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由于PLC以微处理器为核心，故具有微机的许多特点，但它的工作方式却与微机有很大不同。微机一般采用等待命令的工作方式，如常见的键盘扫描方式或I/O扫描方，若有键按下或有I/O变化，则转入相应的子程序，若无则继续扫描等待。
PLC则是采用循环扫描的工作方式。对每个程序，CPU从**条指令开始执行，按指令步序号做周期性的程序循环扫描，如果无跳转指令，则从**条指令开始逐条执行用户程序，直至遇到结束符后又返回**条指令，如此周而复始不断循环，每一个循环称为一个扫描周期。扫描周期的长短主要取决于以下几个因素：一是CPU执行指令的速度；二是执行每条指令占用的时间；三是程序中指令条数的多少。一个扫描周期主要可分为3个阶段。
1．输入刷新阶段
在输入刷新阶段，CPU扫描全部输入端口，读取其状态并写入输入状态寄存器。完成输入端刷新工作后，将关闭输入端口，转入程序执行阶段。在程序执行期间即使输入端状态发生变化，输入状态寄存器的内容也不会改变，而这些变化必须等到下一工作周期的输入刷新阶段才能被读入。
2．程序执行阶段
在程序执行阶段，根据用户输入的控制程序，从**条开始逐步执行，并将相应的逻辑运算结果存入对应的内部辅助寄存器和输出状态寄存器。当**一条控制程序执行完毕后，即转入输入（出？）刷新阶段。
3．输出刷新阶段
当所有指令执行完毕后，将输出状态寄存器中的内容，依次送到输出锁存电路（输出映像寄存器），并通过一定输出方式输出，驱动外部相应执行元件工作，这才形成PLC的实际输出。


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440R-P4NANS 440RP4NANS Guardmaster MSR45E Extension Module
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1C31166G01 1C31166G01
1C31179G02 1C31179G02
1C31181G02 1C31181G02
1C31189G01 1C31189G01
1C31192G01 1C31192G01
1C31197G01 1C31197G05 1C31197G01
1C31219G01 1C31219G01
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1C31224G01 1C31224G01
1C31238H01 1C31238H01
1P00028G01 1P00028G01
1TGE120010R1000
200-CICN
2094-AC09-M02 2094-AC09-M02
2094-AM02-S 2094AM02-S
2094-BMP5-S 2094-BMP5-S
2098-DSD-010X 2098-DSD-010X
2098-DSD-010X-DN 2098-DSD-010X-DN
2150-AGA 1747-KE
269PLUS-100P-120
2706-M1N1
2711-B*10 2711-B*10
2711P-T10C4D2
2711-T10C8X 2711-T10C8X
2711-T9C1X 2711-T9C1X
3300/03 3300/03-01-00
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3300/20 3300/20-13-03-01-00-00
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3300/20-05-03-01-00-00
3300/46 3300/46-46-04-02-01
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330180-50-00
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330709-000-070-10-02-00
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440S-M565055 440SM565055 S

资讯来源：A06B-6240-H104