PLC实质上是一种工业控制用的专用计算机。PLC系统也是由硬件系统和软件系统两大部分组成。其软件主要有以下几个逻辑部件：

（1）继电器逻辑

为适应电气控制的需要，PLC为用户提供继电器逻辑，用逻辑与或非等逻辑运算来处理各种继电器的连接。PLC内部有储单元有“1”和“0”两种状态，对应于“0N”和“0FF”两种状态。因此PLC中所说的继电器是一种逻辑概念的，而不是真正的继电器，有时称为“软继电器”。这些“软继电器”与通常的继电器相比有以下特点：

①体积小、功耗低

②无触点、速度快、寿命长

③有无数个触点，使用中不必考虑接点的容量

PLC一般为用户提供以下几种继电器（以FX2N系列PLC为例）：

输入继电器（X）：把现场信号输入PLC，同时提供无限多个常开、常闭触点供用户编程使用。在程序中只有触点没有线圈，信号由外部信号驱动。编号采用八进制，分别为X000—X007，X010-X017等。

输出继电器（Y）：具备一对物理接点，可以串接在负载回路中，对应物理元件有继电器、晶闸管和晶体管。外部信号不能直接驱动，只能在程序中用指令驱动。编号采用八进制，分别为Y000—Y007，Y010-Y017等。

内部继电器（M）：与外界没有直接联系，仅作运算的中间结果使用。有时也称为辅助继电器或中间继电器。和输出继电器一样，只能由程序驱动。每个辅助继电器有无限多对常开、常闭触点，供编程使用。地址号按十进制分配，通用型辅助继电器有M0-M499共500点，保持型辅助继电器有M500-M1023共524点，特殊型辅助继电器有M8000-M8255共157点。

(2)定时器逻辑

PLC一般采用硬件定时中断，软件计数的方法来实现定时逻辑功能，定时器一般包括：

定时条件：控制定时器操作。

定时语句：指定所使用的定时器，给出定时设定值。

定时器的当前值：记录定时时间。

定时继电器：定时器达到设定的值时为“1“（0N）状态，未开始定时或定时未达到设定值时为“0”（0FF）状态。

其逻辑功能如下表所示:

定时器：T

a.功能：该元件是定时用的，范围为0.001～32.767（1ms定时器）秒、0.01～327.67秒（10ms定时器）、0.1～3276.7秒（100ms定时器）。元件范围按十进制分配如下：

T246～T249：1ms定时器

T200～T245：10ms定时器

T0～T199：100 ms定时器

b.举例：

①梯形图：

②程序清单

LD X000

OUT T0 K123

LD T0

OUT y000

END

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TRABON / LUBRIQUIP 0.015IN3 (0.246CM3) MODULAR DIVIDER VALVE MSP-15T

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PNEUTRONICS DIGITAL PNEUMATIC CONTROL BOARD 990-004765-011

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ZIMMER PNEUMATIC CLAMPING ELEMENT MKS3501A NIB

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YUKEN HYDRAULIC CYLINDER CJT70-FA32B31B-?ABA-EK

SMC CYLINDER CDNAFN63N-220-D

PARKER SCHRADER BELLOWS 150PSI SOLENOID COIL NNB AL41533BJN5300B 103644

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ASCO 24DC-FT 11.2W SOLENOID N262.444 E / 400-425-142

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BIMBA PNEUMATIC CYLINDER 245-DP

DEZURIK CYLINDER ACTUATOR 941816R001 44482

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PMV 800kPA ELECTRO-PNEUMAT?IC DOUBLE ACTING VALVE POSITIONER P-2020 NIB

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CMT 80 PSI, 2.5" STROKE DOUBLE ACTING IMPACT MARKER 288SUBK.OB.25.R

YUKEN CYLINDER CJT70-TC40B320B?-ABA-EK 20A

PARKER SCHRADER BELLOWS 24 VDC SOLENOID PNEUMATIC VALVE W2154L579B

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FESTO PNEUMATIC CYLINDER DZH-50-200PPV-A

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NUMATICS 110/120V .06A 150 PSIG-AIR DOUBLE SOLENOID VALVE, I34BB400KP14D

MAC 24VDC 2.5W 25-150 PSI PNEUMATIC SOLENOID VALVE, 6313D-611-PM-59?1DA

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PARKER JIC P 16 SS FLUID CONNECTING NUT, LOT OF 17

HYDRAULIC THREAD NUT HMV 134/A-101