Understanding from the perspective of signal state detection
In the digital input circuit of the PLC (Programmable Logic Controller), under normal circumstances, when the emergency stop button is not pressed, the input point has a signal (high level or 1 state).
When the emergency stop button is connected to a normally open point, during normal operation, the input point of the PLC can detect this “closed” signal (for example, 1). Once the emergency stop button is pressed, the input point signal becomes “disconnected” (for example, 0), so that the PLC can quickly detect the change in the signal and execute the program logic related to the emergency stop.
From the perspective of program safety and reliability
For the PLC scan cycle, the emergency stop signal access method of the normally open point is more in line with the fault-safety principle. If the normally closed point is used for access, when the line is disconnected or other faults occur, the PLC may misjudge that the emergency stop button is pressed, causing the equipment to stop abnormally. With the normally open point access, the PLC will only execute the emergency stop program when the emergency stop button is actually pressed and the physical connection changes the signal, which can reduce the false action caused by line failure.
Programming habits and logical consistency
In the logic design of the PLC program, the processing method of the normally open emergency stop signal can better cooperate with the logic of the rising edge trigger of other control signals. For example, many control logics are based on the change of the signal from 0 to 1 (rising edge) or from 1 to 0 (falling edge) to trigger the action. After the emergency stop button is connected with the normally open point, the falling edge signal generated by pressing the emergency stop button can be conveniently used to trigger a series of emergency stop related actions, such as cutting off the output, stopping the motor, etc., making the program logic clearer and more unified.
