描述
PLC程序内常开触点的用法（大家注意我说的常开触点是PLC程序内的，不是外部的常开按钮，大家别弄混了），如果外部常开按钮没有按下，Q0.1就没有输出，因为I0.0没有接通。如果外部常开按钮按下，Q0.1就有输出，因为I0.0接通了（PLC程序内，绿色的为接通，红色的为有输出）。这个最好理解。
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