PLC步进指令编程时,先要分析控制过程,确定步进和转移条件,按规则画出状态转换图画,再根据状态转移图出梯形图;最后由梯形图写出程序表( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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