1. Introduction
Programmable Logic Controllers (PLCs) are widely used in industrial control systems due to their compact size, high cost-performance ratio, low power consumption, strong anti-interference capability, and ease of programming. However, in practical applications, the number of output points required for the controlled object is often less than the number of input points. This can lead to situations where the system requires more input points than the PLC's built-in capacity allows. To address this issue, several strategies are commonly employed.
Firstly, additional input modules can be added to expand the number of input points, which increases the overall system cost. Alternatively, selecting a PLC with a larger number of input points may result in unnecessary output point redundancy, leading to resource waste. Another approach involves using peripheral circuits such as decoders, which can reduce initial investment. Finally, software-based solutions allow for expanding input points without additional hardware by utilizing the PLC’s existing resources through clever programming techniques.
This paper introduces two software-based methods for expanding PLC input points, leveraging the flexibility of input and output configurations to enhance system functionality efficiently.
2. Input Point Expansion Method Based on Software Programming
One of the key features of PLCs is the independence of their input and output points. This independence allows for flexible configuration, enabling multiple input and output points to share common terminals or operate independently. By combining these capabilities, it becomes possible to expand the number of input points using principles similar to those of matrix keyboard scanning and input node matrix organization.
2.1 Expanding PLC Input Points Using the Matrix Keyboard Scanning Principle
In this method, m PLC input points are used as row lines, while n output points are used as column lines in a matrix structure. The common terminals of both input and output are connected, and the column lines are controlled via internal software to perform a scan. Simultaneously, the state of the row lines is monitored to determine which input node is activated. This technique allows the expansion of m × n input points from just m input and n output points.
Figure 1 illustrates how a 4×2 input expansion can be achieved using this principle. The working process involves activating specific output points to scan corresponding input nodes. For example, when output Y0 is active, only the input nodes connected to that line will be detected. This time-sharing approach ensures accurate identification of closed input nodes, making it suitable for systems with limited I/O resources.
2.2 Expanding PLC Input Points Using the Input Node Combination Matrix Method
When the PLC does not have sufficient output points to implement the matrix scanning method, an alternative approach based on the combination matrix can be used. In this method, m input points form groups, and n input points serve as detection terminals. Each group contains n nodes, allowing for a total of m × n input points to be expanded.
Figure 2 shows a 3×4 input expansion using this method. Diodes are used to prevent signal interference between nodes. When a specific input node is closed, signals are sent to both the corresponding group terminal and the detection terminal, allowing the PLC to identify the exact node that is activated.
This method is highly adaptable and does not require additional hardware, making it ideal for systems with limited I/O availability. The scan time can be adjusted based on the application requirements, ensuring efficient operation without compromising reliability.
3. Conclusion
Expanding PLC input points through software programming provides an effective and cost-efficient solution without the need for additional hardware. Both methods described in this paper have been tested in laboratory settings and have proven to be reliable, user-friendly, and valuable for real-world applications. These techniques demonstrate the versatility of PLCs in handling complex control tasks while optimizing resource usage.
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