The Mitsubishi FX series PLCs come with 20 or 27 basic logic instructions, 2 step instructions, and over 100 function instructions depending on the model. In this section, we will focus on the FX2N model to explain its basic logic instructions, step instructions, and practical applications. The FX2N includes a total of 27 basic logic instructions, which also covers the 20 basic instructions found in some of the earlier sub-series PLCs.
Among these, the **SET** and **RST** instructions are particularly important.
- **SET (Set Instruction)**: This instruction is used to activate and maintain the state of a target component. Once set, the component remains in that state even if the input condition is no longer met.
- **RST (Reset Instruction)**: This instruction resets the target component, clearing its state and returning it to its initial condition. It can be used on components like Y (output), M (auxiliary relay), S (status relay), T (timer), C (counter), D (data register), V, and Z.
The application of these instructions is illustrated in Figure 1. For example, when X0 is activated, Y0 turns ON and stays ON even if X0 is turned off. However, if X1 is activated, Y0 turns OFF and remains OFF regardless of the status of X1 afterward.
These instructions are widely used in control systems for maintaining states, resetting operations, and managing timers and counters. They are especially useful in scenarios where a specific output needs to stay active or be cleared based on certain conditions.
The **SET** instruction can be applied to Y, M, and S components, while **RST** can be used on a broader range of components including Y, M, S, T, C, D, V, and Z. This makes RST ideal for clearing data registers or resetting internal devices like timers and counters.
Additionally, the same component can be operated by both SET and RST multiple times, and the order in which they are used may vary. However, only the last executed instruction will have an effect on the component's final state.
Understanding how to use these instructions effectively is essential for programming efficient and reliable PLC systems. Whether you're designing a simple automation task or a complex control system, mastering SET and RST will help you manage outputs and internal states more precisely.
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