在输入采样阶段，可编程逻辑控制器以扫描方式依次地读入所有输入状态和数据，并将它们存入I/O映象区中的相应的单元内。输入采样结束后，转入用户程序执行和输出刷新阶段。在这两个阶段中，即使输入状态和数据发生变化，I/O映象区中的相应单元的状态和数据也不会改变。因此，如果输入是脉冲信号，则该脉冲信号的宽度必须大于一个扫描周期，才能保证在任何情况下，该输入均能被读入可靠性高。由于PLC大都采用单片微型计算机，因而集成度高，再加上相应的保护电路及自诊断功能，提高了系统的可靠性。编程容易。PLC的编程多采用继电器控制梯形图及命令语句，其数量比微型机指令要少得多，除中、高档PLC外，一般的小型PLC只有16条左右。
PU519
由于梯形图形象而简单，因此容易掌握、使用方便，甚至不需要计算机知识，就可进行编程。组态灵活。由于PLC采用积木式结构，用户只需要简单地组合，便可灵活地改变控制系统的功能和规模，因此，可适用于任何控制系统。输入/输出功能模块齐全。PLC的优点之一，是针对不同的现场信号(如直流或交流、开关量、数字量或模拟量、电压或电流等)，均有相应的模板可与工业现场的器件(如按钮、开关、传感电流变送器、电机启动器或控制阀等)直接连接，并通过总线与CPU主板连接。
PU519
安装方便。与计算机系统相比，PLC的安装既不需要专用机房，也不需要严格的屏蔽措施。使用时只需把检测器件与执行机构和PLC的I/O接口端子正确连接，便可正常工作。运行速度快。由于PLC的控制是由程序控制执行的，因而不论其可靠性还是运行速度，都是继电器逻辑控制无法相比的。近年来，微处理器的使用，特别是随着单片机大量采用，大大增强了PLC的能力，并且使PLC与微型机控制系统之间的差别越来越小，特别是高档PLC更是如此。
PU519
In the input sampling stage, the programmable logic controller sequentially reads in all input states and data in a scanning manner, and stores them in the corresponding units in the I/O image area. After the input sampling is completed, it enters the user program execution and output refresh phase. In these two stages, even if the input state and data change, the state and data of the corresponding units in the I/O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scanning cycle to ensure that the input can be read with high reliability in any case. Due to the majority of PLCs using single-chip microcomputers, their high integration level, coupled with corresponding protection circuits and self-diagnosis functions, improves the reliability of the system. Programming is easy. The programming of PLC mostly uses relay control ladder diagrams and command statements, with a much smaller number than microcomputer instructions. Except for medium and high-end PLCs, general small PLCs only have about 16.
PU519
Due to its vivid and simple appearance, the ladder diagram is easy to master, easy to use, and can even be programmed without the need for computer knowledge. Flexible configuration. Due to the modular structure of the PLC, users can flexibly change the functionality and scale of the control system by simply combining them, making it suitable for any control system. The input/output functional modules are complete. One of the advantages of PLC is that for different on-site signals (such as DC or AC, switch quantity, digital or analog quantity, voltage or current, etc.), corresponding templates can be directly connected to industrial on-site devices (such as buttons, switches, sensing current transmitters, motor starters or control valves, etc.), and connected to the CPU motherboard through a bus.
PU519
Easy installation. Compared to computer systems, the installation of PLC does not require a dedicated computer room or strict shielding measures. When in use, it is only necessary to correctly connect the detection device to the I/O interface terminals of the actuator and PLC to ensure normal operation. Fast running speed. Due to the fact that PLC control is executed by program control, its reliability and operating speed cannot be compared to relay logic control. In recent years, the use of microprocessors, especially with the widespread adoption of microcontrollers, has greatly enhanced the ability of PLCs and reduced the difference between PLCs and microcomputer control systems, especially high-end PLCs.
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