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从PLC控制系统与电器控制系统比较可知，PLC的用户程序（软件）代替了继电器控制电路（硬件）。因此，对于使用者来说，可以将PLC等效成是许许多多各种各样的“软继电器”和“软接线”的集合，而用户程序就是用“软接线”将“软继电器”及其“触点”按一定要求连接起来的“控制电路”。
为了更好的理解这种等效关系，下面通过一个例子来说明。如图1所示为三相异步电动机单向起动运行的电器控制系统。其中，由输入设备SB1、SB2、FR的触点构成系统的输入部分，由输出设备KM构成系统的输出部分。
图1 三相异步电动机单向运行电器控制系统
a）主电路 b）控制电路
如果用PLC来控制这台三相异步电动机，组成一个PLC控制系统，根据上述分析可知，系统主电路不变，只要将输入设备SB1、SB2、FR的触点与PLC的输入端连接，输出设备KM线圈与PLC的输出端连接，就构成PLC控制系统的输入、输出硬件线路。而控制部分的功能则由PLC的用户程序来实现，其等效电路如图2所示。
图2 PLC的等效电路
图中，输入设备SB1、SB2、FR与PLC内部的“软继电器”X0、X1、X2的“线圈”对应，由输入设备控制相对应的“软继电器”的状态，即通过这些“软继电器”将外部输入设备状态变成PLC内部的状态，这类“软继电器”称为输入继电器；同理，输出设备KM与PLC内部的“软继电器”Y0对应，由“软继电器”Y0状态控制对应的输出设备KM的状态，即通过这些“软继电器”将PLC内部状态输出，以控制外部输出设备，这类“软继电器”称为输出继电器。
因此，PLC用户程序要实现的是：如何用输入继电器X0、X1、X2来控制输出继电器Y0。当控制要求复杂时，程序中还要采用PLC内部的其它类型的“软继电器”，如辅助继电器、定时器、计数器等，以达到控制要求。
要注意的是，PLC等效电路中的继电器并不是实际的物理继电器，它实质上是存储器单元的状态。单元状态为“１”，相当于继电器接通；单元状态为“０”，则相当于继电器断开。因此，我们称这些继电器为“软继电器”。
ABB YB560103-CB DSQC236B DriveUnit
ABB YB560103-CC DSQC236C DriveUnit
ABB 3HAB2207-1 DSQC236D DriveUnit
ABB YB560103-CD DSQC236G DriveUnit
ABB 3HAB2207-1 DSQC236P DriveUnit
ABB YB560103-CE DSQC236T DriveUnit
ABB YB560 103-CL/5 DSQC241 PowerUnit
ABB 3HAB 2210-1 DSQC300 Robot Computer
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ABB 3HAB 2219-1 DSQC316 Main Computer
ABB 3HAB 2220-1 DSQC317 6MB Card
ABB 3HAB 2236-1 DSQC321 4MB Card
ABB 3HAB 5956-1 DSQC323 8MB Card
ABB 3HAB 5957-1 DSQC324 16MB Card
ABB 3HAB 2241-1 DSQC325 Main Computer
ABB 3HAB 2242-1 DSQC326 Robot Computer
ABB 3HAB7229-1 DSQC328 I/O Card
ABB 3HAB 6182-1 DSQC335 Robot Computer
ABB 3HAB8101-3/05D DSQC345B DriveUnit
ABB 3HAB8101-3/08C DSQC345C DriveUnit
ABB 3HAB8101-7/01C DSQC346C DriveUnit
ABB 3HAB8101-8 DSQC346G DriveUnit
ABB cable DSQC351 Interbus-S
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ABB 900056 1Gb SD-card S2 DDR
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3HNE 00312-1 Teach Pendant
3HNE 00313 Teach Pendant
CHAA 3560-AXA Teach Pendant
ABB 3HAB9271-1/01B DriveUnit Dummy DriveUnit Dummy
ABB 57510001-AA DSCA 114 Print out board
ABB 57520001-U DSCA 121 Communication board
ABB 57160001-A DSDI 110 Digital input board
ABB 57160001-K DSDO 110 Digital output board
ABB YB161102-AH DSDX 110 Digital in/output board
ABB 57160001-TP DSDX B001 digital in-output board
ABB 57360001-U DSMB 124 Semiconductor memory board
ABB 57360001-AN DSMB 125 Semiconductor memory board
ABB 57360001-HG DSMB 127 Memory board
ABB 57330001-N DSMC 110 Floppy disk controller
ABB YB161102-AK DSPA 110 Axis computer
ABB 57310256-AF DSPC 150 Central processor
ABB 57310256-BA DSPC 153 Expand. single board computer
ABB 57310001-GP DSPC 157 Expand. single board computer
ABB YB161102-AD DSQC 103 Resolver power board
ABB YB161102-AF DSQC 105 Display board
ABB YB161102-BA DSQC 110 Programming board
ABB YB161102-BS DSQC 115 Resolver power board + 2 analogue
ABB YB161102-BE DSQC 116 Power supply
ABB YB161102-BG DSQC 117 Resolver power board
ABB YB161102-BU DSQC 124 Terminal unit
ABB YB161102-BV DSQC 129 R/D board
ABB YB161102-CH DSQC 136 Safety board
ABB YB161102-CM DSQC 138 Terminal unit
ABB YB560103-AC DSQC 202 Robot computer
ABB YB560103-AS DSQC 208A External axis board
ABB YB560103-AL DSQC 209 Analogue I/O board
ABB YB560103-AM DSQC 210 Safety board
ABB YB560103-AN DSQC 211 Power supply
ABB YB560103-BF DSQC 215 Robot computer
ABB YB560103-AZ DSQC 220 Programming board
ABB YB560103-BD DSQC 223 PC Board digital I/O
ABB YB560103-BE DSQC 224 PC Board combi I/O
ABB YB560103-BK DSQC 227 Winchester interface
ABB YB560103-BD DSQC 228 Safety board
ABB YB560103-BN DSQC 230 Robot computer
ABB YB560103-BS DSQC 233 External axis board
ABB YB560103-BY DSQC 235A Rectifier
ABB YB560103-BZ DSQC 235B Rectifier
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DSQC 236B Drive
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ABB 3HAB2207-1 DSQC 236D Drive unit
ABB YB560103-CD DSQC 236G Drive unit
ABB 3HAB2245-1 DSQC 236H Drive unit
ABB 3HAA3563-AJA DSQC 236P Drive unit
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DSQC 239 Board - ABB RIO
ABB DSQC 241 Power supply
ABB YB560103-CM DSQC 242 Programming board
ABB 3HAA0001-CP DSQC 243 Serial measurement board
ABB 3HAA3563-ACA DSQC 245 Serial measurement board
ABB 3HAA3563-AAA DSQC 248 Programming board
ABB 3HAA3563-AGA DSQC 249A Rectifier
ABB 3HAA3563-AHA DSQC 249B Rectifier
ABB 3HAA3563-AD DSQC 252 System board
E3HAC028756-001 DSQC 253 SMB unit DSQC 573
E3HAA3563-AMA DSQC 253 Serial Measurement Board
E3E 042816 DSQC 253 Exch. Serial mes. DSQC 253W
6369901-218 DSQC 253 Serial Measurement Board
ABB 3HAB2211-1 DSQC 256A System board
ABB 3HAA3563-AUA DSQC 258 Power supply
ABB 3HAB8797-1 DSQC 266B Drive unit
ABB 3HAB8798-1 DSQC 266C Drive unit
ABB 3HAB8801-1 DSQC 266G Drive unit
ABB 3HAB8800-1 DSQC 266H Drive unit
ABB 3HAB8799-1 DSQC 266K Drive unit
ABB 3HAB8802-1 DSQC 266T Drive unit
ABB 3HAA3573-AAA DSQC 300 Board - Computer
ABB 3HAB2210-1 DSQC 300 Board - Computer
ABB 3HAB2233-1 DSQC 300 Board - Computer
ABB 6369901-380 DSQC 300 Board - Computer
ABB 3HAA3573-ABA DSQC 301 Programming board
ABB 3HAB2209-1 DSQC 306 Computer board
ABB 6369901-298 DSQC 306 Computer board
ABB E3HAB2209-1 DSQC 306 Computer board
ABB 3HAB3700-1 DSQC 313 Serial measurement board
ABB 3HAB4259-1 DSQC 313 Serial measurement board
ABB 3HAC9522-1 DSQC 313 Serial Measurement Board
ABB 3HAB2215-1 DSQC 314A Rectifier
ABB 3HAB2216-1 DSQC 314B Rectifier
ABB 3HAB2231-1 DSQC 314C Rectifier DC link
ABB 6369901-336 DSQC 314C Rectifier DC link
ABB E3HAB2231-1 DSQC 314C Rectifier DC link
ABB 3HAB2214-1 DSQC 315 Combi I/O
ABB 3HAB2219-1 DSQC 316 Computer board
ABB 3HAB2209-1 DSQC 317 Memory expansion board
ABB 3HAB2233-1 DSQC 322 Robot computer
ABB 3HAB5956-1 DSQC 323 Memory expansion board
ABB 3HAB5957-1 DSQC 324 Memory expansion board
ABB 3HAB2241-1 DSQC 325 Main computer
ABB 3HAB2242-1 DSQC 326 Robot computer
ABB 3HAB7230-1 DSQC 327 Analogue Digital Combi I/O
ABB 3HAB7229-1 DSQC 328 Digital 24 VDC I/O
ABB 3HAB7215-1 DSQC 331 Panel unit
ABB 3HAC17973-1 DSQC 332 Digital relay I/O
ABB 3HAB5845-1 DSQC 334 Power supply
ABB 3HAB6182-1 DSQC 335 Robot computer
ABB 3HNE00001-1 DSQC 336 Network board NIOC-01
ABB 3HAB8101-1 DSQC 345A Rectifier
ABB 3HAB8101-2 DSQC 345B Rectifier
ABB 3HAB8101-3 DSQC 345C Rectifier
ABB 3HAB8101-4 DSQC 345D Rectifier
ABB 3HAB8101-14 DSQC 345E Rectifier
ABB 3HAB8101-5 DSQC 346A Drive unit
ABB 3HAB8101-6 DSQC 346B Drive unit
ABB 3HAB8101-7 DSQC 346C Drive unit
ABB 3HAB8101-11 DSQC 346E Drive unit
ABB 3HAB8101-8 DSQC 346G Drive unit
ABB 3HAB8101-13 DSQC 346U Drive unit
ABB 3HNE00957-1 DSQC 350 Remote I/O
ABB 3HNE00065-1 DSQC 354 Encoder interface
DSQC 355 I/O Board
ABB 3HAB8101-10 DSQC 358C Drive unit / Rectifier
ABB 3HAB8101-12 DSQC 358E Drive unit / Rectifier
ABB 3HAB8101-15 DSQC 358F Driveunit / Rectifier
ABB 3HAB8101-16 DSQC 358G Drive unit / Rectifier
ABB 3HAC0373-1 DSQC 361 Main computer
ABB 3HAC1462-1 DSQC 363 Robot computer
ABB 3HAC1620-1 DSQC 365 Power supply
ABB 3HAC3180-1 DSQC 373 Robot computer
ABB 3HAC3462-1 DSQC 374 Power supply
ABB 3HAC10597-1 DSQC 386 Devicenet gateway
ABB 3HAC3616-1 DSQC 500 Main computer
ABB 3HAC3619-1 DSQC 503 Axis computer
ABB 3HAC4297-1 DSQC 506 Power supply
ABB 3HAC5393-2 DSQC 508 Battery unit
ABB 3HAC5687-1 DSQC 509 Panel unit
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