输入继电器仅用于接收外部输入信号[例如图2(a)中，按下启动按钮SB2时，输入继电器接通，其动合触点X2就闭合]，它不能由PLC内部其他继电器的触点来驱动。因此梯形图中只出现输入继电器的触点，而不出现其线圈。输出继电器输出程序执行结果给外部输出设备。当梯形图中的输出继电器线圈接通时，就有信号输出，但不是直接驱动输出设备，而要通过输出接口的“硬”继电器、晶体管或晶闸管才能实现。
输出继电器的触点也可供内部编程使用。CHICAGO BLOWER 20″ FAN WITH STAND 230/460 VAC 3 PH 5 HP VANE DAMPER SIZE 20
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