PLC Working Mode As you may already know, the working mode of a PLC is based on cyclic scanning. This means that the PLC continuously scans the program from start to finish, left to right, in a repetitive loop. While this process seems simple at first glance, many beginners make mistakes during actual programming due to a lack of deep understanding of the underlying mechanism. So, how exactly does the cyclic scanning of a PLC work? To better explain this, we can use a two-way circuit as an example. Two-Way Circuit A two-way circuit is a common logic setup. For instance, consider a simple circuit where pressing a button toggles a light on and off. The first time you press the button, the light turns on; the second time, it turns off. This is known as a divide-by-two circuit. There are various ways to implement such a circuit, but for the purpose of this explanation—focusing on the PLC's working mode—we'll use basic logic instructions. Below is a diagram showing the setup: the button is connected to X0, and the lamp is connected to Y0. PLC Cycle Scanning Process A picture is worth a thousand words. Here’s a visual representation of the PLC’s cycle scanning process. Don’t worry if it’s not clear yet—there are detailed breakdowns below. The following four figures illustrate what happens when the button is pressed for the first time. - The light turns on. Figure 1: Program status before the button is pressed. Figure 2: Button is pressed, and the first scan cycle begins. Figure 3: The button is fully pressed (but not released), and the PLC starts the second scan cycle. Figure 4: Program status after the button is released. Now, let’s look at the process of the second button press and release. - The light turns off. Figure 1: Program status before the second button press. Figure 2: Second button press, first scan cycle of the PLC. Figure 3: Button is pressed again, and the PLC begins the second scan cycle. Figure 4: Program status after the button is released. Digit Segment Led Display,Washing Machine Display,Smd Led Display,Bar Segment Led Display Wuxi Ark Technology Electronic Co.,Ltd. , https://www.arkledcn.com
