Digital Logic


   Following on from our description of digital electronics is the subject of digital logic. Using it's simplest definition, digital logic is the implementation of logical statements using digital electronics. This, of course, begs the question , what is a logic statement. A logical statement can be something as simple as ....
    "If it is dark and I am in the room, I will turn on the light"
   in this statement we have linked the decision of turning on the light to two conditions  a) being dark and b) being in the room.  Only if both of these conditions are TRUE will we turn on the light.  In logic statements like this we can talk about conditions and decisions as being TRUE or FALSE and the logic linking them as a combination of simple words such as AND, OR and NOT.  In the above statement we can write it more concisely as
                                            L = A AND B
    where L is the on/off state of the light.  In this case, moving more into the abstract language of logic,  we can say that L is TRUE only when both A AND B are TRUE.  Or, expressing the same relationship a different way
                           L is FALSE when A OR B is FALSE
   Both are perfectly valid ways of expressing the original logic statement.

  Supposing we wanted to create an electronic circuit that automatically implemented this logic function, how would we represent the conditions, decisions and connecting logic statement ?  The simple answer is logic gates.  Logic gates are based on real electronic devices that are designed to implement logic functions. Logic conditions that are being tested are the inputs to these devices and the decisions are the outputs. TRUE and FALSE are represented by two different voltage levels (traditionally 5v for TRUE and 0v for FALSE).  When logic statements are represented in this way they are normally shown as a circuit diagram using symbols to represent each of the common logic functions. It is also common to abbreviate the TRUE and FALSE to '1' and '0' respectively. Some of the most common symbols are shown below with a more comprehensive table of symbols logic AND gate schematic symbol
digital logic AND gate schematic symbol 
digital logic OR gate schematic symbol used in circuit diagrams
 digital logic OR gate schematic symbol used in circuit diagrams

   Using these symbols as the building blocks, complex logic statements can be created to represent the desired logic relationships and using the logic devices described, these relationships can actually be implemented in electronic form


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Copyright 2008