1. Chapter 3
Basic Logic Gates 1

2. Objectives You should be able to:
Describe the operation and use of AND gates and OR gates.
Construct truth tables for AND and OR gates.
Draw timing diagrams for AND and OR gates.
Use timing analysis to describe operation of an ENABLE function. 2

3. Objectives You should be able to:
Sketch external connections to IC chips to implement AND and OR logic circuits.
Explain how to use a logic pulser and a logic probe to troubleshoot digital ICs.
Describe the operation and use of NAND and NOR gates.
Construct truth tables for NAND and NOR gates. 3

4. Objectives You should be able to:
Draw timing diagrams for NAND and NOR gates.
Use the outputs of a Johnson shift counter to generate specialized waveforms using combinations of the five basic gates.
Develop a comparison of Boolean equations and truth tables for the five basic gates. 4

5. The AND Gate
The output, X, is HIGH if input A AND input B are both HIGH
Figure 3-1 5

6. The AND Gate
Truth Table – 6

7. The AND Gate Boolean Equation X = A AND B or X = AB
Can have more than two inputs
Number of combinations = 2N 7

8. The OR Gate
The output at X will be HIGH whenever input A OR input B is HIGH or both are HIGH
Figure 3-5 8

9. The OR Gate
Truth Table - 9

10. The OR Gate Boolean Equation X = A OR B or X = A+B
Can have more than two inputs
Number of combinations = 2N 10

11. Timing Analysis Timing diagram Oscilloscope Logic analyzer
Voltage versus time
Up to two
Logic analyzer
State table
Up to 16 11

12. Timing Analysis
MultiSIM AND gate simulation. 12

13. Timing Analysis
Example of timing analysis –
Determine the output. 13

14. Enable and Disable Functions
Enable function using AND gate
Figure 3-17 14

15. Enable and Disable Functions
Disable function using OR gate
Figure 3-18 15

16. Using Integrated Circuit Logic Gates
Enable and Disable
Pin Connections
VCC and GND
Figure 3-20 16

17. Introduction to Troubleshooting Techniques
The procedure used to find the fault or trouble in a circuit.
Logic Probe
Metal tip
Indicator lamp(s)
Floating - open circuit, neither HIGH nor LOW
Logic Pulser - provide known digital signal to a circuit 17

18. Discussion Points
Describe how a logic probe and pulser could be used to troubleshoot an AND gate.
Describe how a logic probe and pulser could be used to troubleshoot an OR gate. 18

19. The Inverter Used to complement or invert a digital signal Figure 3-2719

20. The Inverter Truth Table Boolean Equation X = A Inversion bar NOT gate20

21. The NAND Gate Same as the AND gate except that its output is inverted
Figure 3-29 21

22. The NAND Gate Truth Table Boolean Equation X = AB
Multiple inputs - the output is always HIGH unless all inputs go HIGH 22

23. The NOR Gate Same as the OR gate except that its output is inverted
Figure 3-36 23

24. The NOR Gate Truth Table Boolean Equation X = A + B
See Table 3-8 in your text 24

25. Logic Gate Waveform Generation
Repetitive waveform
Waveform generator
Johnson shift counter
Figure 3-43 25

26. Discussion Point
Which Johnson counter outputs must be connected to a 3 input AND gate to enable only CP #4? 26

27. Discussion Point
Sketch the output waveform resulting from inputting the Johnson counter outputs shown: 27

28. Using IC Logic Gates
Hex - six gates
Figure 3-60 28

29. Using IC Logic Gates Quad - four gates
three-, four-, and eight-input configurations
Figure 3-61 29

30. Summary of Basic Logic Gates and IEEE/IEC Standard Logic Symbols30

31. Summary of Basic Logic Gates and IEEE/IEC Standard Logic Symbols
Figure 3-65 31

32. Discussion Point
Briefly describe the operation of each of the basic logic gates:
AND OR
NOT (inverter)
NAND
NOR 32

33. Discussion Point Create a truth table for a three input NAND gate.
Write the Boolean equation for a 3 input OR gate. 33

34. Discussion Point
Sketch the output waveform X for the 2 input AND gate shown. 34

35. Discussion Point
Sketch the output waveforms for the Johnson shift counter outputs shown: 35

36. Discussion Point
Determine the problem (if any) with the 7427 NOR IC using the logic probe results shown: 36

37. Summary
The AND gate requires that all inputs are HIGH in order to get a HIGH output
The OR gate outputs a HIGH if any of its inputs are HIGH
An effective way to measure the precise timing relationships of digital waveforms is with an oscilloscope or a logic analyzer 37

38. Summary
Beside providing the basic logic functions, AND and OR gates can also be used to enable or disable a signal to pass from one point to another
There are several integrated circuits available in both TTL and CMOS that provide the basic logic functions 38

39. Summary
Two important troubleshooting tools are the logic pulser and the logic probe. The pulser is used to inject pulses into a circuit under test. The probe reads the level at a point in a circuit to determine is it is HIGH, LOW, or floating
An inverter provides an output that is the complement of its input 39

40. Summary A NAND gate outputs a LOW when all of its inputs are HIGH
A NOR gate outputs a HIGH when all of its inputs are LOW
Specialized waveforms can be created by using a repetitive waveform generator and the basic gates 40

41. Summary
Manufacturers’ data manuals are used by the technician to find the pin configuration and operating characteristics for the ICs used in modern circuitry. 41