PLC通过输入接口可以检测被控对象的各种数据，以这些数据作为PLC对被控制对象进行控制的依据；同时PLC又通过输出接口将处理结果送给被控制对象，以实现控制目的。
由于外部输入设备和输出设备所需的信号电平是多种多样的，而PLC内部CPU的处理的信息只能是标准电平，所以I/O接口要实现这种转换。
I/O接口一般都具有光电隔离和滤波功能，以提高PLC的抗干扰能力。
另外，I/O接口上通常还有状态指示，工作状况直观，便于维护。
PLC提供了多种操作电平和驱动能力的I/O接口，有各种各样功能的I/O接口供用户选用。
I/O接口的主要类型有：数字量（开关量）输入、数字量（开关量）输出、模拟量输入、模拟量输出等。
常用的开关量输入接口按其使用的电源不同有三种类型：直流输入接口、交流输入接口和交/直流输入接口。
常用的开关量输出接口按输出开关器件不同有三种类型：是继电器输出、晶体管输出和双向晶闸管输出。
继电器输出接口可驱动交流或直流负载，但其响应时间长，动作频率低；而晶体管输出和双向晶闸管输出接口的响应速度快，动作频率高，但前者只能用于驱动直流负载，后者只能用于交流负载。
PLC的I/O接口所能接受的输入信号个数和输出信号个数称为PLC输入/输出（I/O）点数。
I/O点数是选择PLC的重要依据之一。
当系统的I/O点数不够时，可通过PLC的I/O扩展接口对系统进行扩展。
2．通信接口PLC配有各种通信接口，这些通信接口一般都带有通信处理器。
PLC通过这些通信接口可与监视器、打印机、其它PLC、计算机等设备实现通信。
PLC与打印机连接，可将过程信息、系统参数等输出打印；与监视器连接，可将控制过程图像显示出来；与其它PLC连接，可组成多机系统或连成网络，实现更大规模控制。
与计算机连接，可组成多级分布式控制系统，实现控制与管理相结合。
远程I/O系统也必须配备相应的通信接口模块。
3．智能接口模块智能接口模块是一独立的计算机系统，它有自己的CPU、系统程序、存储器以及与PLC系统总线相连的接口。
它作为PLC系统的一个模块，通过总线与PLC相连，进行数据交换，并在PLC的协调管理下独立地进行工作。
PLC的智能接口模块种类很多，如：高速计数模块、闭环控制模块、运动控制模块、中断控制模块等。
ALLEN BRADLEY VDK-2 KEYBOARD CIRCUIT CARD VDK2ALLEN BRADLEY/OSAI OS-5630 ConTROL BOARD OS5630ALLEN BRADLEY/OSAI OS-5630 ConTROL BOARD OS5630ALLEN TEL AT1503EV-BU ETHERNET PATCH CABLE 3FT BLUE COREDALLEN-BRADLEY 1606-XL480EP POWER SUPPLY 1606-XL SER.B (USED) A-B ALLEN-BRADLEYALLEN-BRADLEY 1794-0B8 FLEX I/O I/O MODULE SER A REV A01ALLEN-BRADLEY 1794-0E4 FLEX I/O ANALOG OUTPUT SER.B REV.A01ALLEN-BRADLEY 1794-IB8 FLEX I/O 24VDC SINK INPUT SER.A REV.A01ALLEN-BRADLEY SLC 500 POWER SUPPLY 1746-P3 SER AALLEN-BRADLEY SLC 500 POWER SUPPLY 1746-P3 SER AALLFAVOR PRINTED CIRCUIT BOARD 2198496ALLFAVOR PRINTED CIRCUIT BOARD 2198496ALLFAVOR PRINTED CIRCUIT BOARD 2198496ALLFAVOR PRINTED CIRCUIT BOARD 2198496ALLIANCE 512202 PCB MODULE 512203ALLIANCE 512202 PCB MODULE 512203ALLTEC FO-SP112924 PC BOARD POWER ENTRY , CPD3 FOSP112924ALLTECH FO-SP112924 ENTRY BOARD REV AB FO112924-AA PX-F0109499-AHALSTOM 029.045 699 PCB CARD MODULEALSTOM 029.045 699 PCB CARD MODULEALSTOM 029.045 699 PCB CARD MODULEALSTOM 029.045 699 PCB CARD MODULEALSTOM 029.063 242 PCB Pulse AmplifierALSTOM 029.063 242 PCB Pulse AmplifierALSTOM 029.093 500 PCB CARDALSTOM 029.093 500 PCB CARDALSTOM 029.111 890 PCB CARDALSTOM 029.111 890 PCB CARDALSTOM 029.111 890 PCB CARDALSTOM 029.111 890 PCB CARDAlstom Battery Alarm 300 BA30241158AAlstom Battery Alarm 300 BA30241158AAlstom CEGELEC MAE95-08 PCB CIRCUIT MAE 9508Alstom CEGELEC MAE95-08 PCB CIRCUIT MAE 9508Alstom CEGELEC MAE95-08 PCB CIRCUIT MAE 9508Alstom CEGELEC MAE95-08 PCB CIRCUIT MAE 9508ALSTOM ConVERTEAM 029.059 270 PCB CARDALSTOM ConVERTEAM 029.059 270 PCB CARDALSTOM ConVERTEAM 029.059 270 PCB CARDALSTOM ConVERTEAM 029.059 270 PCB CARDALSTOM D-FAULT DETECTION BOARD D9840578ALSTOM D-FAULT DETECTION BOARD D9840578ALSTOM D-FREQUENCY DETECTION BOARD D9840579ALSTOM D-FLAME & FAULT ALARM BOARD D9840581ALSTOM D-LAMP & METER BOARD D9840721ALSTOM D-LAMP & METER BOARD D9840721ALSTOM D-LAMP & METER BOARD D9840721ALSTOM D-LAMP & METER BOARD D9840721ALSTOM GDSSIGMA ConTROL DC UNIT GDS10024005Alstom GDSAlstom GDSALSTOM ISTAT 300 HIGH ACCURACY ANALOGUE WATTS TRANSDUCER 3WGFE6051AALSTOM KCEU1410F12MEE 193796K aag alstom kceu1410f12mee 193796kALSTOM KCGG14201D20EEA...187240K alstom kcgg14201d2deea... 187240KAlstom LGPG 111 LGPG11101P11MEE RELAYALSTOM MAE00-14 4 CHANNEL ANALOG I/P TERMINATION PANELALSTOM MCGG62N1CB0753F OVER CURRENT PROTECTION RELAYALSTOM PIB100 F 3BEB0169 BOARDALSTOM PIB100 F 3BEB0169 BOARDALT-A1-94V1 CIRCUIT BOARD 44-38-0001 AS13 ALTA194V1ALTUS AL 1132 PCB D / FAST BY DHL / FEDEXALTUS PCB 1400-312C AL3201 / FAST SHIP DHL OR FEDEXALTUS PCB A AL3116 / FAST SHIP DHL OR FEDEXALTUS PCB AL1128 C / FAST BY DHL / FEDEXAMBI TECH INDUSTRIES 35-0091 PC BOARD 350091AMBI TECH INDUSTRIES 35-0091 PC BOARD 350091AMERICAN STANDARD TRANE REV.A PC BOARD AMETEK -B CIRCUIT BOARD 90AXC1E BAMETEK -B SEQUENCE CARD 91AF3T24DC4EA8GP BAMETEK 80-STATIC SWITCH ConTROL BOARD OSC4790AMETEK 80-PC BOARD AMETEK 80-PC BOARD AMETEK 80-BOARD AMETEK 80-BOARD AMETEK 80-BOARD AMETEK 80-BOARD AMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK 80485SE OXYGEN SENSOR BOARD REV.GAMETEK AN-1111 CIRCUIT BOARD AN1111 C-C-1019-377AMETEK DNC-TConTROL BOARD DNCT2010020AMETEK DNC-TConTROL BOARD DNCT2010020AMETEK DNC-TConTROL BOARD DNCT2010020AMETEK DNC-TConTROL BOARD DNCT2010020AMETEK DNC-TConTROL BOARD DNCT2010020AMETEK DNC-TConTROL BOARD DNCT2010020AMETEK DNC-TConTROL BOARD DNCT2032020AMETEK DNC-TConTROL BOARD DNCT2032020AMETEK PANALARM FLASHER BOARD 0700014AMETEK PANALARM FLASHER BOARD 0700014AMK AE-EA1 PC BOARD AEEA1AMK AE-EA1 PC BOARD AEEA1AMK KU-EA1 PC BOARD KUEA1AMK KU-EA1 PC BOARD KUEA1AMP 459181-1A CIRCUIT BOARD 4591811AAMP 459181-1A CIRCUIT BOARD 4591811AAMPLIDAN MC.EVA AFP.MLS TONE MODUL PCBAMPLIDAN MC.EVA AFP.MLS TONE MODUL PCBAMPLIDAN MC.EVA AFP.MLS TONE MODUL PCBAMPLIDAN MC.EVA AFP.MLS TONE MODUL PCBAMPLIDAN MD.UDE AFP.BEA 9943 PCBAMPLIDAN MD.UDE AFP.BEA 9943 PCBAMPLIDAN MO.EVA AFP.EVA SUBSTATION MODUL 15306 PCBAMPLIDAN T.0 CBU 9940 PCBAMPLIDAN T.0 CBU 9940 PCBAMSAMOTION FX-USB-AW PLC PROGRAMMING CABLE WITH 1761-CBL-PM02 COMPUTER CABLEAMTRON TECHNOLOGY PCD-RP-101E INTERFACE CARD PCDRP101EANALOG DEVICES RTI-1225 PC BOARDANALOG DEVICES RTI-1225 PC BOARDANALOG DEVICES RTI-1260G-1000 BIPOLAR ConVERTER BOARD RTI1260G1000ANALOG DEVICES RTI-1262 I/O BOARDANALOG DEVICES RTI-1262 I/O BOARDANALOG DEVICES RTI-602 PC BOARD RTI602ANALOG DEVICES RTI-602 PC BOARD RTI602ANDERSON INSTRUMENT EHTST 56000-A50 DISPLAY BOARD 56000A50ANDERSON-NEGELE CURRENT OUTPUT BOARD 2OUTPUTANSCHUTZ PCB CARD RACK BOARD E01ANSCHUTZ PCB CARD RACK BOARD E01ANSCHUTZ PCB CARD E01ANSCHUTZ PCB CARD E01ANSCHUTZ PCB CARD E01ANSCHUTZ PCB CARD E01ANSCHUTZ 132-084 NG002 PCB CARDANSCHUTZ 132-084 NG002 PCB CARDANSCHUTZ DEGREE RUDDER ANGLE INDICATOR 40-0-40 PORT/STBD