当PLC运行时，是通过执行反映控制要求的用户程序来完成控制任务的，需要执行众多的操作，但CPU不可能同时去执行多个操作，它只能按分时操作（(串行工作)方式，每一次执行一个操作，按顺序逐个执行。由于CPU的运算处理速度很快，Emerson 5X00226G02 总线接口模块所以从宏观上来看，PLC外部出现的结果似乎是同时(并行）完成的。这种串行工作过程称为PLC的扫描工作方式。用扫描工作方式执行用户程序时，扫描是从条程序开始，在无中断或跳转控制的情况下，按程序存储顺序的先后，逐条执行用户程序，直到程序结束。然后再从头开始扫描执行，周而复始重复运行。
5X00226G02
PLC的扫描工作方式与电器控制的工作原理明显不同。5X00226G02 总线接口模块电器控制装置采用硬逻辑的并行工作方式，如果某个继电器的线圈通电或断电，那么该继电器的所有常开和常闭触点不论处在控制线路的哪个位置上，都会立即同时动作;而PLC采用扫描工作方式(串行工作方式)，如果某个软继电器的线圈被接通或断开，其所有的触点不会立即动作，必须等扫描到该时才会动作。但由于PLC的扫描速度快，通常PLC与电器控制装置在I/O的处理结果上并没有什么差别。
5X00226G02
When the PLC is running, it completes control tasks by executing user programs that reflect control requirements, requiring the execution of numerous operations. However, the CPU cannot perform multiple operations simultaneously. It can only execute one operation at a time in a time-sharing (serial work) manner, one at a time, in sequence. Due to the fast processing speed of the CPU and the Emerson 5X00226G02 bus interface module, from a macro perspective, the results that appear outside the PLC seem to be completed simultaneously (in parallel). This serial working process is called the scanning working mode of PLC. When using scanning mode to execute user programs, scanning starts from each program and, without interruption or jump control, executes the user programs one by one in the order of program storage until the program ends. Then scan and execute from scratch, repeating the process over and over again.
5X00226G02
The scanning working mode of PLC is significantly different from the working principle of electrical control. The 5X00226G02 bus interface module electrical control device adopts a hard logic parallel working mode. If the coil of a certain relay is powered on or off, all normally open and normally closed contacts of the relay will immediately act simultaneously regardless of which position they are in the control circuit; The PLC adopts a scanning working mode (serial working mode). If the coil of a soft relay is connected or disconnected, all its contacts will not immediately act and must wait until the scanning reaches that point before it can act. However, due to the fast scanning speed of PLC, there is usually no difference in the I/O processing results between PLC and electrical control devices.
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