COMBINATIONAL CIRCUITS USING TTL 74XX ICS Study of logic gates using 74XX ICs, Four-bit parallel adder(IC7483), Comparator(IC 7485), Decoder(IC 74138, IC 74154), BCD-to-7-segment decoder(IC 7447), Encoder(IC 74147), Multiplexer(IC74151), Demultiplexer (IC 74154).

Four-bit Parallel Adder IC 7483

1-bit Full adder Logic Symbol Truth Table Logic Diagram

Four-bit parallel adder Block diagram Logic symbol

Truth table IC’s used are: 74LS83A LS:Low power Schottky TTL 74LS283

Pin Diagram Logic Diagram

Adder Expansion 4-bit Parallel adder can be expanded to handle the addition of two 8-bit numbers by using two 4-bit adders. Carry input of Low-order adder(co) is connected to ground because there is no carry into least significant bit position. Carry output of Low-order adder is connected to carry input of high-order adder.

Example Show how two 74LS83A adders can be connected to form an 8-bit parallel adder. Show output bits for the following 8-bit input numbers: A8A7A6A5A4A3A2A1=10111001, B8B7B6B5B4B3B2B1=10011110

Application Simple Voting system

Comparator (IC 7485) Pin diagram Logic diagram Basic function: Compare the magnitudes of two binary numbers to determine relationship of those quantities. 74HC85 is a 4-bit comparator Cascading inputs: These inputs allow several comparators to be cascaded for comparison of any number of bits greater than four. Pin diagram Logic diagram

Functional table for 74LS85

Cascading comparator To expand the comparator, A<B, A=B and A>B outputs of Lower-order comparator are connected to the corresponding cascading inputs of the next higher-order comparator. Condition: Lower-order comparator A=B input=HIGH A<B and A>B inputs=LOW Use 74HC85 comparators to compare the magnitudes of two 8-bit numbers. Show the comparator with proper interconnections Expand for 16-bit comparator?

Problems 1) The binary numbers A=1011 and B=1010 are applied to the inputs of 74LS85. Determine the outputs. 2) The binary numbers A=11001011 and B=11010100 are applied to 8-bit comparator. Determine the states of output pins 5,6 and 7 on each 74LS85.

Decoder(IC 74138, IC 74154) Basic function: To detect the presence of a specified combination of bits(code) on its inputs and to indicate the presence of that code by a specified output level. It has n inputs to handle n bits and from one to 2n output lines to indicate the presence of one or more n-bit combinations. Applications: It is used to implement Combinational circuit. It is used to convert BCD to 7-segment code. It is used in memories to select particular register.

3-line to 8-line decoder(3 X 8)-74HC138 Pin Diagram Logic Diagram

Function table

4-line to 16-line decoder(4 X 16)-74HC154 Pin Diagram Logic Diagram   Function table

Cascading Decoders-cascading 5-bit number 74HC154 can handle only 4 bits. 5th bit,A4 is connected to chip select inputs, and of one decoder is connected to chip select inputs, and of other decoder .

Application-In computers for Input/Output selection

BCD-to-7 segment Decoder (IC 7447) Logic diagram of Basic 7-segment decoder Pin Diagram = lamp test = ripple blanking input =blanking input/ripple blanking output Logic Diagram-7447 Pin Diagram-7447

Encoder(IC 74147) It has 2n input lines and n output lines. Logic diagram of Decimal to BCD encoder Truth table for Decimal to BCD encoder Gate level diagram of Decimal to BCD encoder

74HC147- priority encoder It is also called as 10-line-to-4-line encoder. 74HC147 is a priority encoder with active-low inputs (0) for decimal digits 1 to 9 and active-low BCD outputs. Logic Diagram-74147 Pin Diagram-74147

Truth table-74147

Application-Keyboard Keys are represented by 10 push-button switches, each with a pull-up resistor to +v. When key is not pressed, line is HIGH. When key is pressed, line is connected to ground making a LOW to the corresponding encoder input.

Multiplexer(IC 74151) “It is a device that allows digital information from several sources to one line”. Gate level Diagram Truth table Logic Diagram Waveforms

8 to 1 MUX-74LS151 =LOW, allows the selected input data to pass through to the output. indicates AND relationship between data select inputs and each of the data inputs 0 through 7. Pin Diagram-74LS151 Logic Diagram

Truth table

Quad 2-input Multiplexer-IC 74HC157 It contains 4-separate 2-input multiplexers. All the multiplexers share common data select line and a common Enable. =LOW, allows selected input data to pass through to the output. =HIGH, prevents data from going through to the output (disables the multiplexer). G1=indicates AND relationship between data select input and data inputs. When data slect=HIGH, B inputs of the multiplexer are selected. When data slect=LOW, A inputs of the multiplexer are selected Logic Diagram Pin Diagram

Applications 1) 7-segment display multiplexer

2) Logic function generator It is used in generation of combinational logic functions in sum-of-product form. a) Implement the logic function specified in truth table by using 74LS151 8-input data selector. Compare this method with a discrete logic gate implementation Sol:

b) Implement the logic function specified in truth table by using 74LS151 8-input data selector. Compare this method with a discrete logic gate implementation Sol:

Demultiplexer 1 to 4 line Demultiplexer

1 to 16 line demultiplexer-74154 Logic Diagram