1761-NET-ENI 产品就亿佳
PLC梯形图编程语言是从继电器接点控制线路图上发展起来的一种编程语言，两者的结构非常类似，但其程序执行过程存在本质的区别。因此，同样作为继电器接点控制系统与梯形图的基本组成3要素——触点、线圈、连线，两者有着本质的不同。 1．触点的性质与特点 梯形图中所使用的输入、输出、内部继电器等编程元件的“常开”、“常闭”触点，其本质是PLC内部某一存储器的数据“位”状态。程序中的“常开”触点是直接使用该位的状态进行逻辑运算处理；“常闭”触点是使用该位的“逻辑非”状态进行处理。它与继电器控制电路的区别在于： ①梯形图中的触点可以在程序中无限次使用，它不像物理继电器那样，受到实际安装触点数量的限制。 ②在任何时刻，梯形图中的“常开”、“常闭”触点的状态是 的，不可能出现两者同时为“l”的情况，“常开”、“常闭”触点存在严格的“非”关系。 2．线圈的性质与特点 梯形图编程所使用的内部继电器、输出等编程元件，虽然采用了与继电器控制线路同样的“线圈”这一名称，但它们并非实际存在的物理继电器。程序对以上线圈的输出控制，只是将PLC内部某一存储器的数据“位”的状态进行赋值而已。数据“位”置“1”对应于线圈的“得电”；数据“位”置“0”对应于“断电”。因此，它与继电器控制电路的区别在于： ①如果需要，梯形图中的“输出线圈”可以在程序中进行多次赋值，即在梯形图中可以使用所谓的“重复线圈”。 ②PLC程序的执行，严格按照梯形图“从上至下”、“从左至右”的时序执行，在同一PLC程序执行循环内，不能改变已经执行完成的指令输出状态（已经执行完成的指令输出状态，只能在下一循环中予以改变）。有效利用PLC的这一程序执行特点，可以设计出许多区别于继电器控制线路的特殊逻辑，如“边沿”处理信号等。 3．连线的性质与特点 梯形图中的“连线”仅代表指令在PLC中的处理顺序关系（“从上至下”、“从左至右”），它不像继电器控制线路那样存在实际电流，因此，在梯形图中的每一输出线圈应有各自独立的逻辑控制“电路”（即明确的逻辑控制关系），不同输出线圈间不能采用继电器控制线路中经常使用的“电桥型连接”方式，试图通过后面的执行条件，改变已经执行完成的指令输出。
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