Safety
Risk of faulty operation: If the normally closed point is used for access, when the line connected to the emergency stop button is broken or loose, the PLC will receive a signal loss, that is, it will mistakenly think that the emergency stop button is pressed. This may cause the equipment to stop suddenly during normal operation, causing unnecessary production interruptions, and may even affect the quality of products in some continuous production processes. For example, in chemical production, an unwarranted emergency stop may prevent the reacting chemical substances from proceeding according to the normal process, resulting in waste of raw materials or unqualified products.
Fault safety hazards: In some occasions with extremely high safety requirements, such as elevator control and large amusement facility control, this misoperation may bring more serious safety hazards. Elevators may suddenly stop running, trapping passengers; amusement facilities may also brake urgently during operation, causing physical harm to tourists.
Program logic processing
Complexity of signal state judgment: In the program, for the normally closed emergency stop signal, it is closed during normal operation (signal is 1), and it is disconnected after the button is pressed (signal is 0). This is the opposite of the signal change logic when the normally open point is connected. This will make the program inconsistent with other normal control signal logic when processing emergency stop logic. For example, in the program part that uses the rising or falling edge of the signal to trigger the action, the falling edge trigger of the normally closed emergency stop signal will appear when the normal state turns to the emergency stop state, but it is the rising edge trigger when resuming normal operation (closing the emergency stop button again). This opposite logic may increase the difficulty of program writing and debugging.
Impact on the scanning cycle: When scanning the input signal, the PLC needs to constantly judge the state of the emergency stop signal. For the normally closed signal, since its normal state and fault state (line fault causes signal loss) will cause the PLC to receive a low-level signal, distinguishing between normal emergency stop operations and line faults requires additional program processing and time overhead, which may have a certain impact on the PLC’s scanning cycle. In some control systems with high real-time requirements, this may cause control delays.
