In PLC programming, the physical contact of the stop button is generally connected as normally open, and the program is written as normally closed. Such a stop button only serves as a signal in this set of equipment.
Why write normally open for emergency stop?
So why is the emergency stop the opposite of the normal stop button? In fact, what is written as “Normal Open” is not only emergency stop, but also contacts with protection functions such as thermal relay and temperature control.
Prevent button failure
If the program is written as normally closed, then the emergency stop button is invalid at this time, and the stop signal cannot be sent to the PLC. It is very dangerous to stop the machine in an emergency. Therefore, write normally open in the emergency stop in the PLC program, and the external wiring is still normally closed contact When it is disconnected, it is a two-way disconnection. When it is disconnected in the program, the actual physical contact is also disconnected, which is the fail-safe protection.
The program is revised, the corresponding circuit should be revised
If it is an ordinary stop button, the external connection is normally open. When it is only used as a signal, there is no need to connect the load. It is only necessary to connect to the PLC. However, if it is a protection connection such as emergency stop or thermal relay, Then not only connect the PLC, but also connect to the actual circuit.
Of course, many people will not write the contact with protection function into the program. It is only an external function and not reflected in the program to prevent program errors from affecting actual operations. This is also a good approach.
