PLC是按集中输入、集中输出，周期性循环扫描的方式进行工作的。它的中心工作过程分输入采样阶段、程序执行阶段、输出刷新阶段。
在输入采样阶段，首先扫描所以输入端子，并将各输入状态存入相对应的输入映像寄存器中，此时输入映像寄存器被刷新，它与外界隔离，其内容保持不变，直到下一个扫描周期的输入采样阶段。
在程序执行阶段，PLC按从左到右、从上到下的步骤顺序执行程序。
在输出刷新阶段中，元件映像寄存器中所有输出继电器的状态一起转存到输出锁存器中，通过一定方式集中输出，*经过输出端子驱动外部负责，在下一个输出刷新阶段开始之前，输出锁存器的状态不会改变。
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TELEMECANIQUE RM3 JA112FG7 TIMER RELAY NIB
VICKERS DGMX2-3-PP-AW-S-40 PRESSURE REDUCING VALVE
TURCK BI40-CP80-VP4X2/S10 10-65 VDC PROXIMITY INDUCTIVE
CAROL 76822.18.06, WIRE, 12 AWG, 600 V, GREEN
CUTLER HAMMER E50KL443, ADJUSTABLE ROLLER, 1 5/8 TO 3 3
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TIMONTA 3-PHASE, 440VAC, 50A, 50/60HZ LINE FILTER FMAC-
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EPSON V13H010L42 PROJECTOR LAMP, NIB
ALLEN BRADLEY 1336-L-SP2C REAL PARTS, NIB, SEALED
PANDUIT HSTT25-C, BLACK HEAT SHRINK TUBING, 1/4" W, 100
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IFM EFECTOR 500 PN5002 PRESSURE SENSOR, NIB
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BOSCH 3-842-503-060 GEAR SPEED REDUCER, 1:15 RATIO, 7 N
