In the world of PLC programming, newbies are often confused by all the terms and conventions. PLC, or programmable logic controller, is known as the "brain" of industrial automation, but it is not easy to master it thoroughly.
Today, we are going to discuss a confusing and controversial concept: Why is the emergency stop button often written as Normal Open (NO) in PLC programs? This question can be called an "unsolved mystery" in the programming world, which not only makes novice programmers rack their brains, but also often triggers "deep thinking" among veteran programmers.
"Normally Open" and "Normally Closed" in the PLC World
To understand this problem, we first need to understand the concepts of "normally open" and "normally closed" in PLC. Simply put, normally open (NO) and normally closed (NC) describe the state of a switch or button when it is not pressed or activated. Normally open, as the name suggests, means that the circuit is open when the button is not pressed; while normally closed means that the circuit is closed when the button is not pressed.
So, the question is, why is the emergency stop button defined as normally open in the PLC program? Intuitively, this seems a bit unreasonable. As a safety device, the emergency stop button is usually designed to disconnect the circuit and immediately stop the machine when pressed. According to this logic, the emergency stop button should be normally closed.
Safety first: the physical and logical layers of emergency stop buttons
On a physical level, emergency stop buttons are often designed as normally closed (NC). This is because, in electrical design, normally closed buttons maintain circuit continuity when no operation is performed. When an emergency stop is required, the button is pressed and the circuit is disconnected, thereby immediately stopping the operation of the machine. An important advantage of this design is that in the event of a button or line failure (such as a broken wire), the machine will automatically stop to ensure safety.
However, when it comes to the logic level of PLC programming, we need to do the opposite. Although the logic layer and physical layer of PLC are intertwined, the processing methods are different. In PLC programming, we usually write the input signal of the emergency stop button as normally open (NO), mainly for the following reasons:
1. Easy to understand and maintain: Logically, the normally open input has a low signal (0) when not pressed and a high signal (1) when pressed. This design is more intuitive for programmers, especially easier to understand and handle during debugging and maintenance.
2. Prevent misoperation: In actual applications, misoperation of the emergency stop button may have serious consequences. By writing the emergency stop button as normally open in the PLC program, it can be ensured that the emergency stop signal will only be triggered when an emergency stop is really needed.
3. Compatibility: Many PLC systems and software tools expect input signals to be normally open by default. This compatibility design can reduce errors caused by inconsistent signal definitions during programming.
"Programming Trap" of PLC Emergency Stop Button
Although it is reasonable to define the emergency stop button as normally open in the PLC program, it is still a "programming trap" for beginners. Many novices will naturally think that the emergency stop button should be normally closed when they first come into contact with PLC programming. This intuitive "misunderstanding" often leads to confusion and errors when debugging the program.
A typical example is that when you are debugging a PLC program, if the emergency stop button is mistakenly defined as a normally closed state, you may find that the system does not work properly in the normal state you expect. The reason is that the PLC program will think that the emergency stop button is pressed, causing the entire system to enter the "stop" state. This error may seem simple, but in an actual industrial environment, it may cause serious production delays and losses.
Don’t be blinded by “normally open”
Therefore, when programming the PLC, it is important to remember that the emergency stop button should be defined as a normally open state. This is an "iron rule" that must be followed. However, this does not mean that we should ignore physical safety considerations in actual operation. On the contrary, we need to find a balance between physical design and logical programming to ensure the safety and reliability of the system.
During the programming process, if you feel that you are confused by these "normally open" and "normally closed", you might as well stop, have a cup of coffee, take a deep breath, and then re-examine your design and code. Sometimes, calming down and thinking about the problem again will make you suddenly enlightened.
Design philosophy of emergency stop button in PLC programming
Why should the emergency stop button in PLC programming be written as normally open? This may seem like a small question, but it contains a profound design philosophy. It not only reflects the trade-off between safety and ease of use, but also reveals the different ways of dealing with the physical world and the logical world.
In actual work, only by understanding and following these rules and conventions can we truly master the complexity of PLC programming. As the "guardians" of industrial automation, we need to always be sensitive and alert to these details. After all, in an industrial environment, safety and efficiency are always the most important.
So, next time when you see the emergency stop button in the PLC program is written as normally open, don't be surprised, this is the wisdom of PLC programming.
If you have more questions, you can leave us a message. We also hope that you can master the world of PLC programming with ease.
