In PLC programming, the physical contact of the stop button is generally connected as normally closed, while in the program, it is written as normally open. This way, the stop button only serves as a signal in this equipment.Why is the emergency stop in PLC programming opposite to that of a regular stop button? In fact, not only the emergency stop but also thermal relays, temperature controls, and other protective contacts are written as normally open. The specific reasons are detailed below:First, the emergency stop button is also connected to the PLC’s DI point as a normally closed contact:
The above image shows a part of the wiring diagram for the emergency stop button connected to the PLC, where you can see it is connected as a normally closed contact! The advantage of connecting it as a normally closed contact is that when the circuit of the emergency stop button is broken, the program can immediately reflect this, or in other words, it is equivalent to the emergency stop button being pressed, turning the normally closed contact into a normally open contact, thus achieving monitoring of the disconnection state!Next, let’s take a look at the PLC program:
In this program, the start condition 1 (I0.2) and start condition 2 (I0.3) are both connected to the PLC as normally open buttons, while the emergency stop button (E_StopPB) is connected to the PLC as a normally closed button! — From the circle ① in the image, we can see that when none of the three buttons are pressed, only E_StopPB is connected. When all three buttons are pressed, as shown in circle ②, the two states of start conditions 1 and 2 are connected, while the state of E_StopPB shows as disconnected!The difference between these two states explains why the emergency stop button is wired as normally closed at the PLC hardware level, while it is used as normally open in the PLC program! — This is because the digital input (DI) points of the PLC are designed such that when the peripheral circuit is connected, the program internally shows as connected, and when the peripheral circuit is disconnected, the program internally shows as disconnected. This is a result of the hardware design of the PLC’s DI points!Additionally, because the emergency stop signal is a very important point, people utilize the hardware characteristics of the PLC’s digital input (DI) points to stipulate that the wiring of the emergency stop button should be connected as a normally closed point! — Because the normally closed point keeps the circuit continuously connected, the program must use a normally open point, ensuring that when there is no emergency stop, the program logic can remain connected!