general principles<br>After the model and specification of PLC are generally determined, the basic specifications and parameters of each component of PLC can be determined one by one according to the control requirements, and the model of each component module can be selected. The following principles shall be followed when selecting module model.<br>1. Convenience: Generally speaking, as a PLC, there are many kinds of modules that can meet the control requirements. The selection should be based on the principle of simplifying the circuit design, facilitating the use and minimizing the external controller components.<br>For example, for the input module, the input form that can be directly connected with the external detection element should be selected first, avoiding the use of interface circuit. For the output module, the output module that can directly drive the load shall be selected first, and the intermediate relay and other elements shall be minimized.<br>2. Generality: when selecting models, the unity and generality of each component module of PLC shall be considered to avoid too many types of modules. In this way, it is not only beneficial to purchase and reduce spare parts, but also can increase the interchangeability of all components of the system, providing convenience for design, commissioning and maintenance.<br>3. Compatibility: when selecting each component module of PLC system, the compatibility shall be fully considered. In order to avoid the problem of poor compatibility, the manufacturers of main components of PLC system should not be too many. If possible, try to choose products from the same manufacturer.<br>
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