Compared with other control equipment,the most important feature of PLC is”scanning”.After the PLC is powered on,it automatically repeats program scan and I/O scan,that is,three stages of input sampling,user program execution,and output refresh.Completing the above three stages is called a scan cycle.In the input sampling stage,PLC reads all input states and data in sequence by scanning,and stores them in the corresponding units in the I/O image area.,even if the input state and data change,the state and data of the corresponding unit in the I/O image area will not change.In the user program execution stage,the PLC scans the program(ladder diagram)sequentially from top to bottom,first left to right,and refreshes the status of the corresponding unit in the RAM storage area or I/O image area according to the result of the logic operation.In the output refresh stage,all output latch circuits are refreshed according to the corresponding state and data in the I/O image area,and then the corresponding peripherals are driven by the output circuit.At this time,it is the real output of the PLC.It can be seen that the process and results of PLC program control are different from the relay logic circuit,especially involving the sequence of steps and the capture of pulse signals,which are closely related to the scanning principle of PLC.In practice,a large number of procedural problems stem from this,often resulting in incredible results.Therefore,to design a PLC program,you must be proficient in the basic principles of PLC.
与其它控制设备比较,PLC最重要的特征是“扫描”。PLC上电后,自动重复执行程序扫描和I/O扫描,即输入采样、用户程序执行和输出刷新三个阶段。完成上述三个阶段称作一个扫描周期。在输入采样阶段,PLC以扫描方式依次地读入所有输入状态和数据,并将它们存入I/O映象区中相应的单元内,输入采样结束后,转入用户程序执行和输出刷新阶段,即使输入状态和数据发生变化,I/O映象区中的相应单元的状态和数据也不会改变。在用户程序执行阶段,PLC按由上而下、先左后右的顺序依次地扫描程序(梯形图),根据逻辑运算的结果,刷新RAM存储区或I/O映象区对应单元的状态。在输出刷新阶段,根据I/O映象区内对应的状态和数据刷新所有的输出锁存电路,再经输出电路驱动相应的外设,这时,才是PLC的真正输出。可见,采用PLC程序控制的过程和结果与继电器逻辑回路是有区别的,特别是涉及到梯级的次序、脉冲信号的捕捉等,与PLC的扫描原理是密切相关的。实践中,大量的程序问题均源于此,常常会出现不可思议的结果。因此,设计PLC程序,必须精通PLC的基本原理。
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