描述
PLC的一个扫描周期必须经过输入采样、程序执行和输出刷新三个阶段及其他一些辅助阶段,其中的输入采样和输出刷新在有的场合也称为I/O刷新。
输入采样阶段:首先以扫描方式将所有输入端子的通(ON)断(OFF)状态转换成电平的高低状态“1”或“0”并保存入输入锁存器中;然后,将其写入各自对应的输入状态寄存器中,即刷新输入;随即关闭输入端口,进入程序执行阶段。
程序执行阶段:按用户程序指令存放的先后顺序扫描执行每条指令,经相应的运算和处理后,其结果再写入输出状态寄存器中,输出状态寄存器中所有的内容随着程序的执行而改变。
输出刷新阶段:当所有指令执行完毕,输出状态寄存器的通(ON)断(OFF)状态在输出刷新阶段送至输出锁存器中,并通过一定的方式(继电器、晶体管或晶闸管)输出,驱动相应被控设备工作。
PLC的上述工作方式称之为扫描工作方式,即PLC运行时,需要进行大量的操作,这使得PLC中的CPU只能根据分时操作原理,按一定的顺序,每一时刻执行一个操作。当PLC运行时,在经过初始化后,即进入扫描工作方式,且周而复始地重复进行,因此也称PLC的工作方式为“循环扫描工作方式”。
可以看出,PLC在初始化后,进入循环扫描。PLC一次扫描的过程,包括公共处理、执行程序、循环时间计算处理、I/O刷新、RS-232端口服务和外设端口服务共五个阶段,其所需时间称为一个工作周期(或扫描周期)。显然,PLC的扫描周期与用户程序的长短和该PLC的扫描速度紧密相关。
PLC在进入循环扫描前的初始化过程,主要是将所有输入/输出、内部辅助、特殊辅助、辅助记忆继电器区域清零,并完成定时器预置,识别扩展单元等任务,以保证它们在进入循环扫描后,正确无误地工作。
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