Table 4-8 shows a binary logic truth table for the NOT function which lists the results of the NOT function on the Input A.In relay-based logic circuits,a normally closed(NC)set of contracts is used to perform the NOT function,as shown in Figure 4-2.If the electric relay A is not energized,there is electrical current flow or logic continuity through the normally closed contacts,so output relay coil is energized or On.In other words,if input A is logic 0 or not On then the output Z is logic 1 or On.If input A is logic 1 or On,the normally closed contacts are opened,there is no current flow or logic flow in the circuit,relay Z is off,and output Z is 0.
OR Function
A logical OR function that has two or more inputs and a single output operates in accordance with the following definition:The output of an OR function assumes the 1 state if one or more inputs assume the 1 state. The inputs to a logic function OR gate can be designated by A,B,…,N and the output by Z.We make the assumption that the inputs and outputs can take only one of two possible values,either 0 or 1.The logic expression for this function is Z=A+B+…+N.Table 4-9 gives a two-input truth table for a two-input OR function.
Here is an example of OR logic in process control:If the water level in a hot water heater is low,or the temperature in the tank is too high,a logic system can be designed that will turn off the heater in the system using logic circuits or relays.Figure 4-3 shows this application,which uses relays to perform the logic function.
Logic Function Symbols
Two common sets of symbols are used in process control applications to represent logic function:graphic and ladder.The graphic symbols are generally used on engineering drawings to convey the overall logic plan for a discrete or batch control system.They are based on the standard ISA-5.2-1976(R1992)-Binary Logic Diagrams for Process Operations.The ladder logic symbols are used to describe a logic control plan if relays are being used to implement the control system or if programmable controller ladder logic is used to implement the control plan.Figure 4-5 compares these two sets of symbols for the most common logic functions encountered in logic control design.
Ladder Diagrams
Ladder diagrams are a traditional method for describing electrical logic controls.These circuits are called ladder diagrams because they look like ladders with rungs.Each rung of a ladder diagram is numbered so we can easily cross-reference between the sections on the drawing that describe
the control.
The utility of ladder diagrams can best be appreciated by investigating a simple control example.Figure 4-6 shows a typical process-level control application.In this application,let us assume that the flow into the tank is random.Let us also assume that we need to control the level in the tank by opening or closing the outlet valve(LV-1)based on the tank’s level as it is sensed by a level switch(LSH-1).
或功能
逻辑或功能，它有两个或多个输入和一个输出其运作依据以下的定义：一个或函数的输出假设1的状态，如果一个或多个输入假设1的状态。或门的投入逻辑功能可以指定的A，B，...，N和z的输出我们所作出的假设是，输入和输出可以只有两个可能的值之一，0或1，逻辑这个函数表达式是Z =甲+乙+ ... + N的表4-9给出了两个两年或函数的输入，输入真值表。
下面是在过程控制或逻辑例如：如果在热水器水位低，或在燃料箱温度过高，逻辑系统可以设计成将关闭系统中的加热器使用逻辑电路或relays.Figure 4-3显示了该应用程序，它使用继电器执行逻辑功能。
逻辑函数符号
常见的两种符号套用于过程控制等应用为代表的逻辑功能：图形和ladder.The图形符号一般都在工程图纸用来传递的总体逻辑计划离散或批量控制system.They是基于标准ISA -5.2-1976（R1992），二进制逻辑图的过程Operations.The梯形逻辑符号是用来描述逻辑继电器控制计划，如果被用来实现控制系统的可编程序控制器或梯形逻辑是用于实现控制计划。图4-5比较了最常见的逻辑功能符号的这两套中遇到的逻辑控制设计。
梯形图
阶梯图是一种用于描述电子电路逻辑controls.These传统方法被称为阶梯图，因为他们喜欢与一个阶梯rungs.Each梯形图梯子看的编号，所以我们可以很容易地跨在绘图参考章节之间的描述
控制。
在梯形图实用的最好办法是调查一个简单的控制example.Figure 4-6升值是一个典型的进程级控制application.In此应用程序，让我们假设入柜流动random.Let我们还假设我们需要控制在坦克通过打开或关闭出口阀（吕- 1）的基础上，坦克的水平，因为它是由一个水平感应开关（唇形- 1）的水平。
