1. 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).

2. Four-bit Parallel Adder
IC 7483

3. 1-bit Full adder
Logic Symbol
Truth Table
Logic Diagram

4. Four-bit parallel adder
Block diagram
Logic symbol

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

6. Pin Diagram
Logic Diagram

7. 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.

8. 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= , B8B7B6B5B4B3B2B1=

9. Application
Simple Voting system

10. 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

11. Functional table for 74LS85

12. 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?

13. 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= and B= are applied to 8-bit comparator. Determine the states of output pins 5,6 and 7 on each 74LS85.

14. 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.

15. 3-line to 8-line decoder(3 X 8)-74HC138
Pin Diagram
Logic Diagram

16. Function table

17. 4-line to 16-line decoder(4 X 16)-74HC154
Pin Diagram
Logic Diagram
Function table

18. 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 .

19. Application-In computers for Input/Output selection

20. 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

21. 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

22. 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

23. Truth table-74147

24. 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.

25. 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

26. 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

27. Truth table

28. 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

29. Applications
1) 7-segment display multiplexer

30. 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:

31. 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:

32. Demultiplexer
1 to 4 line Demultiplexer

33. 1 to 16 line demultiplexer-74154
Logic Diagram