它采用可以编制程序的存储器，用来在其内部存储执行逻辑运算、顺序运算、计时、计数和算术运算等操作的指令，并能通过数字式或模拟式的输入和输出，控制各种类型的机械或生产过程。
PLC及其有关的外围设备都应按照易于与工业控制系统形成一个整体，易于扩展其功能的原则而设计。
在工业生产的各个领域，机械加工企业为了提高生产效率，采用机械化流水作业的方式，对不同类型的零件分别组成的自动生产线。
随着产品机型的更新换代，生产线承担的加工对象也随之改变，这就需要改变控制程序，使生产线的机械设备按新的工艺过程运行，而继电接触器控制器系统是采用固定接线，很难适应这个要求。
大型自动生产线的控制系统使用的继电器数量很多，随着PLC的应用日益普及,其使用方法简单,便于掌握,且可靠性,抗干扰性很强,自身具有完善的功能.模块化的结构,使其在工业生产线上的应用越来越广泛。
发展趋势：1.向高速度、大容量方向发展 为了提高PLC的处理能力，要求PLC具有的响应速度和更大的存储容量。
目前，有的PLC的扫描速度可达0.1ms/k步左右。
PLC的扫描速度已成为很重要的一个性能指标。
在存储容量方面，有的PLC可达几十兆字节。
为了扩大存储容量，有的公司已使用了磁泡存储器或硬盘。
2.向超大型、超小型两个方向发展 当前中小型PLC比较多，为了适应市场的多种需要，今后PLC要向多品种方向发展，特别是向超大型和超小型两个方向发展。
现已有I/O点数达14336点的超大型PLC，其使用32位微处理器，多CPU并行工作和大容量存储器，功能强。
ABB CAL 16-11B AUXILIARY SWITCH SK8290022B 10A 600VACABB CAL5-11 AUXILIARY SWITCH 1SBN010020R1011 690V 16AABB CI801 PROFIBUS DPV1 COMMUNICATION MODULE 3BSE022366R1ABB CI820 S800 REDUNDANT FCI 3BSE013200R1ABB CM-IVN Coupling Unit 1SVR650669R9400ABB CM-IVN.S Coupling Unit 1SVR750669R9400ABB CM-IWN.4S INSULATION MonITORING RELAY 24-240VAC/DCABB CM-IWN.5 Multifunction Insulation Monitoring Relay 1SVR650660R0400ABB CM-IWS.1S INSULATION MonITORING RELAY 1SVR730660R0100 24-240V AC/DCABB CM-MSS THERMISTOR MOTOR PROTECTION MonITORING RELAY 1SVR430801R1100ABB CM-MSS THERMISTOR MOTOR PROTECTION RELAY 1SVR430811R1300ABB CM-MSS THERMISTOR MOTOR PROTECTION UNIT 1SVR430720R0300 24-240VAC/DCABB CM-MSS.51P PTC THERMISTOR MOTOR PROTECTION RELAY 1SVR740712R1300 24-240V AC/ABB CM-PFS PHASE MonITORING RELAY 1SVR430824R9300ABB CM-PVS.31 RMS MonITORING RELAY 1SVR630794R1300ABB CM-TCS.13 MULTIFUNCTION TEMPERATURE MonITORING RELAY 1SVR630740R0300 24-240VABB COMMANDER CR100 CR101/B021/STD CHART RECORDER 85-265VACABB COMMANDER PR100 STRIP CHART RECORDER PR10B/B0/000000/113ST 10-30VDCABB CP410 M ConTROL PANEL 1SBP260181R1001 24VDCABB CP511 PROGRAMMABLE LOGIC ConTROLLER 1SBP260173R1001 24V DCABB CP552 HMI OPERATOR PANEL 1SBP260177R1001 24VDCABB CP-E 24/10.0 POWER SUPPLY 1SVR427035R0000 24VDC 10AABB CP-E 24/5.0 POWER SUPPLY 1SVR427034R0000 24VDC 5AABB CP-E 24/5.0 POWER SUPPLY 1SVR427034R0000 24VDC 5A SCREW RUSTEDABB CT-AHS OFF DELAY WITH AUXILIARY SUPPLY TIME RELAY 1SVR430113R0100ABB CT-AHS.22 OFF DELAY WITH AUX VOLTAGE TIME RELAY 1SVR630110R3300ABB CT-AHS.22 OFF DELAY WITH AUX VOLTAGE TIME RELAY 1SVR630110R3300 24-240VAC/24ABB CT-ERS.22S ON DELAY TIMING RELAY 1SVR730100R3300?ABB CT-MBS MULTI RANGE TIME RELAY 1SVR430012R0200 110-240VAC/24V AC-DCABB CT-MBS.22S Multifunction Time Relay 1SVR730010R3200ABB CT-MFE MULTI FUNCTION TIME RELAY 1SVR550029R8100 0.05S-100H 24-240VAC/DCABB CT-MFE TIME RELAY 1SVR550029R8100 24-240V AC/DC 0.05S-100HABB CT-MFS.21S MULTIFUNCTION TIME RELAY 1SVR730010R0200 24-240VAC/DCABB CT-MXS.22 MULTI FUNCTION TIME RELAY 1SVR630030R3300 24-240VAC/24-48VDCABB CT-SDE STAR DELTA TIME RELAY 1SVR550210R4100 110-130VACABB CT-SDS.22 STAR DELTA TIMER RELAY 1SVR630210R3300 24-240VAC/24-48VDCABB CYLON UNITRON UC32 PROGRAMMABLE ConTROLLER UC32.16ABB CYLON UNITRON UC32 PROGRAMMABLE ConTROLLER UC32.8ABB DCF506-0140-51 OVERVOLTAGE PROTECTION FOR FIELD SUPPLYABB DI524 DO DIGITAL INPUT MODULE 1SAP240000R0001ABB DI524-B8 DIGITAL INPUT MODULE 1SAP240000R0001ABB DI801 3BSE020508R1 DIGITAL INPUT MODULEABB DI801 DIGITAL INPUT MODULE 3BSE020508R1ABB DI803 DIGITAL INPUT MODULE 3BSE022362R1ABB DI810 DIGITAL INPUT MODULE 3BSE008508R1ABB DO801 DIGITAL OUTPUT MODULE 3BSE020510R1ABB DO802 3BSE022364R1 DIGITAL OUTPUT RELAYABB DO820 Digital Output Relay 3BSE008514R1ABB DP25C3P-1 3-POLE DEFINITE PURPOSE ConTACTOR COIL VOLTAGE 120VABB DP25C3P-1 3-POLE MAGNETIC ConTACTOR 35A COIL VOLTAGE 120VACABB DPT-CB010 AUTOMATIC TRANSFER SWITCH CB01063-2GABB DRIVES USART86-INT CURRENT LOOP MODULE 57275847ABB DSAI 133 PCB 3BYN571201-AC/00 3BYN 266801-005ABB DSAX 110 PCB 3BYN266801-009 3BYN571201-Y/00ABB DSBC 174 PCB 3BSE012211R1 3BSC980004R615ABB DSBC 174 PCB 3BSE012311R1 3BSC980004R615ABB DSCA 114 COMMUNICATION BOARD 57510001-AA/5ABB DSDI 110A PCB 3BYN571601 3BYN 266801-008ABB DSDO 110 PCB IN 2668 1002-32 PCBABB DSDO 110 PCB IN5716-1001-AP/04 IN2668 1002-32ABB DSDX 110 DIGITAL I/O PCB YB 16 11 02-AH/3 LOT OF 5 UNITABB DSPC 157 MAIN COMPUTER BOARD 57310001-GP/2ABB DSQC 116 ConTROLLER MODULE GOSSEN KonSTANTER S126 S 5 EU 20ABB DSQC 129 ConTROLLER BOARD YB161102-BV/2 LOT OF 2ABB DSQC 230 CPU MODULE YB560103-BNABB DSQC 230 PCBABB DSQC 236A SERVO DRIVE ConTROL BOARD YB560103-CA/7ABB DSQC 256 A ROBOTICS ConTROL BOARD 3HAB 2211-1/0ABB DSTA 001B ConNECTION UNIT FOR ANALOG MODULE 3BSE018316R1ABB DSTA001 ConNECTION FOR ANALOG CARD 3BYN571201-AB/00ABB DSTX 150 62250001-EABB DSTX 151 57350001-EB/1 HR7700048 DSTX 152 57350001-EF/1ABB DSTX 152 57350001-EF/1 HR7700049 PCB DSTX 151 57350001-EB/1ABB E 320 DU OVERLOAD RELAY 100-320AABB E93232 FUSE HOLDER 10.3X38MM 32A 400VABB EI 813F ETHERNET MODULE 10baseT 3BDH000022R1ABB ELR72 ELECTRonIC DIFFERENTIAL RELAY 115VAC/DC 230VACABB EZ 3563/640ABB FAIO-01 Analog IO Extension Revision A 3AUA0000141402AABB FI 830F FIELDBUS MODULE PROFIBUS-DP 3BDH000032R1ABB GH R 512 0000 VO Relay Output Unit R51 VDE 0160 GHR5120000VOABB K44E ConTROL RELAY FPH1421001R0444 COIL VOLTAGE 110-127VABB K44E ConTROL RELAY FPH1421001R0448 415-480VABB K62E ConTACTOR RELAY FPH1421001R0624 COIL VOLTAGE 110-127VABB M102-P MOTOR ConTROL AND PROTECTION UNIT 1TNA920711R0002IPD 24VDCABB Marine CMA 73 ASV Control GVT 3605894 Automatic Power Management Unit GVT360ABB MRP31 A3 Modbus RTU Interface 1SAJ251000R0001 MRP31-A3ABB MS325 690V 1SAM150000R1006 MANUAL MOTOR STARTERABB MS325 690V 1SAM150000R1011 MANUAL MOTOR STARTERABB NAMC-11 PCB 3BSE015488R1ABB NDBU-85C DDCS Branching 8+1 ChannelABB NI 40-41 NUMERICAL 3-PHASE OVERCURRENT AND EARTH FAULT RELAY 1MYN742837-C 18ABB OESA 630DV12PL SWITCH FUSE 630A 64250175ABB OFS 260 ELECTRonIC FUSE MonITORABB OFS 690 ELECTRonIC FUSE MonITORABB PM554-T-ETH A4 CENTRAL PROCESSING UNIT 1TNE968900R0110ABB PM825 PROCESSOR MODULE 3BSE010796R1 BODY DAMAGE BROKEN DISCOUNTED PRICEABB PQFM_COBO PQF LV ACTIVE LINE FILTER ConTROLLER 2GCA291950A0075 100-440VABB PQFMGR TS PQF MANAGER 2GCA291723A0075 100-460VABB PR222DS/P LSIG TRIP UNIT 800AABB PR512/P SWITCHBOARD PROTECTION UNITAbb Procontic CS 31 07KT93B Central Processing Unit GJR5251300R0101 07 KT 93ABB Procontic CS 31 ICST08A9 Analog I Remote Unit 24 VDC 200mAABB PROConTIC CS31 ECZ base MODULEABB PROConTIC CS31 I/O REMOTE UNIT ICSO08R1 230VACABB PROConTIC K200 07EA200 GJV3 0724 18 R1ABB PSM0004 PCB 2RCA025059A0001P 2RCA024585MABB R1541 INTERFACE RELAY 220V GHR1541601R1 LOT OF 7 PCABB RB122A 24VAC/DC INTERFACE RELAY 1SNA645012R2500ABB REA 101 ARC PROTECTION RELAY 26-60VDCABB REJ601 REJ601BD446BB1NH FEEDER PROTECTION RELAY 24-240V AC/DCABB REM 610 MOTOR PROTECTION RELAY 1MRS050692ABB REM543 Machine Protection Terminal REM543CG216AAAA Rev B REM 543ABB REM615E_G FRONT DISPLAY PANEL HBMCAEAGAAC1BNN1XGABB RET541 TRANSFORMER TERMINALABB RIIO-01 Modbus Power Module 3AXD50000033791ABB RVC Power Factor Controller RVC8-1/5A 2GCA294985A0050 100-440VABB RVC6-1/5A POWER FACTOR ConTROLLER 2GCA288097A0050 100-440VABB S 202 B 32A 2-POLE CIRCUIT BREAKER GB10963 2CD S252 001 R0325ABB S5 MOTOR OPERATOR 1SDA013876R1 220-250VDC/220-240VACABB SACE 1SDA038323R1 GEARED MOTOR DEVICE 110/130V-CC/CA 1.25AABB SACE E1 AIR CIRCUIT BREAKER 1250A OCR SACE PR111/PABB SACE E3S/MS 32 3-POLE CIRCUIT BREAKER 1SDA058979R1 3200AABB SACE EKIP DIP ConTROL DISPLAY UNITABB SACE EKIP TOUCH LSIG DISPLAY UNITABB SACE PR111/P 2000A OVERCURRENT RELEASE PROTECTION UNITABB SACE PR111/P 3200A OVERCURRENT RELEASE PROTECTION UNITABB SACE PR111/P 4000A OVERCURRENT RELEASE PROTECTION UNITABB SACE PR111/P LSIG TRIP UNIT 2000AABB SACE PR111/P LSIG TRIP UNIT 2500AABB SACE PR111/P LSIG TRIP UNIT 800A