PLC在输入取样阶段，首先按顺序将所有输入端子的通断状态或输入数据读入，并将其存入(写入)各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。在程序执行阶段，即使输入状态有变化，输入状态寄存器的内容也不会改变。变化了的输入信号状态只能在下一个扫描周期的取样阶段被读入。
PLC在程序执行阶段，按用户程序指令存放的先后顺序扫描执行每条指令，所需的执行条件可从输入状态寄存器、内部继电器(寄存器)和当前输出状态寄存器中读入，经过相应的运算和处理后，其结果再次写入输出状态寄存器中。所以，输出状态寄存器中所有的内容将随着程序的执行而改变。
当所有指令执行完毕，输出状态寄存器的通断状态在输出刷新阶段送至输出端并通过一定方式(继电器、晶体管或晶闸管等)输出，驱动相应输出设备工作，这就是PLC的实际输出。
经过这三个阶段，完成一个扫描周期。扫描周期的长短与用户程序的指令条数及执行各条指令所需的时间有关，一般不超过100ms。
GE PC Card ic3600 IC3600AIAD1B IC3600AIAD NO DOA'S
ABB BAILEY CONTROLS 6639049d1 SLC01 main board
ALLEN BRADLEY 2705-P11J1L 2705P11J1L OPERATOR INTERFACE
ABB OES600J3 600A DISCONNECT SWITCH W/ FUSES LPJ-450SP
TRANE TR1 VFD 25HP 18.5KW TR16027GT4CN1STR0DLF00A00C0
NEW FOXBORO 136-5-1 13651 136 COMPUTING RELAY
NEW FOXBORO 136-1 1361 136 COMPUTING RELAY NEW
Reliance Snubber Resistor 2SR21200 D2-3291 230v NEW
NEW ALLEN BRADLEY 810-A09A 810A09A SER B RELAY NIB
FOXBORO 2AX-A4-S 2AXA4S CONTROL CARD AUTO-MANUAL
Reliance Snubber Resistor 2SR20600 D2-3291 230v NEW
TOSHIBA VFS7-4150UPLH TRANSISTOR INVERTER 25 Kva 20HP
MICRO MOTION RFT9712 1KN REMOTE FLOW TRANSMITTER
NEW Abb PC POWER SUPPLY BOARD NPOW42-62c NPOW42
NEW DOLAN JENNER SS950-48000 48" SafeScan Light Bar set
==============================================================================
Rexroth MAC071C-0-US-4-C/095-B-0/WI520LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI521LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI522LV
Rexroth MAC071C-0-US-4-C/095-B-0/WI522LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI524LV
Rexroth MAC071C-0-US-4-C/095-B-0/WI524LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI527BX/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI538LV
Rexroth MAC071C-0-US-4-C/095-B-0/WI538LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI553LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI560LV
Rexroth MAC071C-0-US-4-C/095-B-0/WI560LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI568LV
Rexroth MAC071C-0-US-4-C/095-B-0/WI572LV/S001
Rexroth MAC071C-0-US-4-C/095-B-0/WI581BX
Rexroth MAC071C-0-US-4-C/095-B-0/WI594LV/S001
Rexroth MAC071C-0-US-4-C/095-B-1/AM154SG
Rexroth MAC071C-0-US-4-C/095-B-1/AM154SG/S001
Rexroth MAC071C-0-US-4-C/095-B-1/DI524LV
Rexroth MAC071C-0-US-4-C/095-B-1/WI511LV/S001
Rexroth MAC071C-0-US-4-C/095-B-1/WI515LV
Rexroth MAC071C-0-US-4-C/095-B-1/WI516LV
Rexroth MAC071C-0-US-4-C/095-B-1/WI517LV
Rexroth MAC071C-0-US-4-C/095-B-1/WI517LV/S001
Rexroth MAC071C-0-US-4-C/095-B-1/WI518LV
Rexroth MAC071C-0-US-4-C/095-B-1/WI520LV
Rexroth MAC071C-0-US-4-C/095-B-1/WI520LV/S001
Rexroth MAC071C-0-US-4-C/095-B-1/WI521LV/S001
