Page 66 - Computer science 868 Class 12
P. 66

02


                                                                       COMPUTER HARDWARE

















                      Learning Objectives


                  2.1  Fundamental Gates or Basic Logic Gates       2.2  Universal Gates
                  2.3  Half Adder and Full Adder                    2.4  Encoder
                  2.5  Decoder                                      2.6  Multiplexer





              A logic gate is a digital circuit that depicts a logical relationship between the input and output voltages of the circuit.
              They are the building blocks of a digital circuit.
              A physical gate acts as a barrier and controls entry into a building. Similarly, logic gates are used to control the flow of
              information based on logical relations.  A logic gate can accept one or more inputs but always produces single output.
              They produce signal 1 as output when the input logic requirement is satisfied, 0 otherwise.
              Logic gates require a power supply. In both input and output 0 volts represents off(0) and 5 volts represent on(1). Each
              logic gate is depicted with a specific graphical symbol.


                  2.1 FUNDAMENTAL GATES OR BASIC LOGIC GATES
              The three basic logical operations namely conjunction (and), disjunction (or) and negation (not) are represented by
              their corresponding logic gates defined below. These gates are also called fundamental gates.

              2.1.1 NOT Gate
              It requires a single input and is also called a unary gate. Not gate inverts the output. Input 1 is changed to 0 and
                                                               –
              0 to 1. Not gate is represented as a complement (') or   (bar) at the top. The gate symbol and truth table of Not gate
              is given below.

                                                                   A          A'
                                                                   0           1
                                    Symbol of NOT gate             1           0            +        A        L
                                                              Truth table of NOT gate
                                                                                            -
              From the switching circuit of the NOT gate we find that the lamp glows when switch
              A is opened as electricity reaches the bulb. But when the switch is closed, the current
              does not reach the bulb and it does not glow.                                      Switching circuit
                                                                                               diagram of NOT gate


                6464  Touchpad Computer Science-XII
   61   62   63   64   65   66   67   68   69   70   71