Low-cost PLC input point expansion method - Database & Sql Blog Articles

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By utilizing the built-in input and output points of a PLC, it's possible to expand the actual number of input points without adding extra hardware. This approach enhances the cost-effectiveness of the system. Both methods introduced in this article have been tested in the lab, proving to be reliable, easy to understand, and highly applicable in real-world scenarios.

1. Introduction

PLCs are widely used in industrial control 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 input points required for a task may exceed the available ones, while the output points are often sufficient. To address this imbalance, several strategies can be employed:

1) Adding dedicated input modules increases the system cost and reduces efficiency. 2) Using a PLC with more input points can lead to unused output points, causing resource waste. 3) Incorporating peripheral circuits such as decoders can reduce initial investment. 4) A software-based approach allows expanding input points using existing PLC resources without additional hardware, making it an efficient solution.

This paper introduces two software-based methods for expanding PLC input points by leveraging both input and output points.

2. Software-Based Input Point Expansion Method

One key feature of PLCs is that their input and output points are independent, allowing flexible combinations. By applying matrix keyboard scanning principles or input node combination matrices, we can effectively increase the number of input points.

2.1 Expanding Input Points Using Matrix Keyboard Scanning

By using m input points as row lines and n output points as column lines, we can create a matrix that enables the detection of up to m × n input points. The PLC controls the column lines, while the row lines detect the state of the input nodes. This method is particularly useful when there are spare output points available.

Figure 1 illustrates a 4×2 input expansion using this principle. When y0 is active, only s0 is detected; when y1 is active, s1 is detected. This process ensures that each input node can be uniquely identified based on the state of the output and input lines.

The scan time depends on the PLC type—shorter for transistor or solid-state outputs, and longer for relay outputs to avoid contact wear.

2.2 Expanding Input Points Using Input Node Combination Matrix

When no output points are available, an alternative is to use a combination matrix. By grouping m input points and connecting them to n detection terminals, we can expand the total number of input points to m × n. Diodes are used to prevent signal interference between nodes.

In Figure 2, three input groups and four detection terminals allow the identification of eight unique input nodes. When s0 closes, signals appear at x0 and x6, indicating its status. This method is highly adaptable and suitable for most PLC systems.

3. Conclusion

Using the internal input and output points of a PLC to expand the number of input points without additional hardware improves system cost-efficiency. Both methods have been validated in laboratory settings, demonstrating reliability, simplicity, and significant application value in industrial automation systems.