KH3-71150

详情描述：

KH3-71150
西门子的计数器C1、C2等大能接受什么频率的脉冲呢？什么时候必须采用高速计数呢？这个是看PLC的什么指标算出来的？
答：PLC的计数器在计数脉冲的上升沿时计数。计数器实际上是一种“软件”计数器，与PLC的扫描工作方式有关。PLC在每个扫描周期开始的时候读取数字量输入的值。如果前一扫描周期读取的是0、本次扫描周期读取的是1，操作系统就知道出现了计数脉冲的上升沿，将计数器的当前值加1或减1。下图假设PLC的扫描周期和计数脉冲的周期都是恒定的。如果计数脉冲的周期小于2倍扫描周期，就会丢失计数脉冲的上升沿。实际上PLC的扫描周期不是恒定的，由于程序的跳转或中断等原因，都会使扫描周期增大，导致丢失计数脉冲的上升沿。计数脉冲的高电平和低电平脉冲的宽度小于扫描周期，也会丢失脉冲的上升沿。实际工作中可以用STEP7读取扫描周期大致的平均值，计数器的大计数脉冲的频率应比它的两倍的倒数小一些。小得越多越保险。如果不能满足要求，只有改用高速计数器。
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