When RLO transitions from "0" to "1", FR <counter> clears the edge-detecting flag that is used for setting and selecting upwards or downwards count of the addressed counter. Enable counter is not required to set a counter or for normal counting This means that in spite of a constant RLO of 1 for the Set Counter Preset Value, Counter Up, or Counter Down, these instructions are not executed again after the enable.
允许指令(FR)只有在逻辑操作结果(RLO)由0跳变为1时,才对计数器进行允许操作.CPU只在正边沿信号时执行FR指令.
例子：
STL Explanation
A I 2.0 //检查输入 I 2.0的信号状态.
FR C3 //当A 2.0信号状态从0变为1的时候启用计数器 C3
