4119A输入采样阶段在输入采样阶段，PLC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映象区中的相应得单元内。4119A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，/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。4119A用户程序执行阶段在用户程序执行阶段，PLC总是按由上而下的顺序依次地扫描用户程序(梯形图)。4119A在扫描每一条梯形图时， 又总是先扫描梯形图左边的由各触点构成的控制线路，并按先左后右、先上后下的顺序对由触点构成的控制线路进行逻辑运算,然后根据逻辑运算的结果，刷新该逻辑线圈在系统RAM存储区中对应位的状态; 4119A或者刷新该输出线圈在I/O映象区中对应位的状态;或者确定是否要执行该梯形图所规定的特殊功能指令。 4119A input sampling stage In the input sampling stage, the PLC sequentially reads 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 of 4119A is completed, it enters the user program execution and output refresh stage. In these two stages, even if the input state and data change, the state and data of the corresponding units in the/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 in any case. 4119A User Program Execution Phase During the user program execution phase, the PLC always scans the user program (ladder diagram) in a top-down order. When scanning each ladder diagram, 4119A always scans the control circuit composed of each contact on the left side of the ladder diagram first, and performs logical operations on the control circuit composed of the contacts in the order of left to right, top to bottom. Then, based on the results of the logical operations, the state of the corresponding bit of the logical coil in the system RAM storage area is refreshed; 4119A or refresh the state of the corresponding bit of the output coil in the I/O image area; Or determine whether to execute the special functional instructions specified in the ladder diagram
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