type programmer, which measures the PLC220V power supply voltage (L, N terminal). The value is normal. Then open the PLC to check whether the internal insurance is blown, and the insurance is intact. Because there is no PLC details, it is not possible to determine whether there is a problem with the PLC internal circuit or the peripheral circuit.

Because the peripheral line and the main box are all plug-connected, the operator adopts the elimination method to disconnect all the peripheral lines (air plugs P1, P2A, P3A) except the main motor in the box, and then connect the debugging board. Switch to the debug file. Power-on test, the result is that the whole machine returns to normal, indicating that the PLC is in good condition, so it is judged that the fault is in the peripheral circuit connected to the three plugs. Then switch the switch to the operation file, and then plug in P3A, P2A, P1 and turn on the test respectively. The result is that when the P1 is connected, the PLC indicates nothing, and it can be determined that the fault is in the peripheral circuit connected to P1 (folding part). After careful examination of this section, it was found that the IN24 wire was exposed and grounded. After exposing the exposed connector, turn on the power and test the car. The fault is removed and it returns to normal.