Logic Levels

Logic Levels

Boolean logic variable ‘0’ or ‘1’ is not used to represent actual numbers but it is used to represent the state of voltage variable called logical level. Commonly used representation of logic levels are shown in Table below.

Table: Representation of Logic Levels for Boolean Variables

Logic 0	Logic 1
False	True
Open switch	Close switch

Low	High
No	Yes
OFF	ON

Inputs and outputs of logic gates can occur only in two levels. These two levels are termed HIGH and LOW, or TRUE and FALSE, or ON and OFF, or simply 1 and 0. There are two different ways to assign a signal value to logic level such as positive logic and negative logic. 

1. Positive Logic: If higher of the two voltage levels represents a logic ‘1’ and the lower of the two levels represents a logic ‘0’, then the logic system is referred to as a positive logic system. Figure 1 shows the positive logic system.

Figure 1: Positive Logic System

2. Negative Logic: If the higher of the two voltage levels represents a logic ‘0’ and the lower of the two levels represents a logic ‘1’, then the logic system is referred to as a negative logic system. Figure 2 shows the representation of negative logic systems.

Figure 2: Negative Logic System

3. Mixed Logic: In mixed logic, the assignment of logical values to voltage values is not fixed, and it can be decided by the logic designers. Mixed logic provides a simplified mechanism for the analysis and design of digital circuits. The proper use of mixed logic notation provides logic expressions and logic diagrams that are analogue to each other. Also, a mixed logic diagram provides clear information as to the operation of a circuit.

REFERENCES

1.        Digital Design by M. Morris Mano, Michael D Ciletti, Pearson

2.        Digital Fundamentals by Thomas L. Floyd, R. P. Jain, Pearson

3.        Digital Circuits and Design by S Salivahanan, S Arivazhagan, Vikas Publishing House Pvt Ltd.

4.        Digital Systems by Ronald J. Tocci, Neal S. Widmer, Gregory L. Moss, Pearson

5.        Digital Electronics Principles And Integrated Circuits by Anil K. Maini

6.        Fundamentals of Digital Circuits by Anand Kumar

7.        Digital Electronics by John Morris 

8.        Digital Electronics : An Introduction To Theory And Practice By William Gothmann.